diff --git a/bits/40_harb.js b/bits/40_harb.js
index de95b76ce..6e6605115 100644
--- a/bits/40_harb.js
+++ b/bits/40_harb.js
@@ -235,7 +235,11 @@ function dbf_to_sheet(buf, opts)/*:Worksheet*/ {
 }
 
 function dbf_to_workbook(buf, opts)/*:Workbook*/ {
-	try { return sheet_to_workbook(dbf_to_sheet(buf, opts), opts); }
+	try {
+		var o = sheet_to_workbook(dbf_to_sheet(buf, opts), opts);
+		o.bookType = "dbf";
+		return o;
+	}
 	catch(e) { if(opts && opts.WTF) throw e; }
 	return ({SheetNames:[],Sheets:{}});
 }
@@ -546,6 +550,7 @@ var SYLK = /*#__PURE__*/(function() {
 		keys(ws).forEach(function(k) { o[k] = ws[k]; });
 		var outwb = sheet_to_workbook(o, opts);
 		keys(wb).forEach(function(k) { outwb[k] = wb[k]; });
+		outwb.bookType = "sylk";
 		return outwb;
 	}
 
@@ -664,7 +669,11 @@ var DIF = /*#__PURE__*/(function() {
 	}
 
 	function dif_to_sheet(str/*:string*/, opts)/*:Worksheet*/ { return aoa_to_sheet(dif_to_aoa(str, opts), opts); }
-	function dif_to_workbook(str/*:string*/, opts)/*:Workbook*/ { return sheet_to_workbook(dif_to_sheet(str, opts), opts); }
+	function dif_to_workbook(str/*:string*/, opts)/*:Workbook*/ {
+		var o = sheet_to_workbook(dif_to_sheet(str, opts), opts);
+		o.bookType = "dif";
+		return o;
+	}
 
 	var sheet_to_dif = /*#__PURE__*/(function() {
 		var push_field = function pf(o/*:Array<string>*/, topic/*:string*/, v/*:number*/, n/*:number*/, s/*:string*/) {
diff --git a/bits/45_rtf.js b/bits/45_rtf.js
index 3a1b7eeec..dd4b42f97 100644
--- a/bits/45_rtf.js
+++ b/bits/45_rtf.js
@@ -1,78 +1,97 @@
-function rtf_to_workbook(d/*:RawData*/, opts)/*:Workbook*/ {
-	switch(opts.type) {
-		case 'base64': return rtf_to_book_str(Base64_decode(d), opts);
-		case 'binary': return rtf_to_book_str(d, opts);
-		case 'buffer': return rtf_to_book_str(has_buf && Buffer.isBuffer(d) ? d.toString('binary') : a2s(d), opts);
-		case 'array':  return rtf_to_book_str(cc2str(d), opts);
-	}
-	throw new Error("Unrecognized type " + opts.type);
+function rtf_to_sheet(d, opts) {
+  switch (opts.type) {
+    case "base64":
+      return rtf_to_sheet_str(Base64_decode(d), opts);
+    case "binary":
+      return rtf_to_sheet_str(d, opts);
+    case "buffer":
+      return rtf_to_sheet_str(has_buf && Buffer.isBuffer(d) ? d.toString("binary") : a2s(d), opts);
+    case "array":
+      return rtf_to_sheet_str(cc2str(d), opts);
+  }
+  throw new Error("Unrecognized type " + opts.type);
 }
-
-/* TODO: RTF technically can store multiple tables, even if Excel does not */
-function rtf_to_book_str(str/*:string*/, opts)/*:Workbook*/ {
-	var o = opts || {};
-	var sname = o.sheet || "Sheet1";
-	var ws/*:Worksheet*/ = o.dense ? ([]/*:any*/) : ({}/*:any*/);
-	var wb/*:Workbook*/ = { SheetNames: [ sname ], Sheets: {} };
-	wb.Sheets[sname] = ws;
-
-	var rows = str.match(/\\trowd[\s\S]*?\\row\b/g);
-	if(!rows.length) throw new Error("RTF missing table");
-	var range/*:Range*/ = ({s: {c:0, r:0}, e: {c:0, r:rows.length - 1}}/*:any*/);
-	rows.forEach(function(rowtf, R) {
-		if(Array.isArray(ws)) ws[R] = [];
-		var rtfre = /\\[\w\-]+\b/g;
-		var last_index = 0;
-		var res;
-		var C = -1;
-		var payload = [];
-		while((res = rtfre.exec(rowtf))) {
-			var data = rowtf.slice(last_index, rtfre.lastIndex - res[0].length);
-			if(data.charCodeAt(0) == 0x20) data = data.slice(1);
-			if(data.length) payload.push(data);
-			switch(res[0]) {
-				case "\\cell":
-					++C;
-					if(payload.length) {
-						// TODO: value parsing, including codepage adjustments
-						var cell = {v: payload.join(""), t:"s"};
-						if(cell.v == "TRUE" || cell.v == "FALSE") { cell.v = cell.v == "TRUE"; cell.t = "b"; }
-						else if(!isNaN(fuzzynum(cell.v))) { cell.t = 'n'; if(o.cellText !== false) cell.w = cell.v; cell.v = fuzzynum(cell.v); }
-
-						if(Array.isArray(ws)) ws[R][C] = cell;
-						else ws[encode_cell({r:R, c:C})] = cell;
-					}
-					payload = [];
-					break;
-				case "\\par": // NOTE: Excel serializes both "\r" and "\n" as "\\par"
-					payload.push("\n");
-					break;
-			}
-			last_index = rtfre.lastIndex;
-		}
-		if(C > range.e.c) range.e.c = C;
-	});
-	ws['!ref'] = encode_range(range);
-	return wb;
+function rtf_to_sheet_str(str, opts) {
+  var o = opts || {};
+  var ws = o.dense ? [] : {};
+  var rows = str.match(/\\trowd[\s\S]*?\\row\b/g);
+  if (!rows)
+    throw new Error("RTF missing table");
+  var range = { s: { c: 0, r: 0 }, e: { c: 0, r: rows.length - 1 } };
+  rows.forEach(function(rowtf, R) {
+    if (Array.isArray(ws))
+      ws[R] = [];
+    var rtfre = /\\[\w\-]+\b/g;
+    var last_index = 0;
+    var res;
+    var C = -1;
+    var payload = [];
+    while ((res = rtfre.exec(rowtf)) != null) {
+      var data = rowtf.slice(last_index, rtfre.lastIndex - res[0].length);
+      if (data.charCodeAt(0) == 32)
+        data = data.slice(1);
+      if (data.length)
+        payload.push(data);
+      switch (res[0]) {
+        case "\\cell":
+          ++C;
+          if (payload.length) {
+            var cell = { v: payload.join(""), t: "s" };
+            if (cell.v == "TRUE" || cell.v == "FALSE") {
+              cell.v = cell.v == "TRUE";
+              cell.t = "b";
+            } else if (!isNaN(fuzzynum(cell.v))) {
+              cell.t = "n";
+              if (o.cellText !== false)
+                cell.w = cell.v;
+              cell.v = fuzzynum(cell.v);
+            }
+            if (Array.isArray(ws))
+              ws[R][C] = cell;
+            else
+              ws[encode_cell({ r: R, c: C })] = cell;
+          }
+          payload = [];
+          break;
+        case "\\par":
+          payload.push("\n");
+          break;
+      }
+      last_index = rtfre.lastIndex;
+    }
+    if (C > range.e.c)
+      range.e.c = C;
+  });
+  ws["!ref"] = encode_range(range);
+  return ws;
 }
-
-/* TODO: standardize sheet names as titles for tables */
-function sheet_to_rtf(ws/*:Worksheet*//*::, opts*/)/*:string*/ {
-	var o = ["{\\rtf1\\ansi"];
-	var r = safe_decode_range(ws['!ref']), cell/*:Cell*/;
-	var dense = Array.isArray(ws);
-	for(var R = r.s.r; R <= r.e.r; ++R) {
-		o.push("\\trowd\\trautofit1");
-		for(var C = r.s.c; C <= r.e.c; ++C) o.push("\\cellx" + (C+1));
-		o.push("\\pard\\intbl");
-		for(C = r.s.c; C <= r.e.c; ++C) {
-			var coord = encode_cell({r:R,c:C});
-			cell = dense ? (ws[R]||[])[C]: ws[coord];
-			if(!cell || cell.v == null && (!cell.f || cell.F)) continue;
-			o.push(" " + (cell.w || (format_cell(cell), cell.w)).replace(/[\r\n]/g, "\\par "));
-			o.push("\\cell");
-		}
-		o.push("\\pard\\intbl\\row");
-	}
-	return o.join("") + "}";
+function rtf_to_workbook(d, opts) {
+  var wb = sheet_to_workbook(rtf_to_sheet(d, opts), opts);
+  wb.bookType = "rtf";
+  return wb;
+}
+function sheet_to_rtf(ws, opts) {
+  var o = ["{\\rtf1\\ansi"];
+  if (!ws["!ref"])
+    return o[0] + "}";
+  var r = safe_decode_range(ws["!ref"]), cell;
+  var dense = Array.isArray(ws);
+  for (var R = r.s.r; R <= r.e.r; ++R) {
+    o.push("\\trowd\\trautofit1");
+    for (var C = r.s.c; C <= r.e.c; ++C)
+      o.push("\\cellx" + (C + 1));
+    o.push("\\pard\\intbl");
+    for (C = r.s.c; C <= r.e.c; ++C) {
+      var coord = encode_cell({ r: R, c: C });
+      cell = dense ? (ws[R] || [])[C] : ws[coord];
+      if (!cell || cell.v == null && (!cell.f || cell.F)) {
+        o.push(" \\cell");
+        continue;
+      }
+      o.push(" " + (cell.w || (format_cell(cell), cell.w) || "").replace(/[\r\n]/g, "\\par "));
+      o.push("\\cell");
+    }
+    o.push("\\pard\\intbl\\row");
+  }
+  return o.join("") + "}";
 }
diff --git a/bits/75_xlml.js b/bits/75_xlml.js
index ee8e97d9e..6dad076a7 100644
--- a/bits/75_xlml.js
+++ b/bits/75_xlml.js
@@ -919,6 +919,7 @@ function parse_xlml_xml(d, _opts)/*:Workbook*/ {
 	out.SSF = dup(table_fmt);
 	out.Props = Props;
 	out.Custprops = Custprops;
+	out.bookType = "xlml";
 	return out;
 }
 
diff --git a/bits/79_html.js b/bits/79_html.js
index a821ced5e..fbd4df4fc 100644
--- a/bits/79_html.js
+++ b/bits/79_html.js
@@ -97,9 +97,14 @@ var HTML_END = '</body></html>';
 function html_to_workbook(str/*:string*/, opts)/*:Workbook*/ {
 	var mtch = str.match(/<table[\s\S]*?>[\s\S]*?<\/table>/gi);
 	if(!mtch || mtch.length == 0) throw new Error("Invalid HTML: could not find <table>");
-	if(mtch.length == 1) return sheet_to_workbook(html_to_sheet(mtch[0], opts), opts);
+	if(mtch.length == 1) {
+		var w = sheet_to_workbook(html_to_sheet(mtch[0], opts), opts);
+		w.bookType = "html";
+		return w;
+	}
 	var wb = book_new();
 	mtch.forEach(function(s, idx) { book_append_sheet(wb, html_to_sheet(s, opts), "Sheet" + (idx+1)); });
+	wb.bookType = "html";
 	return wb;
 }
 
@@ -215,7 +220,9 @@ function parse_dom_table(table/*:HTMLElement*/, _opts/*:?any*/)/*:Worksheet*/ {
 }
 
 function table_to_book(table/*:HTMLElement*/, opts/*:?any*/)/*:Workbook*/ {
-	return sheet_to_workbook(parse_dom_table(table, opts), opts);
+	var o = sheet_to_workbook(parse_dom_table(table, opts), opts);
+	//o.bookType = "dom"; // TODO: define a type for this
+	return o;
 }
 
 function is_dom_element_hidden(element/*:HTMLElement*/)/*:boolean*/ {
diff --git a/bits/80_parseods.js b/bits/80_parseods.js
index f6423a94e..7daa7904c 100644
--- a/bits/80_parseods.js
+++ b/bits/80_parseods.js
@@ -770,9 +770,12 @@ function parse_ods(zip/*:ZIPFile*/, opts/*:?ParseOpts*/)/*:Workbook*/ {
 	if(!content) throw new Error("Missing content.xml in ODS / UOF file");
 	var wb = parse_content_xml(utf8read(content), opts, Styles);
 	if(safegetzipfile(zip, 'meta.xml')) wb.Props = parse_core_props(getzipdata(zip, 'meta.xml'));
+	wb.bookType = "ods";
 	return wb;
 }
 function parse_fods(data/*:string*/, opts/*:?ParseOpts*/)/*:Workbook*/ {
-	return parse_content_xml(data, opts);
+	var wb = parse_content_xml(data, opts);
+	wb.bookType = "fods";
+	return wb;
 }
 
diff --git a/bits/83_numbers.js b/bits/83_numbers.js
index c7594f660..e43e8e4ec 100644
--- a/bits/83_numbers.js
+++ b/bits/83_numbers.js
@@ -396,7 +396,7 @@ function parse_old_storage(buf, sst, rsst, v) {
   var ret;
   switch (buf[2]) {
     case 0:
-      break;
+      return void 0;
     case 2:
       ret = { t: "n", v: ieee };
       break;
@@ -456,7 +456,7 @@ function parse_new_storage(buf, sst, rsst) {
   var ret;
   switch (buf[1]) {
     case 0:
-      break;
+      return void 0;
     case 2:
       ret = { t: "n", v: d128 };
       break;
@@ -761,6 +761,7 @@ function parse_TN_DocumentArchive(M, root) {
   });
   if (out.SheetNames.length == 0)
     throw new Error("Empty NUMBERS file");
+  out.bookType = "numbers";
   return out;
 }
 function parse_numbers_iwa(cfb) {
@@ -961,6 +962,8 @@ function write_numbers_iwa(wb, opts) {
     throw new Error("Too many messages");
   }
   var entry = CFB.find(cfb, dependents[1].location);
+  if (!entry)
+    throw "Could not find ".concat(dependents[1].location, " in Numbers template");
   var x = parse_iwa_file(decompress_iwa_file(entry.content));
   var docroot;
   for (var xi = 0; xi < x.length; ++xi) {
@@ -968,8 +971,12 @@ function write_numbers_iwa(wb, opts) {
     if (packet.id == 1)
       docroot = packet;
   }
+  if (docroot == null)
+    throw "Could not find message ".concat(1, " in Numbers template");
   var sheetrootref = parse_TSP_Reference(parse_shallow(docroot.messages[0].data)[1][0].data);
   entry = CFB.find(cfb, dependents[sheetrootref].location);
+  if (!entry)
+    throw "Could not find ".concat(dependents[sheetrootref].location, " in Numbers template");
   x = parse_iwa_file(decompress_iwa_file(entry.content));
   for (xi = 0; xi < x.length; ++xi) {
     packet = x[xi];
@@ -985,6 +992,8 @@ function write_numbers_iwa(wb, opts) {
   entry.size = entry.content.length;
   sheetrootref = parse_TSP_Reference(sheetref[2][0].data);
   entry = CFB.find(cfb, dependents[sheetrootref].location);
+  if (!entry)
+    throw "Could not find ".concat(dependents[sheetrootref].location, " in Numbers template");
   x = parse_iwa_file(decompress_iwa_file(entry.content));
   for (xi = 0; xi < x.length; ++xi) {
     packet = x[xi];
@@ -993,6 +1002,8 @@ function write_numbers_iwa(wb, opts) {
   }
   sheetrootref = parse_TSP_Reference(parse_shallow(docroot.messages[0].data)[2][0].data);
   entry = CFB.find(cfb, dependents[sheetrootref].location);
+  if (!entry)
+    throw "Could not find ".concat(dependents[sheetrootref].location, " in Numbers template");
   x = parse_iwa_file(decompress_iwa_file(entry.content));
   for (xi = 0; xi < x.length; ++xi) {
     packet = x[xi];
@@ -1005,6 +1016,8 @@ function write_numbers_iwa(wb, opts) {
     pb[7][0].data = write_varint49(range.e.c + 1);
     var cruidsref = parse_TSP_Reference(pb[46][0].data);
     var oldbucket = CFB.find(cfb, dependents[cruidsref].location);
+    if (!oldbucket)
+      throw "Could not find ".concat(dependents[cruidsref].location, " in Numbers template");
     var _x = parse_iwa_file(decompress_iwa_file(oldbucket.content));
     {
       for (var j = 0; j < _x.length; ++j) {
@@ -1047,6 +1060,8 @@ function write_numbers_iwa(wb, opts) {
       var row_headers = parse_shallow(store[1][0].data);
       var row_header_ref = parse_TSP_Reference(row_headers[2][0].data);
       oldbucket = CFB.find(cfb, dependents[row_header_ref].location);
+      if (!oldbucket)
+        throw "Could not find ".concat(dependents[cruidsref].location, " in Numbers template");
       _x = parse_iwa_file(decompress_iwa_file(oldbucket.content));
       {
         if (_x[0].id != row_header_ref)
@@ -1065,6 +1080,8 @@ function write_numbers_iwa(wb, opts) {
       oldbucket.size = oldbucket.content.length;
       var col_header_ref = parse_TSP_Reference(store[2][0].data);
       oldbucket = CFB.find(cfb, dependents[col_header_ref].location);
+      if (!oldbucket)
+        throw "Could not find ".concat(dependents[cruidsref].location, " in Numbers template");
       _x = parse_iwa_file(decompress_iwa_file(oldbucket.content));
       {
         if (_x[0].id != col_header_ref)
@@ -1109,6 +1126,8 @@ function write_numbers_iwa(wb, opts) {
       var sstref = parse_TSP_Reference(store[4][0].data);
       (function() {
         var sentry = CFB.find(cfb, dependents[sstref].location);
+        if (!sentry)
+          throw "Could not find ".concat(dependents[sstref].location, " in Numbers template");
         var sx = parse_iwa_file(decompress_iwa_file(sentry.content));
         var sstroot;
         for (var sxi = 0; sxi < sx.length; ++sxi) {
@@ -1116,6 +1135,8 @@ function write_numbers_iwa(wb, opts) {
           if (packet2.id == sstref)
             sstroot = packet2;
         }
+        if (sstroot == null)
+          throw "Could not find message ".concat(sstref, " in Numbers template");
         var sstdata = parse_shallow(sstroot.messages[0].data);
         {
           sstdata[3] = [];
@@ -1141,6 +1162,8 @@ function write_numbers_iwa(wb, opts) {
           var tileref = parse_TSP_Reference(tl[2][0].data);
           (function() {
             var tentry = CFB.find(cfb, dependents[tileref].location);
+            if (!tentry)
+              throw "Could not find ".concat(dependents[tileref].location, " in Numbers template");
             var tx = parse_iwa_file(decompress_iwa_file(tentry.content));
             var tileroot;
             for (var sxi = 0; sxi < tx.length; ++sxi) {
diff --git a/bits/85_parsezip.js b/bits/85_parsezip.js
index 9066232ab..6fc25d157 100644
--- a/bits/85_parsezip.js
+++ b/bits/85_parsezip.js
@@ -247,6 +247,8 @@ function parse_zip(zip/*:ZIP*/, opts/*:?ParseOpts*/)/*:Workbook*/ {
 		if(dir.vba.length > 0) out.vbaraw = getzipdata(zip,strip_front_slash(dir.vba[0]),true);
 		else if(dir.defaults && dir.defaults.bin === CT_VBA) out.vbaraw = getzipdata(zip, 'xl/vbaProject.bin',true);
 	}
+	// TODO: pass back content types metdata for xlsm/xlsx resolution
+	out.bookType = xlsb ? "xlsb" : "xlsx";
 	return out;
 }
 
diff --git a/demos/README.md b/demos/README.md
index 96c9d8687..20d692624 100644
--- a/demos/README.md
+++ b/demos/README.md
@@ -20,7 +20,7 @@ can be installed with Bash on Windows or with `cygwin`.
 **JavaScript APIs**
 - [`XMLHttpRequest and fetch`](xhr/)
 - [`Clipboard Data`](https://docs.sheetjs.com/docs/getting-started/demos/clipboard)
-- [`Typed Arrays and Math`](array/)
+- [`Typed Arrays for Machine Learning`](https://docs.sheetjs.com/docs/getting-started/demos/ml)
 
 **Frameworks**
 - [`angularjs`](angular/)
diff --git a/demos/array/Makefile b/demos/array/Makefile
deleted file mode 100644
index abeba97b5..000000000
--- a/demos/array/Makefile
+++ /dev/null
@@ -1,7 +0,0 @@
-.PHONY: init
-init:
-	npm i
-
-.PHONY: tfjs
-tfjs: init
-	node tf.js
diff --git a/demos/array/README.md b/demos/array/README.md
index 18a50d328..dcf14b3f2 100644
--- a/demos/array/README.md
+++ b/demos/array/README.md
@@ -1,131 +1,6 @@
 # Typed Arrays and Math
 
-ECMAScript version 6 introduced Typed Arrays, array-like objects designed for
-low-level optimizations and predictable operations.  They are supported in most
-modern browsers and form the basis of various APIs, including NodeJS Buffers,
-WebGL buffers, WebAssembly, and tensors in linear algebra and math libraries.
-
-This demo covers conversions between worksheets and Typed Arrays.  It also tries
-to cover common numerical libraries that work with data arrays.
-
-Excel supports a subset of the IEEE754 Double precision floating point numbers,
-but many libraries only support `Float32` Single precision values. `Math.fround`
-rounds `Number` values to the nearest single-precision floating point value.
-
-## Working with Data in Typed Arrays
-
-Typed arrays are not true Array objects.  The array of array utility functions
-like `aoa_to_sheet` will not handle arrays of Typed Arrays.
-
-#### Exporting Typed Arrays to a Worksheet
-
-A single typed array can be converted to a pure JS array with `Array.from`:
-
-```js
-var column = Array.from(dataset_typedarray);
-```
-
-`aoa_to_sheet` expects a row-major array of arrays.  To export multiple data
-sets, "transpose" the data:
-
-```js
-/* assuming data is an array of typed arrays */
-var aoa = [];
-for(var i = 0; i < data.length; ++i) {
-  for(var j = 0; j < data[i].length; ++j) {
-    if(!aoa[j]) aoa[j] = [];
-    aoa[j][i] = data[i][j];
-  }
-}
-/* aoa can be directly converted to a worksheet object */
-var ws = XLSX.utils.aoa_to_sheet(aoa);
-```
-
-#### Importing Data from a Spreadsheet
-
-`sheet_to_json` with the option `header:1` will generate a row-major array of
-arrays that can be transposed.  However, it is more efficient to walk the sheet
-manually:
-
-```js
-/* find worksheet range */
-var range = XLSX.utils.decode_range(ws['!ref']);
-var out = []
-/* walk the columns */
-for(var C = range.s.c; C <= range.e.c; ++C) {
-  /* create the typed array */
-  var ta = new Float32Array(range.e.r - range.s.r + 1);
-  /* walk the rows */
-  for(var R = range.s.r; R <= range.e.r; ++R) {
-    /* find the cell, skip it if the cell isn't numeric or boolean */
-    var cell = ws[XLSX.utils.encode_cell({r:R, c:C})];
-    if(!cell || cell.t != 'n' && cell.t != 'b') continue;
-    /* assign to the typed array */
-    ta[R - range.s.r] = cell.v;
-  }
-  out.push(ta);
-}
-```
-
-If the data set has a header row, the loop can be adjusted to skip those rows.
-
-
-## Demos
-
-Each example focuses on single-variable linear regression.  Sample worksheets
-will start with a label row.  The first column is the x-value and the second
-column is the y-value.  A sample spreadsheet can be generated randomly:
-
-```js
-var aoo = [];
-for(var i = 0; i < 100; ++i) aoo.push({x:i, y:2 * i + Math.random()});
-var ws = XLSX.utils.json_to_sheet(aoo);
-var wb = XLSX.utils.book_new(); XLSX.utils.book_append_sheet(wb, ws, "Sheet1");
-XLSX.writeFile(wb, "linreg.xlsx");
-```
-
-Some libraries provide utility functions that work with plain arrays of numbers.
-When possible, they should be preferred over manual conversion.
-
-Reshaping raw float arrays and exporting to a worksheet is straightforward:
-
-```js
-function array_to_sheet(farray, shape, headers) {
-  /* generate new AOA from the float array */
-  var aoa = [];
-  for(var j = 0; j < shape[0]; ++j) {
-    aoa[j] = [];
-    for(var i = 0; i < shape[1]; ++i) aoa[j][i] = farray[j * shape[1] + i];
-  }
-
-  /* add headers and generate worksheet */
-  if(headers) aoa.unshift(headers);
-  return XLSX.utils.aoa_to_sheet(aoa);
-}
-```
-
-#### TensorFlow
-
-[TensorFlow](https://js.tensorflow.org/) `tensor` objects can be created from
-arrays of arrays:
-
-```js
-var tensor = tf.tensor2d(aoa).transpose();
-var col1 = tensor.slice([0,0], [1,tensor.shape[1]]).flatten();
-var col2 = tensor.slice([1,0], [1,tensor.shape[1]]).flatten();
-```
-
-`stack` should be used to create the 2-d tensor for export:
-
-```js
-var tensor = tf.stack([col1, col2]).transpose();
-var shape = tensor.shape;
-var farray = tensor.dataSync();
-var ws = array_to_sheet(farray, shape, ["header1", "header2"]);
-```
-
-The demo generates a sample dataset and uses a simple linear predictor with
-least-squares scoring to calculate regression coefficients.  The tensors are
-exported to a new file.
+[The new demo](https://docs.sheetjs.com/docs/getting-started/demos/ml) includes 
+interactive examples as well as strategies for CSV and JS Array interchange.
 
 [![Analytics](https://ga-beacon.appspot.com/UA-36810333-1/SheetJS/js-xlsx?pixel)](https://github.com/SheetJS/js-xlsx)
diff --git a/demos/array/linest.js b/demos/array/linest.js
deleted file mode 100644
index 4d38b85fd..000000000
--- a/demos/array/linest.js
+++ /dev/null
@@ -1,27 +0,0 @@
-/* xlsx.js (C) 2013-present SheetJS -- http://sheetjs.com */
-/*global module, require, XLSX:true */
-if(typeof require !== 'undefined' && typeof XLSX === 'undefined') XLSX = require('xlsx');
-
-function generate_random_file(n) {
-	if(!n) n = 100;
-	var aoo = [];
-	var x_ = 0, y_ = 0, xx = 0, xy = 0;
-	for(var i = 0; i < n; ++i) {
-		var y = Math.fround(2 * i + Math.random());
-		aoo.push({x:i, y:y});
-		x_ += i / n; y_ += y / n; xx += i*i; xy += i * y;
-	}
-	var m = Math.fround((xy - n * x_ * y_)/(xx - n * x_ * x_));
-	console.log(m, Math.fround(y_ - m * x_), "JS Pre");
-	var ws = XLSX.utils.json_to_sheet(aoo);
-	var wb = XLSX.utils.book_new();
-	XLSX.utils.book_append_sheet(wb, ws, "Sheet1");
-	ws = XLSX.utils.aoa_to_sheet([[2, 0]]);
-	XLSX.utils.sheet_set_array_formula(ws, "A1:B1", "LINEST(Sheet1!B2:B101,Sheet1!A2:A101)");
-	XLSX.utils.book_append_sheet(wb, ws, "Sheet2");
-
-	XLSX.writeFile(wb, "linreg.xlsx");
-}
-if(typeof module !== 'undefined') module.exports = {
-	generate_random_file: generate_random_file
-};
diff --git a/demos/array/package.json b/demos/array/package.json
deleted file mode 100644
index 2630a5bca..000000000
--- a/demos/array/package.json
+++ /dev/null
@@ -1,6 +0,0 @@
-{
-  "dependencies": {
-    "@tensorflow/tfjs": "^3.16.0",
-    "xlsx": "https://cdn.sheetjs.com/xlsx-latest/xlsx-latest.tgz"
-  }
-}
diff --git a/demos/array/tf.js b/demos/array/tf.js
deleted file mode 100644
index 3452a77f3..000000000
--- a/demos/array/tf.js
+++ /dev/null
@@ -1,78 +0,0 @@
-/* xlsx.js (C) 2013-present SheetJS -- http://sheetjs.com */
-/* eslint-env node */
-var XLSX = require('xlsx');
-var tf = require('@tensorflow/tfjs');
-var linest = require('./linest');
-
-/* generate linreg.xlsx with 100 random points */
-var N = 100;
-linest.generate_random_file(N);
-
-/* get the first worksheet as an array of arrays, skip the first row */
-var wb = XLSX.readFile('linreg.xlsx');
-var ws = wb.Sheets[wb.SheetNames[0]];
-var aoa = XLSX.utils.sheet_to_json(ws, {header:1, raw:true}).slice(1);
-
-/* calculate the coefficients in JS */
-(function(aoa) {
-	var x_ = 0, y_ = 0, xx = 0, xy = 0, n = aoa.length;
-	for(var i = 0; i < n; ++i) {
-		x_ += aoa[i][0] / n;
-		y_ += aoa[i][1] / n;
-		xx += aoa[i][0] * aoa[i][0];
-		xy += aoa[i][0] * aoa[i][1];
-	}
-	var m = Math.fround((xy - n * x_ * y_)/(xx - n * x_ * x_));
-	console.log(m, Math.fround(y_ - m * x_), "JS Post");
-})(aoa);
-
-/* build X and Y vectors */
-var tensor = tf.tensor2d(aoa).transpose();
-console.log(tensor.shape);
-var xs = tensor.slice([0,0], [1,tensor.shape[1]]).flatten();
-var ys = tensor.slice([1,0], [1,tensor.shape[1]]).flatten();
-
-/* set up variables with initial guess */
-var x_ = xs.mean().dataSync()[0];
-var y_ = ys.mean().dataSync()[0];
-var a = tf.variable(tf.scalar(y_/x_));
-var b = tf.variable(tf.scalar(Math.random()));
-
-/* linear predictor */
-function predict(x) { return tf.tidy(function() { return a.mul(x).add(b); }); }
-/* mean square scoring */
-function loss(yh, y) { return yh.sub(y).square().mean(); }
-
-/* train */
-for(var j = 0; j < 5; ++j) {
-	var learning_rate = 0.0001 /(j+1), iterations = 1000;
-	var optimizer = tf.train.sgd(learning_rate);
-
-	for(var i = 0; i < iterations; ++i) optimizer.minimize(function() {
-		var pred = predict(xs);
-		var L = loss(pred, ys);
-		return L
-	});
-
-	/* compute the coefficient */
-	var m = a.dataSync()[0], b_ = b.dataSync()[0];
-	console.log(m, b_, "TF " + iterations * (j+1));
-}
-
-/* export data to aoa */
-var yh = predict(xs);
-var tfdata = tf.stack([xs, ys, yh]).transpose();
-var shape = tfdata.shape;
-var tfarr = tfdata.dataSync();
-var tfaoa = [];
-for(j = 0; j < shape[0]; ++j) {
-	tfaoa[j] = [];
-	for(i = 0; i < shape[1]; ++i) tfaoa[j][i] = tfarr[j * shape[1] + i];
-}
-
-/* add headers and export */
-tfaoa.unshift(["x", "y", "pred"]);
-var new_ws = XLSX.utils.aoa_to_sheet(tfaoa);
-var new_wb = XLSX.utils.book_new();
-XLSX.utils.book_append_sheet(new_wb, new_ws, "Sheet1");
-XLSX.writeFile(new_wb, "tfjs.xls");
diff --git a/modules/45_rtf.js b/modules/45_rtf.js
new file mode 100644
index 000000000..dd4b42f97
--- /dev/null
+++ b/modules/45_rtf.js
@@ -0,0 +1,97 @@
+function rtf_to_sheet(d, opts) {
+  switch (opts.type) {
+    case "base64":
+      return rtf_to_sheet_str(Base64_decode(d), opts);
+    case "binary":
+      return rtf_to_sheet_str(d, opts);
+    case "buffer":
+      return rtf_to_sheet_str(has_buf && Buffer.isBuffer(d) ? d.toString("binary") : a2s(d), opts);
+    case "array":
+      return rtf_to_sheet_str(cc2str(d), opts);
+  }
+  throw new Error("Unrecognized type " + opts.type);
+}
+function rtf_to_sheet_str(str, opts) {
+  var o = opts || {};
+  var ws = o.dense ? [] : {};
+  var rows = str.match(/\\trowd[\s\S]*?\\row\b/g);
+  if (!rows)
+    throw new Error("RTF missing table");
+  var range = { s: { c: 0, r: 0 }, e: { c: 0, r: rows.length - 1 } };
+  rows.forEach(function(rowtf, R) {
+    if (Array.isArray(ws))
+      ws[R] = [];
+    var rtfre = /\\[\w\-]+\b/g;
+    var last_index = 0;
+    var res;
+    var C = -1;
+    var payload = [];
+    while ((res = rtfre.exec(rowtf)) != null) {
+      var data = rowtf.slice(last_index, rtfre.lastIndex - res[0].length);
+      if (data.charCodeAt(0) == 32)
+        data = data.slice(1);
+      if (data.length)
+        payload.push(data);
+      switch (res[0]) {
+        case "\\cell":
+          ++C;
+          if (payload.length) {
+            var cell = { v: payload.join(""), t: "s" };
+            if (cell.v == "TRUE" || cell.v == "FALSE") {
+              cell.v = cell.v == "TRUE";
+              cell.t = "b";
+            } else if (!isNaN(fuzzynum(cell.v))) {
+              cell.t = "n";
+              if (o.cellText !== false)
+                cell.w = cell.v;
+              cell.v = fuzzynum(cell.v);
+            }
+            if (Array.isArray(ws))
+              ws[R][C] = cell;
+            else
+              ws[encode_cell({ r: R, c: C })] = cell;
+          }
+          payload = [];
+          break;
+        case "\\par":
+          payload.push("\n");
+          break;
+      }
+      last_index = rtfre.lastIndex;
+    }
+    if (C > range.e.c)
+      range.e.c = C;
+  });
+  ws["!ref"] = encode_range(range);
+  return ws;
+}
+function rtf_to_workbook(d, opts) {
+  var wb = sheet_to_workbook(rtf_to_sheet(d, opts), opts);
+  wb.bookType = "rtf";
+  return wb;
+}
+function sheet_to_rtf(ws, opts) {
+  var o = ["{\\rtf1\\ansi"];
+  if (!ws["!ref"])
+    return o[0] + "}";
+  var r = safe_decode_range(ws["!ref"]), cell;
+  var dense = Array.isArray(ws);
+  for (var R = r.s.r; R <= r.e.r; ++R) {
+    o.push("\\trowd\\trautofit1");
+    for (var C = r.s.c; C <= r.e.c; ++C)
+      o.push("\\cellx" + (C + 1));
+    o.push("\\pard\\intbl");
+    for (C = r.s.c; C <= r.e.c; ++C) {
+      var coord = encode_cell({ r: R, c: C });
+      cell = dense ? (ws[R] || [])[C] : ws[coord];
+      if (!cell || cell.v == null && (!cell.f || cell.F)) {
+        o.push(" \\cell");
+        continue;
+      }
+      o.push(" " + (cell.w || (format_cell(cell), cell.w) || "").replace(/[\r\n]/g, "\\par "));
+      o.push("\\cell");
+    }
+    o.push("\\pard\\intbl\\row");
+  }
+  return o.join("") + "}";
+}
diff --git a/modules/45_rtf.ts b/modules/45_rtf.ts
new file mode 100644
index 000000000..8b206a5e5
--- /dev/null
+++ b/modules/45_rtf.ts
@@ -0,0 +1,98 @@
+import { WorkBook, WorkSheet, Range, CellObject } from '../';
+import type { utils } from "../";
+
+declare var encode_cell: typeof utils.encode_cell;
+declare var encode_range: typeof utils.encode_range;
+declare var format_cell: typeof utils.format_cell;
+declare var safe_decode_range: typeof utils.decode_range;
+declare function sheet_to_workbook(s: WorkSheet, o?: any): WorkBook;
+declare function cc2str(d: any): string;
+declare function a2s(a: any): string;
+declare var has_buf: boolean;
+declare function Base64_decode(s: string): string;
+declare function fuzzynum(s: string): number;
+
+function rtf_to_sheet(d/*:RawData*/, opts)/*:Worksheet*/ {
+	switch(opts.type) {
+		case 'base64': return rtf_to_sheet_str(Base64_decode(d), opts);
+		case 'binary': return rtf_to_sheet_str(d, opts);
+		case 'buffer': return rtf_to_sheet_str(has_buf && Buffer.isBuffer(d) ? d.toString('binary') : a2s(d), opts);
+		case 'array':  return rtf_to_sheet_str(cc2str(d), opts);
+	}
+	throw new Error("Unrecognized type " + opts.type);
+}
+
+/* TODO: this is a stub */
+function rtf_to_sheet_str(str: string, opts)/*:Worksheet*/ {
+	var o = opts || {};
+	// ESBuild issue 2375
+	var ws: WorkSheet = o.dense ? [] : ({}/*:any*/);
+
+	var rows = str.match(/\\trowd[\s\S]*?\\row\b/g);
+	if(!rows) throw new Error("RTF missing table");
+	var range: Range = {s: {c:0, r:0}, e: {c:0, r:rows.length - 1}};
+	rows.forEach(function(rowtf, R) {
+		if(Array.isArray(ws)) ws[R] = [];
+		var rtfre = /\\[\w\-]+\b/g;
+		var last_index = 0;
+		var res;
+		var C = -1;
+		var payload: string[] = [];
+		while((res = rtfre.exec(rowtf)) != null) {
+			var data = rowtf.slice(last_index, rtfre.lastIndex - res[0].length);
+			if(data.charCodeAt(0) == 0x20) data = data.slice(1);
+			if(data.length) payload.push(data);
+			switch(res[0]) {
+				case "\\cell":
+					++C;
+					if(payload.length) {
+						// TODO: value parsing, including codepage adjustments
+						var cell: CellObject = {v: payload.join(""), t:"s"};
+						if(cell.v == "TRUE" || cell.v == "FALSE") { cell.v = cell.v == "TRUE"; cell.t = "b"; }
+						else if(!isNaN(fuzzynum(cell.v as string))) { cell.t = 'n'; if(o.cellText !== false) cell.w = cell.v as string; cell.v = fuzzynum(cell.v as string); }
+
+						if(Array.isArray(ws)) ws[R][C] = cell;
+						else ws[encode_cell({r:R, c:C})] = cell;
+					}
+					payload = [];
+					break;
+				case "\\par": // NOTE: Excel serializes both "\r" and "\n" as "\\par"
+					payload.push("\n");
+					break;
+			}
+			last_index = rtfre.lastIndex;
+		}
+		if(C > range.e.c) range.e.c = C;
+	});
+	ws['!ref'] = encode_range(range);
+	return ws;
+}
+
+function rtf_to_workbook(d/*:RawData*/, opts): WorkBook {
+	var wb: WorkBook = sheet_to_workbook(rtf_to_sheet(d, opts), opts);
+	wb.bookType = "rtf";
+	return wb;
+}
+
+/* TODO: this is a stub */
+function sheet_to_rtf(ws: WorkSheet, opts): string {
+	var o: string[] = ["{\\rtf1\\ansi"];
+	if(!ws["!ref"]) return o[0] + "}";
+	var r = safe_decode_range(ws['!ref']), cell: CellObject;
+	var dense = Array.isArray(ws);
+	for(var R = r.s.r; R <= r.e.r; ++R) {
+		o.push("\\trowd\\trautofit1");
+		for(var C = r.s.c; C <= r.e.c; ++C) o.push("\\cellx" + (C+1));
+		o.push("\\pard\\intbl");
+		for(C = r.s.c; C <= r.e.c; ++C) {
+			var coord = encode_cell({r:R,c:C});
+			cell = dense ? (ws[R]||[])[C]: ws[coord];
+			if(!cell || cell.v == null && (!cell.f || cell.F)) { o.push(" \\cell"); continue; }
+			o.push(" " + (cell.w || (format_cell(cell), cell.w) || "").replace(/[\r\n]/g, "\\par "));
+			o.push("\\cell");
+		}
+		o.push("\\pard\\intbl\\row");
+	}
+	return o.join("") + "}";
+}
+
diff --git a/modules/83_numbers.js b/modules/83_numbers.js
index c7594f660..e43e8e4ec 100644
--- a/modules/83_numbers.js
+++ b/modules/83_numbers.js
@@ -396,7 +396,7 @@ function parse_old_storage(buf, sst, rsst, v) {
   var ret;
   switch (buf[2]) {
     case 0:
-      break;
+      return void 0;
     case 2:
       ret = { t: "n", v: ieee };
       break;
@@ -456,7 +456,7 @@ function parse_new_storage(buf, sst, rsst) {
   var ret;
   switch (buf[1]) {
     case 0:
-      break;
+      return void 0;
     case 2:
       ret = { t: "n", v: d128 };
       break;
@@ -761,6 +761,7 @@ function parse_TN_DocumentArchive(M, root) {
   });
   if (out.SheetNames.length == 0)
     throw new Error("Empty NUMBERS file");
+  out.bookType = "numbers";
   return out;
 }
 function parse_numbers_iwa(cfb) {
@@ -961,6 +962,8 @@ function write_numbers_iwa(wb, opts) {
     throw new Error("Too many messages");
   }
   var entry = CFB.find(cfb, dependents[1].location);
+  if (!entry)
+    throw "Could not find ".concat(dependents[1].location, " in Numbers template");
   var x = parse_iwa_file(decompress_iwa_file(entry.content));
   var docroot;
   for (var xi = 0; xi < x.length; ++xi) {
@@ -968,8 +971,12 @@ function write_numbers_iwa(wb, opts) {
     if (packet.id == 1)
       docroot = packet;
   }
+  if (docroot == null)
+    throw "Could not find message ".concat(1, " in Numbers template");
   var sheetrootref = parse_TSP_Reference(parse_shallow(docroot.messages[0].data)[1][0].data);
   entry = CFB.find(cfb, dependents[sheetrootref].location);
+  if (!entry)
+    throw "Could not find ".concat(dependents[sheetrootref].location, " in Numbers template");
   x = parse_iwa_file(decompress_iwa_file(entry.content));
   for (xi = 0; xi < x.length; ++xi) {
     packet = x[xi];
@@ -985,6 +992,8 @@ function write_numbers_iwa(wb, opts) {
   entry.size = entry.content.length;
   sheetrootref = parse_TSP_Reference(sheetref[2][0].data);
   entry = CFB.find(cfb, dependents[sheetrootref].location);
+  if (!entry)
+    throw "Could not find ".concat(dependents[sheetrootref].location, " in Numbers template");
   x = parse_iwa_file(decompress_iwa_file(entry.content));
   for (xi = 0; xi < x.length; ++xi) {
     packet = x[xi];
@@ -993,6 +1002,8 @@ function write_numbers_iwa(wb, opts) {
   }
   sheetrootref = parse_TSP_Reference(parse_shallow(docroot.messages[0].data)[2][0].data);
   entry = CFB.find(cfb, dependents[sheetrootref].location);
+  if (!entry)
+    throw "Could not find ".concat(dependents[sheetrootref].location, " in Numbers template");
   x = parse_iwa_file(decompress_iwa_file(entry.content));
   for (xi = 0; xi < x.length; ++xi) {
     packet = x[xi];
@@ -1005,6 +1016,8 @@ function write_numbers_iwa(wb, opts) {
     pb[7][0].data = write_varint49(range.e.c + 1);
     var cruidsref = parse_TSP_Reference(pb[46][0].data);
     var oldbucket = CFB.find(cfb, dependents[cruidsref].location);
+    if (!oldbucket)
+      throw "Could not find ".concat(dependents[cruidsref].location, " in Numbers template");
     var _x = parse_iwa_file(decompress_iwa_file(oldbucket.content));
     {
       for (var j = 0; j < _x.length; ++j) {
@@ -1047,6 +1060,8 @@ function write_numbers_iwa(wb, opts) {
       var row_headers = parse_shallow(store[1][0].data);
       var row_header_ref = parse_TSP_Reference(row_headers[2][0].data);
       oldbucket = CFB.find(cfb, dependents[row_header_ref].location);
+      if (!oldbucket)
+        throw "Could not find ".concat(dependents[cruidsref].location, " in Numbers template");
       _x = parse_iwa_file(decompress_iwa_file(oldbucket.content));
       {
         if (_x[0].id != row_header_ref)
@@ -1065,6 +1080,8 @@ function write_numbers_iwa(wb, opts) {
       oldbucket.size = oldbucket.content.length;
       var col_header_ref = parse_TSP_Reference(store[2][0].data);
       oldbucket = CFB.find(cfb, dependents[col_header_ref].location);
+      if (!oldbucket)
+        throw "Could not find ".concat(dependents[cruidsref].location, " in Numbers template");
       _x = parse_iwa_file(decompress_iwa_file(oldbucket.content));
       {
         if (_x[0].id != col_header_ref)
@@ -1109,6 +1126,8 @@ function write_numbers_iwa(wb, opts) {
       var sstref = parse_TSP_Reference(store[4][0].data);
       (function() {
         var sentry = CFB.find(cfb, dependents[sstref].location);
+        if (!sentry)
+          throw "Could not find ".concat(dependents[sstref].location, " in Numbers template");
         var sx = parse_iwa_file(decompress_iwa_file(sentry.content));
         var sstroot;
         for (var sxi = 0; sxi < sx.length; ++sxi) {
@@ -1116,6 +1135,8 @@ function write_numbers_iwa(wb, opts) {
           if (packet2.id == sstref)
             sstroot = packet2;
         }
+        if (sstroot == null)
+          throw "Could not find message ".concat(sstref, " in Numbers template");
         var sstdata = parse_shallow(sstroot.messages[0].data);
         {
           sstdata[3] = [];
@@ -1141,6 +1162,8 @@ function write_numbers_iwa(wb, opts) {
           var tileref = parse_TSP_Reference(tl[2][0].data);
           (function() {
             var tentry = CFB.find(cfb, dependents[tileref].location);
+            if (!tentry)
+              throw "Could not find ".concat(dependents[tileref].location, " in Numbers template");
             var tx = parse_iwa_file(decompress_iwa_file(tentry.content));
             var tileroot;
             for (var sxi = 0; sxi < tx.length; ++sxi) {
diff --git a/modules/83_numbers.ts b/modules/83_numbers.ts
index 0b6c2761d..7af319036 100644
--- a/modules/83_numbers.ts
+++ b/modules/83_numbers.ts
@@ -184,9 +184,9 @@ interface IWAMessage {
 	data: Uint8Array;
 }
 interface IWAArchiveInfo {
-	id?: number;
+	id: number;
 	merge?: boolean;
-	messages?: IWAMessage[];
+	messages: IWAMessage[];
 }
 /** Extract all messages from a IWA file */
 function parse_iwa_file(buf: Uint8Array): IWAArchiveInfo[] {
@@ -248,7 +248,7 @@ function parse_snappy_chunk(type: number, buf: Uint8Array): Uint8Array {
 	var ptr: Ptr = [0];
 
 	var usz = parse_varint49(buf, ptr);
-	var chunks = [];
+	var chunks: Uint8Array[] = [];
 	while(ptr[0] < buf.length) {
 		var tag = buf[ptr[0]] & 0x3;
 		if(tag == 0) {
@@ -295,7 +295,7 @@ function parse_snappy_chunk(type: number, buf: Uint8Array): Uint8Array {
 
 /** Decompress IWA file */
 function decompress_iwa_file(buf: Uint8Array): Uint8Array {
-	var out = [];
+	var out: Uint8Array[] = [];
 	var l = 0;
 	while(l < buf.length) {
 		var t = buf[l++];
@@ -336,7 +336,7 @@ function compress_iwa_file(buf: Uint8Array): Uint8Array {
 //<<export { decompress_iwa_file, compress_iwa_file };
 
 /** Parse "old storage" (version 0..3) */
-function parse_old_storage(buf: Uint8Array, sst: string[], rsst: string[], v: 0|1|2|3): CellObject {
+function parse_old_storage(buf: Uint8Array, sst: string[], rsst: string[], v: 0|1|2|3): CellObject | void {
 	var dv = u8_to_dataview(buf);
 	var flags = dv.getUint32(4, true);
 
@@ -352,7 +352,7 @@ function parse_old_storage(buf: Uint8Array, sst: string[], rsst: string[], v: 0|
 
 	var ret: CellObject;
 	switch(buf[2]) {
-		case 0: break; // return { t: "z" }; // blank?
+		case 0: return void 0; // return { t: "z" }; // blank?
 		case 2: ret = { t: "n", v: ieee }; break; // number
 		case 3: ret = { t: "s", v: sst[sidx] }; break; // string
 		case 5: ret = { t: "d", v: dt }; break; // date-time
@@ -371,7 +371,7 @@ function parse_old_storage(buf: Uint8Array, sst: string[], rsst: string[], v: 0|
 }
 
 /** Parse "new storage" (version 5) */
-function parse_new_storage(buf: Uint8Array, sst: string[], rsst: string[]): CellObject {
+function parse_new_storage(buf: Uint8Array, sst: string[], rsst: string[]): CellObject | void {
 	var dv = u8_to_dataview(buf);
 	var flags = dv.getUint32(8, true);
 
@@ -388,7 +388,7 @@ function parse_new_storage(buf: Uint8Array, sst: string[], rsst: string[]): Cell
 
 	var ret: CellObject;
 	switch(buf[1]) {
-		case 0: break; // return { t: "z" }; // blank?
+		case 0: return void 0; // return { t: "z" }; // blank?
 		case 2: ret = { t: "n", v: d128 }; break; // number
 		case 3: ret = { t: "s", v: sst[sidx] }; break; // string
 		case 5: ret = { t: "d", v: dt }; break; // date-time
@@ -438,7 +438,7 @@ function write_old_storage(cell: CellObject, sst: string[]): Uint8Array {
 	return out.slice(0, l);
 }
 //<<export { write_new_storage, write_old_storage };
-function parse_cell_storage(buf: Uint8Array, sst: string[], rsst: string[]): CellObject {
+function parse_cell_storage(buf: Uint8Array, sst: string[], rsst: string[]): CellObject | void {
 	switch(buf[0]) {
 		case 0: case 1:
 		case 2: case 3: return parse_old_storage(buf, sst, rsst, buf[0]);
@@ -475,7 +475,7 @@ function parse_TST_TableDataList(M: MessageSpace, root: IWAMessage): string[] {
 	var type = varint_to_i32(pb[1][0].data);
 
 	var entries = pb[3];
-	var data = [];
+	var data: Array<string> = [];
 	(entries||[]).forEach(entry => {
 		// .TST.TableDataList.ListEntry
 		var le = parse_shallow(entry.data);
@@ -505,7 +505,7 @@ interface TileRowInfo {
 	/** Row Index */
 	R: number;
 	/** Cell Storage */
-	cells?: Uint8Array[];
+	cells: Uint8Array[];
 }
 /** Parse .TSP.TileRowInfo */
 function parse_TST_TileRowInfo(u8: Uint8Array, type: TileStorageType): TileRowInfo {
@@ -673,6 +673,7 @@ function parse_TN_DocumentArchive(M: MessageSpace, root: IWAMessage): WorkBook {
 		});
 	});
 	if(out.SheetNames.length == 0) throw new Error("Empty NUMBERS file");
+	out.bookType = "numbers";
 	return out;
 }
 
@@ -694,7 +695,7 @@ function parse_numbers_iwa(cfb: CFB$Container): WorkBook {
 
 	/* find document root */
 	if(M?.[1]?.[0]?.meta?.[1]?.[0].data && varint_to_i32(M[1][0].meta[1][0].data) == 10000) throw new Error("Pages documents are not supported");
-	var docroot: IWAMessage = M?.[1]?.[0]?.meta?.[1]?.[0].data && varint_to_i32(M[1][0].meta[1][0].data) == 1 && M[1][0];
+	var docroot: IWAMessage | false = M?.[1]?.[0]?.meta?.[1]?.[0].data && varint_to_i32(M[1][0].meta[1][0].data) == 1 && M[1][0];
 	if(!docroot) indices.forEach((idx) => {
 		M[idx].forEach((iwam) => {
 			var mtype = varint_to_i32(iwam.meta[1][0].data) >>> 0;
@@ -789,7 +790,7 @@ function write_numbers_iwa(wb: WorkBook, opts: any): CFB$Container {
 	/* TODO: support multiple worksheets, larger ranges, more data types, etc */
 	var ws = wb.Sheets[wb.SheetNames[0]];
 	if(wb.SheetNames.length > 1) console.error("The Numbers writer currently writes only the first table");
-	var range = decode_range(ws["!ref"]);
+	var range = decode_range(ws["!ref"] as string);
 	range.s.r = range.s.c = 0;
 
 	/* Actual NUMBERS 12.0 limit ALL1000000 */
@@ -853,16 +854,19 @@ function write_numbers_iwa(wb: WorkBook, opts: any): CFB$Container {
 
 	/* .TN.DocumentArchive */
 	var entry = CFB.find(cfb, dependents[1].location);
+	if(!entry) throw `Could not find ${dependents[1].location} in Numbers template`;
 	var x = parse_iwa_file(decompress_iwa_file(entry.content as Uint8Array));
-	var docroot: IWAArchiveInfo;
+	var docroot!: IWAArchiveInfo;
 	for(var xi = 0; xi < x.length; ++xi) {
 		var packet = x[xi];
 		if(packet.id == 1) docroot = packet;
 	}
+	if(docroot == null) throw `Could not find message ${1} in Numbers template`;
 
 	/* .TN.SheetArchive */
 	var sheetrootref = parse_TSP_Reference(parse_shallow(docroot.messages[0].data)[1][0].data);
 	entry = CFB.find(cfb, dependents[sheetrootref].location);
+	if(!entry) throw `Could not find ${dependents[sheetrootref].location} in Numbers template`;
 	x = parse_iwa_file(decompress_iwa_file(entry.content as Uint8Array));
 	for(xi = 0; xi < x.length; ++xi) {
 		packet = x[xi];
@@ -880,6 +884,7 @@ function write_numbers_iwa(wb: WorkBook, opts: any): CFB$Container {
 	/* .TST.TableInfoArchive */
 	sheetrootref = parse_TSP_Reference(sheetref[2][0].data);
 	entry = CFB.find(cfb, dependents[sheetrootref].location);
+	if(!entry) throw `Could not find ${dependents[sheetrootref].location} in Numbers template`;
 	x = parse_iwa_file(decompress_iwa_file(entry.content as Uint8Array));
 	for(xi = 0; xi < x.length; ++xi) {
 		packet = x[xi];
@@ -889,6 +894,7 @@ function write_numbers_iwa(wb: WorkBook, opts: any): CFB$Container {
 	/* .TST.TableModelArchive */
 	sheetrootref = parse_TSP_Reference(parse_shallow(docroot.messages[0].data)[2][0].data);
 	entry = CFB.find(cfb, dependents[sheetrootref].location);
+	if(!entry) throw `Could not find ${dependents[sheetrootref].location} in Numbers template`;
 	x = parse_iwa_file(decompress_iwa_file(entry.content as Uint8Array));
 	for(xi = 0; xi < x.length; ++xi) {
 		packet = x[xi];
@@ -903,6 +909,7 @@ function write_numbers_iwa(wb: WorkBook, opts: any): CFB$Container {
 
 		var cruidsref = parse_TSP_Reference(pb[46][0].data);
 		var oldbucket = CFB.find(cfb, dependents[cruidsref].location);
+		if(!oldbucket) throw `Could not find ${dependents[cruidsref].location} in Numbers template`;
 		var _x = parse_iwa_file(decompress_iwa_file(oldbucket.content as Uint8Array));
 		{
 			for(var j = 0; j < _x.length; ++j) {
@@ -940,6 +947,7 @@ function write_numbers_iwa(wb: WorkBook, opts: any): CFB$Container {
 			var row_headers = parse_shallow(store[1][0].data);
 			var row_header_ref = parse_TSP_Reference(row_headers[2][0].data);
 			oldbucket = CFB.find(cfb, dependents[row_header_ref].location);
+			if(!oldbucket) throw `Could not find ${dependents[cruidsref].location} in Numbers template`;
 			_x = parse_iwa_file(decompress_iwa_file(oldbucket.content as Uint8Array));
 			{
 				if(_x[0].id != row_header_ref) throw "Bad HeaderStorageBucket";
@@ -956,6 +964,7 @@ function write_numbers_iwa(wb: WorkBook, opts: any): CFB$Container {
 
 			var col_header_ref = parse_TSP_Reference(store[2][0].data);
 			oldbucket = CFB.find(cfb, dependents[col_header_ref].location);
+			if(!oldbucket) throw `Could not find ${dependents[cruidsref].location} in Numbers template`;
 			_x = parse_iwa_file(decompress_iwa_file(oldbucket.content as Uint8Array));
 			{
 				if(_x[0].id != col_header_ref) throw "Bad HeaderStorageBucket";
@@ -996,12 +1005,14 @@ function write_numbers_iwa(wb: WorkBook, opts: any): CFB$Container {
 			var sstref = parse_TSP_Reference(store[4][0].data);
 			(() => {
 				var sentry = CFB.find(cfb, dependents[sstref].location);
+				if(!sentry) throw `Could not find ${dependents[sstref].location} in Numbers template`;
 				var sx = parse_iwa_file(decompress_iwa_file(sentry.content as Uint8Array));
-				var sstroot: IWAArchiveInfo;
+				var sstroot!: IWAArchiveInfo;
 				for(var sxi = 0; sxi < sx.length; ++sxi) {
 					var packet = sx[sxi];
 					if(packet.id == sstref) sstroot = packet;
 				}
+				if(sstroot == null) throw `Could not find message ${sstref} in Numbers template`;
 
 				var sstdata = parse_shallow(sstroot.messages[0].data);
 				{
@@ -1028,8 +1039,9 @@ function write_numbers_iwa(wb: WorkBook, opts: any): CFB$Container {
 					var tileref = parse_TSP_Reference(tl[2][0].data);
 					(() => {
 						var tentry = CFB.find(cfb, dependents[tileref].location);
+						if(!tentry) throw `Could not find ${dependents[tileref].location} in Numbers template`;
 						var tx = parse_iwa_file(decompress_iwa_file(tentry.content as Uint8Array));
-						var tileroot: IWAArchiveInfo;
+						var tileroot!: IWAArchiveInfo;
 						for(var sxi = 0; sxi < tx.length; ++sxi) {
 							var packet = tx[sxi];
 							if(packet.id == tileref) tileroot = packet;
diff --git a/modules/Makefile b/modules/Makefile
index c0e205239..210764df6 100644
--- a/modules/Makefile
+++ b/modules/Makefile
@@ -3,7 +3,7 @@ LIBFILES=src/types.ts
 TSFILES=$(wildcard *.ts)
 ENTRIES=$(subst .ts,.js,$(TSFILES))
 
-BAREJS=04_base64.js 51_xlsxmeta.js 51_xlsbmeta.js 59_vba.js 64_ftab.js 83_numbers.js
+BAREJS=04_base64.js 45_rtf.js 51_xlsxmeta.js 51_xlsbmeta.js 59_vba.js 64_ftab.js 83_numbers.js
 
 .PHONY: all
 all: $(ENTRIES) xlsx.zahl.js
diff --git a/test.js b/test.js
index 7b23b2ae5..a76eccf38 100644
--- a/test.js
+++ b/test.js
@@ -1430,6 +1430,18 @@ describe('parse features', function() {
 "11111,1934-06-03,1934-06-03,1934-06-03"
 		].join("\n"));
 	}); });
+
+	it('bookType metadata', function() {
+	[
+		// TODO: keep in sync with BookType, support other formats
+		"xlsx"/*, "xlsm" */, "xlsb"/* xls / xla / biff# */, "xlml", "ods", "fods"/*, "csv", "txt", */, "sylk", "html", "dif", "rtf"/*, "prn", "eth"*/, "dbf", "numbers"
+	].forEach(function(r) {
+		if(!XLSX_ZAHL) XLSX_ZAHL=require("./dist/xlsx.zahl");
+		var ws = X.utils.aoa_to_sheet([ ["a", "b", "c"], [1, 2, 3] ]);
+		var wb = X.utils.book_new(); X.utils.book_append_sheet(wb, ws, "Sheet1");
+		var data = X.write(wb, {type: TYPE, bookType: r, WTF: true, numbers:XLSX_ZAHL });
+		assert.equal(X.read(data, {type: TYPE, WTF: true}).bookType, r);
+	}); });
 });
 
 describe('write features', function() {
diff --git a/test.mjs b/test.mjs
index 862b3fac2..b85edd5ed 100644
--- a/test.mjs
+++ b/test.mjs
@@ -1421,6 +1421,17 @@ describe('parse features', function() {
 "11111,1934-06-03,1934-06-03,1934-06-03"
 		].join("\n"));
 	}); });
+
+	it('bookType metadata', function() {
+	[
+		// TODO: keep in sync with BookType, support other formats
+		"xlsx"/*, "xlsm" */, "xlsb"/* xls / xla / biff# */, "xlml", "ods", "fods"/*, "csv", "txt", */, "sylk", "html", "dif", "rtf"/*, "prn", "eth"*/, "dbf", "numbers"
+	].forEach(function(r) {
+		var ws = X.utils.aoa_to_sheet([ ["a", "b", "c"], [1, 2, 3] ]);
+		var wb = X.utils.book_new(); X.utils.book_append_sheet(wb, ws, "Sheet1");
+		var data = X.write(wb, {type: TYPE, bookType: r, WTF: true, numbers:XLSX_ZAHL });
+		assert.equal(X.read(data, {type: TYPE, WTF: true}).bookType, r);
+	}); });
 });
 
 describe('write features', function() {
diff --git a/test.mts b/test.mts
index e9228ae72..ee6731ea8 100644
--- a/test.mts
+++ b/test.mts
@@ -1386,6 +1386,17 @@ describe('parse features', function() {
 "11111,1934-06-03,1934-06-03,1934-06-03"
 		].join("\n"));
 	}); });
+
+	it('bookType metadata', function() {
+	([
+		// TODO: keep in sync with BookType, support other formats
+		"xlsx"/*, "xlsm" */, "xlsb"/* xls / xla / biff# */, "xlml", "ods", "fods"/*, "csv", "txt", */, "sylk", "html", "dif", "rtf"/*, "prn", "eth"*/, "dbf", "numbers"
+	] as X.BookType[]).forEach(function(r: X.BookType) {
+		var ws = X.utils.aoa_to_sheet([ ["a", "b", "c"], [1, 2, 3] ]);
+		var wb = X.utils.book_new(); X.utils.book_append_sheet(wb, ws, "Sheet1");
+		var data = X.write(wb, {type: TYPE, bookType: r, WTF: true, numbers:XLSX_ZAHL });
+		assert.equal(X.read(data, {type: TYPE, WTF: true}).bookType, r);
+	}); });
 });
 
 describe('write features', function() {
@@ -1754,6 +1765,33 @@ describe('roundtrip features', function() {
 		}
 	});	});
 
+	it('should preserve date system', function() {([
+		"biff5", "ods", "slk", "xls", "xlsb", "xlsx", "xml"
+	] as X.BookType[]).forEach(function(ext) {
+		// TODO: check actual date codes and actual date values
+		var wb0 = X.read(fs.readFileSync("./test_files/1904/1900." + ext), {type: TYPE});
+		assert.ok(!wb0.Workbook?.WBProps?.date1904);
+		var wb1 = X.read(X.write(wb0, {type: TYPE, bookType: ext}), {type: TYPE});
+		assert.ok(!wb1.Workbook?.WBProps?.date1904);
+
+		var wb2 = X.utils.book_new(); X.utils.book_append_sheet(wb2, X.utils.aoa_to_sheet([[1]]), "Sheet1");
+		wb2.Workbook = { WBProps: { date1904: false } };
+		assert.ok(!wb2.Workbook?.WBProps?.date1904);
+		var wb3 = X.read(X.write(wb2, {type: TYPE, bookType: ext}), {type: TYPE});
+		assert.ok(!wb3.Workbook?.WBProps?.date1904);
+
+		var wb4 = X.read(fs.readFileSync("./test_files/1904/1904." + ext), {type: TYPE});
+		assert.ok(wb4.Workbook?.WBProps?.date1904);
+		var wb5 = X.read(X.write(wb4, {type: TYPE, bookType: ext}), {type: TYPE});
+		assert.ok(wb5.Workbook?.WBProps?.date1904); // xlsb, xml
+
+		var wb6 = X.utils.book_new(); X.utils.book_append_sheet(wb6, X.utils.aoa_to_sheet([[1]]), "Sheet1");
+		wb6.Workbook = { WBProps: { date1904: true } };
+		assert.ok(wb6.Workbook?.WBProps?.date1904);
+		var wb7 = X.read(X.write(wb6, {type: TYPE, bookType: ext}), {type: TYPE});
+		assert.ok(wb7.Workbook?.WBProps?.date1904);
+	}); });
+
 });
 
 //function password_file(x){return x.match(/^password.*\.xls$/); }
diff --git a/test.ts b/test.ts
index 9d2c8d8cb..ea02f3306 100644
--- a/test.ts
+++ b/test.ts
@@ -1386,6 +1386,17 @@ Deno.test('parse features', async function(t) {
 "11111,1934-06-03,1934-06-03,1934-06-03"
 		].join("\n"));
 	}); });
+
+	await t.step('bookType metadata', async function(t) {
+	([
+		// TODO: keep in sync with BookType, support other formats
+		"xlsx"/*, "xlsm" */, "xlsb"/* xls / xla / biff# */, "xlml", "ods", "fods"/*, "csv", "txt", */, "sylk", "html", "dif", "rtf"/*, "prn", "eth"*/, "dbf", "numbers"
+	] as X.BookType[]).forEach(function(r: X.BookType) {
+		var ws = X.utils.aoa_to_sheet([ ["a", "b", "c"], [1, 2, 3] ]);
+		var wb = X.utils.book_new(); X.utils.book_append_sheet(wb, ws, "Sheet1");
+		var data = X.write(wb, {type: TYPE, bookType: r, WTF: true, numbers:XLSX_ZAHL });
+		assert.equal(X.read(data, {type: TYPE, WTF: true}).bookType, r);
+	}); });
 });
 
 Deno.test('write features', async function(t) {
@@ -1754,6 +1765,33 @@ Deno.test('roundtrip features', async function(t) {
 		}
 	});	});
 
+	await t.step('should preserve date system', async function(t) {([
+		"biff5", "ods", "slk", "xls", "xlsb", "xlsx", "xml"
+	] as X.BookType[]).forEach(function(ext) {
+		// TODO: check actual date codes and actual date values
+		var wb0 = X.read(fs.readFileSync("./test_files/1904/1900." + ext), {type: TYPE});
+		assert.assert(!wb0.Workbook?.WBProps?.date1904);
+		var wb1 = X.read(X.write(wb0, {type: TYPE, bookType: ext}), {type: TYPE});
+		assert.assert(!wb1.Workbook?.WBProps?.date1904);
+
+		var wb2 = X.utils.book_new(); X.utils.book_append_sheet(wb2, X.utils.aoa_to_sheet([[1]]), "Sheet1");
+		wb2.Workbook = { WBProps: { date1904: false } };
+		assert.assert(!wb2.Workbook?.WBProps?.date1904);
+		var wb3 = X.read(X.write(wb2, {type: TYPE, bookType: ext}), {type: TYPE});
+		assert.assert(!wb3.Workbook?.WBProps?.date1904);
+
+		var wb4 = X.read(fs.readFileSync("./test_files/1904/1904." + ext), {type: TYPE});
+		assert.assert(wb4.Workbook?.WBProps?.date1904);
+		var wb5 = X.read(X.write(wb4, {type: TYPE, bookType: ext}), {type: TYPE});
+		assert.assert(wb5.Workbook?.WBProps?.date1904); // xlsb, xml
+
+		var wb6 = X.utils.book_new(); X.utils.book_append_sheet(wb6, X.utils.aoa_to_sheet([[1]]), "Sheet1");
+		wb6.Workbook = { WBProps: { date1904: true } };
+		assert.assert(wb6.Workbook?.WBProps?.date1904);
+		var wb7 = X.read(X.write(wb6, {type: TYPE, bookType: ext}), {type: TYPE});
+		assert.assert(wb7.Workbook?.WBProps?.date1904);
+	}); });
+
 });
 
 //function password_file(x){return x.match(/^password.*\.xls$/); }
diff --git a/testnocp.ts b/testnocp.ts
index aead2ab84..5d207cc37 100644
--- a/testnocp.ts
+++ b/testnocp.ts
@@ -1385,6 +1385,17 @@ Deno.test('parse features', async function(t) {
 "11111,1934-06-03,1934-06-03,1934-06-03"
 		].join("\n"));
 	}); });
+
+	await t.step('bookType metadata', async function(t) {
+	([
+		// TODO: keep in sync with BookType, support other formats
+		"xlsx"/*, "xlsm" */, "xlsb"/* xls / xla / biff# */, "xlml", "ods", "fods"/*, "csv", "txt", */, "sylk", "html", "dif", "rtf"/*, "prn", "eth"*/, "dbf", "numbers"
+	] as X.BookType[]).forEach(function(r: X.BookType) {
+		var ws = X.utils.aoa_to_sheet([ ["a", "b", "c"], [1, 2, 3] ]);
+		var wb = X.utils.book_new(); X.utils.book_append_sheet(wb, ws, "Sheet1");
+		var data = X.write(wb, {type: TYPE, bookType: r, WTF: true, numbers:XLSX_ZAHL });
+		assert.equal(X.read(data, {type: TYPE, WTF: true}).bookType, r);
+	}); });
 });
 
 Deno.test('write features', async function(t) {
@@ -1753,6 +1764,33 @@ Deno.test('roundtrip features', async function(t) {
 		}
 	});	});
 
+	await t.step('should preserve date system', async function(t) {([
+		"biff5", "ods", "slk", "xls", "xlsb", "xlsx", "xml"
+	] as X.BookType[]).forEach(function(ext) {
+		// TODO: check actual date codes and actual date values
+		var wb0 = X.read(fs.readFileSync("./test_files/1904/1900." + ext), {type: TYPE});
+		assert.assert(!wb0.Workbook?.WBProps?.date1904);
+		var wb1 = X.read(X.write(wb0, {type: TYPE, bookType: ext}), {type: TYPE});
+		assert.assert(!wb1.Workbook?.WBProps?.date1904);
+
+		var wb2 = X.utils.book_new(); X.utils.book_append_sheet(wb2, X.utils.aoa_to_sheet([[1]]), "Sheet1");
+		wb2.Workbook = { WBProps: { date1904: false } };
+		assert.assert(!wb2.Workbook?.WBProps?.date1904);
+		var wb3 = X.read(X.write(wb2, {type: TYPE, bookType: ext}), {type: TYPE});
+		assert.assert(!wb3.Workbook?.WBProps?.date1904);
+
+		var wb4 = X.read(fs.readFileSync("./test_files/1904/1904." + ext), {type: TYPE});
+		assert.assert(wb4.Workbook?.WBProps?.date1904);
+		var wb5 = X.read(X.write(wb4, {type: TYPE, bookType: ext}), {type: TYPE});
+		assert.assert(wb5.Workbook?.WBProps?.date1904); // xlsb, xml
+
+		var wb6 = X.utils.book_new(); X.utils.book_append_sheet(wb6, X.utils.aoa_to_sheet([[1]]), "Sheet1");
+		wb6.Workbook = { WBProps: { date1904: true } };
+		assert.assert(wb6.Workbook?.WBProps?.date1904);
+		var wb7 = X.read(X.write(wb6, {type: TYPE, bookType: ext}), {type: TYPE});
+		assert.assert(wb7.Workbook?.WBProps?.date1904);
+	}); });
+
 });
 
 //function password_file(x){return x.match(/^password.*\.xls$/); }
diff --git a/tests/core.js b/tests/core.js
index e5a089570..4fefe5e1c 100644
--- a/tests/core.js
+++ b/tests/core.js
@@ -723,6 +723,7 @@ describe('output formats', function() {
 		["fods",   true,   true],
 		["csv",    true,   true],
 		["txt",    true,   true],
+		["rtf",   false,   true],
 		["sylk",  false,   true],
 		["eth",   false,   true],
 		["html",   true,   true],
@@ -1429,6 +1430,17 @@ describe('parse features', function() {
 "11111,1934-06-03,1934-06-03,1934-06-03"
 		].join("\n"));
 	}); });
+
+	it('bookType metadata', function() {
+	[
+		// TODO: keep in sync with BookType, support other formats
+		"xlsx"/*, "xlsm" */, "xlsb"/* xls / xla / biff# */, "xlml", "ods", "fods"/*, "csv", "txt", */, "sylk", "html", "dif", "rtf"/*, "prn", "eth"*/, "dbf", "numbers"
+	].forEach(function(r) {
+		var ws = X.utils.aoa_to_sheet([ ["a", "b", "c"], [1, 2, 3] ]);
+		var wb = X.utils.book_new(); X.utils.book_append_sheet(wb, ws, "Sheet1");
+		var data = X.write(wb, {type: TYPE, bookType: r, WTF: true, numbers:XLSX_ZAHL });
+		assert.equal(X.read(data, {type: TYPE, WTF: true}).bookType, r);
+	}); });
 });
 
 describe('write features', function() {
@@ -2531,6 +2543,23 @@ describe('js -> file -> js', function() {
 	});
 });
 
+describe('rtf', function() {
+	it('roundtrip should be idempotent', function() {
+		var ws = X.utils.aoa_to_sheet([
+			[1,2,3],
+			[true, false, null, "sheetjs"],
+			["foo", "bar", fixdate, "0.3"],
+			["baz", null, "q\"ux"]
+		]);
+		var wb1 = X.utils.book_new();
+		X.utils.book_append_sheet(wb1, ws, "Sheet1");
+		var rtf1 = X.write(wb1, {bookType: "rtf", type: "string"});
+		var wb2 = X.read(rtf1, {type: "string"});
+		var rtf2 = X.write(wb2, {bookType: "rtf", type: "string"});
+		assert.equal(rtf1, rtf2);
+	});
+});
+
 describe('corner cases', function() {
 	it('output functions', function() {
 		var ws = X.utils.aoa_to_sheet([
diff --git a/types/index.d.ts b/types/index.d.ts
index 2d37afb4a..ebbed34b5 100644
--- a/types/index.d.ts
+++ b/types/index.d.ts
@@ -285,6 +285,9 @@ export interface WorkBook {
     Workbook?: WBProps;
 
     vbaraw?: any;
+
+    /** Original file type (when parsed with `read` or `readFile`) */
+    bookType?: BookType;
 }
 
 export interface SheetProps {
diff --git a/xlsx.flow.js b/xlsx.flow.js
index 123ce0e02..d8f8ebc5c 100644
--- a/xlsx.flow.js
+++ b/xlsx.flow.js
@@ -7845,7 +7845,11 @@ function dbf_to_sheet(buf, opts)/*:Worksheet*/ {
 }
 
 function dbf_to_workbook(buf, opts)/*:Workbook*/ {
-	try { return sheet_to_workbook(dbf_to_sheet(buf, opts), opts); }
+	try {
+		var o = sheet_to_workbook(dbf_to_sheet(buf, opts), opts);
+		o.bookType = "dbf";
+		return o;
+	}
 	catch(e) { if(opts && opts.WTF) throw e; }
 	return ({SheetNames:[],Sheets:{}});
 }
@@ -8156,6 +8160,7 @@ var SYLK = /*#__PURE__*/(function() {
 		keys(ws).forEach(function(k) { o[k] = ws[k]; });
 		var outwb = sheet_to_workbook(o, opts);
 		keys(wb).forEach(function(k) { outwb[k] = wb[k]; });
+		outwb.bookType = "sylk";
 		return outwb;
 	}
 
@@ -8274,7 +8279,11 @@ var DIF = /*#__PURE__*/(function() {
 	}
 
 	function dif_to_sheet(str/*:string*/, opts)/*:Worksheet*/ { return aoa_to_sheet(dif_to_aoa(str, opts), opts); }
-	function dif_to_workbook(str/*:string*/, opts)/*:Workbook*/ { return sheet_to_workbook(dif_to_sheet(str, opts), opts); }
+	function dif_to_workbook(str/*:string*/, opts)/*:Workbook*/ {
+		var o = sheet_to_workbook(dif_to_sheet(str, opts), opts);
+		o.bookType = "dif";
+		return o;
+	}
 
 	var sheet_to_dif = /*#__PURE__*/(function() {
 		var push_field = function pf(o/*:Array<string>*/, topic/*:string*/, v/*:number*/, n/*:number*/, s/*:string*/) {
@@ -10322,83 +10331,102 @@ function parse_FilePass(blob, length/*:number*/, opts) {
 }
 
 
-function rtf_to_workbook(d/*:RawData*/, opts)/*:Workbook*/ {
-	switch(opts.type) {
-		case 'base64': return rtf_to_book_str(Base64_decode(d), opts);
-		case 'binary': return rtf_to_book_str(d, opts);
-		case 'buffer': return rtf_to_book_str(has_buf && Buffer.isBuffer(d) ? d.toString('binary') : a2s(d), opts);
-		case 'array':  return rtf_to_book_str(cc2str(d), opts);
-	}
-	throw new Error("Unrecognized type " + opts.type);
-}
-
-/* TODO: RTF technically can store multiple tables, even if Excel does not */
-function rtf_to_book_str(str/*:string*/, opts)/*:Workbook*/ {
-	var o = opts || {};
-	var sname = o.sheet || "Sheet1";
-	var ws/*:Worksheet*/ = o.dense ? ([]/*:any*/) : ({}/*:any*/);
-	var wb/*:Workbook*/ = { SheetNames: [ sname ], Sheets: {} };
-	wb.Sheets[sname] = ws;
-
-	var rows = str.match(/\\trowd[\s\S]*?\\row\b/g);
-	if(!rows.length) throw new Error("RTF missing table");
-	var range/*:Range*/ = ({s: {c:0, r:0}, e: {c:0, r:rows.length - 1}}/*:any*/);
-	rows.forEach(function(rowtf, R) {
-		if(Array.isArray(ws)) ws[R] = [];
-		var rtfre = /\\[\w\-]+\b/g;
-		var last_index = 0;
-		var res;
-		var C = -1;
-		var payload = [];
-		while((res = rtfre.exec(rowtf))) {
-			var data = rowtf.slice(last_index, rtfre.lastIndex - res[0].length);
-			if(data.charCodeAt(0) == 0x20) data = data.slice(1);
-			if(data.length) payload.push(data);
-			switch(res[0]) {
-				case "\\cell":
-					++C;
-					if(payload.length) {
-						// TODO: value parsing, including codepage adjustments
-						var cell = {v: payload.join(""), t:"s"};
-						if(cell.v == "TRUE" || cell.v == "FALSE") { cell.v = cell.v == "TRUE"; cell.t = "b"; }
-						else if(!isNaN(fuzzynum(cell.v))) { cell.t = 'n'; if(o.cellText !== false) cell.w = cell.v; cell.v = fuzzynum(cell.v); }
-
-						if(Array.isArray(ws)) ws[R][C] = cell;
-						else ws[encode_cell({r:R, c:C})] = cell;
-					}
-					payload = [];
-					break;
-				case "\\par": // NOTE: Excel serializes both "\r" and "\n" as "\\par"
-					payload.push("\n");
-					break;
-			}
-			last_index = rtfre.lastIndex;
-		}
-		if(C > range.e.c) range.e.c = C;
-	});
-	ws['!ref'] = encode_range(range);
-	return wb;
-}
-
-/* TODO: standardize sheet names as titles for tables */
-function sheet_to_rtf(ws/*:Worksheet*//*::, opts*/)/*:string*/ {
-	var o = ["{\\rtf1\\ansi"];
-	var r = safe_decode_range(ws['!ref']), cell/*:Cell*/;
-	var dense = Array.isArray(ws);
-	for(var R = r.s.r; R <= r.e.r; ++R) {
-		o.push("\\trowd\\trautofit1");
-		for(var C = r.s.c; C <= r.e.c; ++C) o.push("\\cellx" + (C+1));
-		o.push("\\pard\\intbl");
-		for(C = r.s.c; C <= r.e.c; ++C) {
-			var coord = encode_cell({r:R,c:C});
-			cell = dense ? (ws[R]||[])[C]: ws[coord];
-			if(!cell || cell.v == null && (!cell.f || cell.F)) continue;
-			o.push(" " + (cell.w || (format_cell(cell), cell.w)).replace(/[\r\n]/g, "\\par "));
-			o.push("\\cell");
-		}
-		o.push("\\pard\\intbl\\row");
-	}
-	return o.join("") + "}";
+function rtf_to_sheet(d, opts) {
+  switch (opts.type) {
+    case "base64":
+      return rtf_to_sheet_str(Base64_decode(d), opts);
+    case "binary":
+      return rtf_to_sheet_str(d, opts);
+    case "buffer":
+      return rtf_to_sheet_str(has_buf && Buffer.isBuffer(d) ? d.toString("binary") : a2s(d), opts);
+    case "array":
+      return rtf_to_sheet_str(cc2str(d), opts);
+  }
+  throw new Error("Unrecognized type " + opts.type);
+}
+function rtf_to_sheet_str(str, opts) {
+  var o = opts || {};
+  var ws = o.dense ? [] : {};
+  var rows = str.match(/\\trowd[\s\S]*?\\row\b/g);
+  if (!rows)
+    throw new Error("RTF missing table");
+  var range = { s: { c: 0, r: 0 }, e: { c: 0, r: rows.length - 1 } };
+  rows.forEach(function(rowtf, R) {
+    if (Array.isArray(ws))
+      ws[R] = [];
+    var rtfre = /\\[\w\-]+\b/g;
+    var last_index = 0;
+    var res;
+    var C = -1;
+    var payload = [];
+    while ((res = rtfre.exec(rowtf)) != null) {
+      var data = rowtf.slice(last_index, rtfre.lastIndex - res[0].length);
+      if (data.charCodeAt(0) == 32)
+        data = data.slice(1);
+      if (data.length)
+        payload.push(data);
+      switch (res[0]) {
+        case "\\cell":
+          ++C;
+          if (payload.length) {
+            var cell = { v: payload.join(""), t: "s" };
+            if (cell.v == "TRUE" || cell.v == "FALSE") {
+              cell.v = cell.v == "TRUE";
+              cell.t = "b";
+            } else if (!isNaN(fuzzynum(cell.v))) {
+              cell.t = "n";
+              if (o.cellText !== false)
+                cell.w = cell.v;
+              cell.v = fuzzynum(cell.v);
+            }
+            if (Array.isArray(ws))
+              ws[R][C] = cell;
+            else
+              ws[encode_cell({ r: R, c: C })] = cell;
+          }
+          payload = [];
+          break;
+        case "\\par":
+          payload.push("\n");
+          break;
+      }
+      last_index = rtfre.lastIndex;
+    }
+    if (C > range.e.c)
+      range.e.c = C;
+  });
+  ws["!ref"] = encode_range(range);
+  return ws;
+}
+function rtf_to_workbook(d, opts) {
+  var wb = sheet_to_workbook(rtf_to_sheet(d, opts), opts);
+  wb.bookType = "rtf";
+  return wb;
+}
+function sheet_to_rtf(ws, opts) {
+  var o = ["{\\rtf1\\ansi"];
+  if (!ws["!ref"])
+    return o[0] + "}";
+  var r = safe_decode_range(ws["!ref"]), cell;
+  var dense = Array.isArray(ws);
+  for (var R = r.s.r; R <= r.e.r; ++R) {
+    o.push("\\trowd\\trautofit1");
+    for (var C = r.s.c; C <= r.e.c; ++C)
+      o.push("\\cellx" + (C + 1));
+    o.push("\\pard\\intbl");
+    for (C = r.s.c; C <= r.e.c; ++C) {
+      var coord = encode_cell({ r: R, c: C });
+      cell = dense ? (ws[R] || [])[C] : ws[coord];
+      if (!cell || cell.v == null && (!cell.f || cell.F)) {
+        o.push(" \\cell");
+        continue;
+      }
+      o.push(" " + (cell.w || (format_cell(cell), cell.w) || "").replace(/[\r\n]/g, "\\par "));
+      o.push("\\cell");
+    }
+    o.push("\\pard\\intbl\\row");
+  }
+  return o.join("") + "}";
 }
 function hex2RGB(h) {
 	var o = h.slice(h[0]==="#"?1:0).slice(0,6);
@@ -18728,6 +18756,7 @@ function parse_xlml_xml(d, _opts)/*:Workbook*/ {
 	out.SSF = dup(table_fmt);
 	out.Props = Props;
 	out.Custprops = Custprops;
+	out.bookType = "xlml";
 	return out;
 }
 
@@ -21619,9 +21648,14 @@ var HTML_END = '</body></html>';
 function html_to_workbook(str/*:string*/, opts)/*:Workbook*/ {
 	var mtch = str.match(/<table[\s\S]*?>[\s\S]*?<\/table>/gi);
 	if(!mtch || mtch.length == 0) throw new Error("Invalid HTML: could not find <table>");
-	if(mtch.length == 1) return sheet_to_workbook(html_to_sheet(mtch[0], opts), opts);
+	if(mtch.length == 1) {
+		var w = sheet_to_workbook(html_to_sheet(mtch[0], opts), opts);
+		w.bookType = "html";
+		return w;
+	}
 	var wb = book_new();
 	mtch.forEach(function(s, idx) { book_append_sheet(wb, html_to_sheet(s, opts), "Sheet" + (idx+1)); });
+	wb.bookType = "html";
 	return wb;
 }
 
@@ -21737,7 +21771,9 @@ function parse_dom_table(table/*:HTMLElement*/, _opts/*:?any*/)/*:Worksheet*/ {
 }
 
 function table_to_book(table/*:HTMLElement*/, opts/*:?any*/)/*:Workbook*/ {
-	return sheet_to_workbook(parse_dom_table(table, opts), opts);
+	var o = sheet_to_workbook(parse_dom_table(table, opts), opts);
+	//o.bookType = "dom"; // TODO: define a type for this
+	return o;
 }
 
 function is_dom_element_hidden(element/*:HTMLElement*/)/*:boolean*/ {
@@ -22528,10 +22564,13 @@ function parse_ods(zip/*:ZIPFile*/, opts/*:?ParseOpts*/)/*:Workbook*/ {
 	if(!content) throw new Error("Missing content.xml in ODS / UOF file");
 	var wb = parse_content_xml(utf8read(content), opts, Styles);
 	if(safegetzipfile(zip, 'meta.xml')) wb.Props = parse_core_props(getzipdata(zip, 'meta.xml'));
+	wb.bookType = "ods";
 	return wb;
 }
 function parse_fods(data/*:string*/, opts/*:?ParseOpts*/)/*:Workbook*/ {
-	return parse_content_xml(data, opts);
+	var wb = parse_content_xml(data, opts);
+	wb.bookType = "fods";
+	return wb;
 }
 
 /* OpenDocument */
@@ -23413,7 +23452,7 @@ function parse_old_storage(buf, sst, rsst, v) {
   var ret;
   switch (buf[2]) {
     case 0:
-      break;
+      return void 0;
     case 2:
       ret = { t: "n", v: ieee };
       break;
@@ -23473,7 +23512,7 @@ function parse_new_storage(buf, sst, rsst) {
   var ret;
   switch (buf[1]) {
     case 0:
-      break;
+      return void 0;
     case 2:
       ret = { t: "n", v: d128 };
       break;
@@ -23778,6 +23817,7 @@ function parse_TN_DocumentArchive(M, root) {
   });
   if (out.SheetNames.length == 0)
     throw new Error("Empty NUMBERS file");
+  out.bookType = "numbers";
   return out;
 }
 function parse_numbers_iwa(cfb) {
@@ -23978,6 +24018,8 @@ function write_numbers_iwa(wb, opts) {
     throw new Error("Too many messages");
   }
   var entry = CFB.find(cfb, dependents[1].location);
+  if (!entry)
+    throw "Could not find ".concat(dependents[1].location, " in Numbers template");
   var x = parse_iwa_file(decompress_iwa_file(entry.content));
   var docroot;
   for (var xi = 0; xi < x.length; ++xi) {
@@ -23985,8 +24027,12 @@ function write_numbers_iwa(wb, opts) {
     if (packet.id == 1)
       docroot = packet;
   }
+  if (docroot == null)
+    throw "Could not find message ".concat(1, " in Numbers template");
   var sheetrootref = parse_TSP_Reference(parse_shallow(docroot.messages[0].data)[1][0].data);
   entry = CFB.find(cfb, dependents[sheetrootref].location);
+  if (!entry)
+    throw "Could not find ".concat(dependents[sheetrootref].location, " in Numbers template");
   x = parse_iwa_file(decompress_iwa_file(entry.content));
   for (xi = 0; xi < x.length; ++xi) {
     packet = x[xi];
@@ -24002,6 +24048,8 @@ function write_numbers_iwa(wb, opts) {
   entry.size = entry.content.length;
   sheetrootref = parse_TSP_Reference(sheetref[2][0].data);
   entry = CFB.find(cfb, dependents[sheetrootref].location);
+  if (!entry)
+    throw "Could not find ".concat(dependents[sheetrootref].location, " in Numbers template");
   x = parse_iwa_file(decompress_iwa_file(entry.content));
   for (xi = 0; xi < x.length; ++xi) {
     packet = x[xi];
@@ -24010,6 +24058,8 @@ function write_numbers_iwa(wb, opts) {
   }
   sheetrootref = parse_TSP_Reference(parse_shallow(docroot.messages[0].data)[2][0].data);
   entry = CFB.find(cfb, dependents[sheetrootref].location);
+  if (!entry)
+    throw "Could not find ".concat(dependents[sheetrootref].location, " in Numbers template");
   x = parse_iwa_file(decompress_iwa_file(entry.content));
   for (xi = 0; xi < x.length; ++xi) {
     packet = x[xi];
@@ -24022,6 +24072,8 @@ function write_numbers_iwa(wb, opts) {
     pb[7][0].data = write_varint49(range.e.c + 1);
     var cruidsref = parse_TSP_Reference(pb[46][0].data);
     var oldbucket = CFB.find(cfb, dependents[cruidsref].location);
+    if (!oldbucket)
+      throw "Could not find ".concat(dependents[cruidsref].location, " in Numbers template");
     var _x = parse_iwa_file(decompress_iwa_file(oldbucket.content));
     {
       for (var j = 0; j < _x.length; ++j) {
@@ -24064,6 +24116,8 @@ function write_numbers_iwa(wb, opts) {
       var row_headers = parse_shallow(store[1][0].data);
       var row_header_ref = parse_TSP_Reference(row_headers[2][0].data);
       oldbucket = CFB.find(cfb, dependents[row_header_ref].location);
+      if (!oldbucket)
+        throw "Could not find ".concat(dependents[cruidsref].location, " in Numbers template");
       _x = parse_iwa_file(decompress_iwa_file(oldbucket.content));
       {
         if (_x[0].id != row_header_ref)
@@ -24082,6 +24136,8 @@ function write_numbers_iwa(wb, opts) {
       oldbucket.size = oldbucket.content.length;
       var col_header_ref = parse_TSP_Reference(store[2][0].data);
       oldbucket = CFB.find(cfb, dependents[col_header_ref].location);
+      if (!oldbucket)
+        throw "Could not find ".concat(dependents[cruidsref].location, " in Numbers template");
       _x = parse_iwa_file(decompress_iwa_file(oldbucket.content));
       {
         if (_x[0].id != col_header_ref)
@@ -24126,6 +24182,8 @@ function write_numbers_iwa(wb, opts) {
       var sstref = parse_TSP_Reference(store[4][0].data);
       (function() {
         var sentry = CFB.find(cfb, dependents[sstref].location);
+        if (!sentry)
+          throw "Could not find ".concat(dependents[sstref].location, " in Numbers template");
         var sx = parse_iwa_file(decompress_iwa_file(sentry.content));
         var sstroot;
         for (var sxi = 0; sxi < sx.length; ++sxi) {
@@ -24133,6 +24191,8 @@ function write_numbers_iwa(wb, opts) {
           if (packet2.id == sstref)
             sstroot = packet2;
         }
+        if (sstroot == null)
+          throw "Could not find message ".concat(sstref, " in Numbers template");
         var sstdata = parse_shallow(sstroot.messages[0].data);
         {
           sstdata[3] = [];
@@ -24158,6 +24218,8 @@ function write_numbers_iwa(wb, opts) {
           var tileref = parse_TSP_Reference(tl[2][0].data);
           (function() {
             var tentry = CFB.find(cfb, dependents[tileref].location);
+            if (!tentry)
+              throw "Could not find ".concat(dependents[tileref].location, " in Numbers template");
             var tx = parse_iwa_file(decompress_iwa_file(tentry.content));
             var tileroot;
             for (var sxi = 0; sxi < tx.length; ++sxi) {
@@ -24496,6 +24558,8 @@ function parse_zip(zip/*:ZIP*/, opts/*:?ParseOpts*/)/*:Workbook*/ {
 		if(dir.vba.length > 0) out.vbaraw = getzipdata(zip,strip_front_slash(dir.vba[0]),true);
 		else if(dir.defaults && dir.defaults.bin === CT_VBA) out.vbaraw = getzipdata(zip, 'xl/vbaProject.bin',true);
 	}
+	// TODO: pass back content types metdata for xlsm/xlsx resolution
+	out.bookType = xlsb ? "xlsb" : "xlsx";
 	return out;
 }
 
diff --git a/xlsx.js b/xlsx.js
index c530dbf26..0815982e8 100644
--- a/xlsx.js
+++ b/xlsx.js
@@ -7755,7 +7755,11 @@ function dbf_to_sheet(buf, opts) {
 }
 
 function dbf_to_workbook(buf, opts) {
-	try { return sheet_to_workbook(dbf_to_sheet(buf, opts), opts); }
+	try {
+		var o = sheet_to_workbook(dbf_to_sheet(buf, opts), opts);
+		o.bookType = "dbf";
+		return o;
+	}
 	catch(e) { if(opts && opts.WTF) throw e; }
 	return ({SheetNames:[],Sheets:{}});
 }
@@ -8066,6 +8070,7 @@ var SYLK = (function() {
 		keys(ws).forEach(function(k) { o[k] = ws[k]; });
 		var outwb = sheet_to_workbook(o, opts);
 		keys(wb).forEach(function(k) { outwb[k] = wb[k]; });
+		outwb.bookType = "sylk";
 		return outwb;
 	}
 
@@ -8184,7 +8189,11 @@ var DIF = (function() {
 	}
 
 	function dif_to_sheet(str, opts) { return aoa_to_sheet(dif_to_aoa(str, opts), opts); }
-	function dif_to_workbook(str, opts) { return sheet_to_workbook(dif_to_sheet(str, opts), opts); }
+	function dif_to_workbook(str, opts) {
+		var o = sheet_to_workbook(dif_to_sheet(str, opts), opts);
+		o.bookType = "dif";
+		return o;
+	}
 
 	var sheet_to_dif = (function() {
 		var push_field = function pf(o, topic, v, n, s) {
@@ -10231,88 +10240,103 @@ function parse_FilePass(blob, length, opts) {
 }
 
 
-var RTF = (function() {
-	function rtf_to_sheet(d, opts) {
-		switch(opts.type) {
-			case 'base64': return rtf_to_sheet_str(Base64_decode(d), opts);
-			case 'binary': return rtf_to_sheet_str(d, opts);
-			case 'buffer': return rtf_to_sheet_str(has_buf && Buffer.isBuffer(d) ? d.toString('binary') : a2s(d), opts);
-			case 'array':  return rtf_to_sheet_str(cc2str(d), opts);
-		}
-		throw new Error("Unrecognized type " + opts.type);
-	}
-
-	/* TODO: this is a stub */
-	function rtf_to_sheet_str(str, opts) {
-		var o = opts || {};
-		var ws = o.dense ? ([]) : ({});
-
-		var rows = str.match(/\\trowd[\s\S]*?\\row\b/g);
-		if(!rows.length) throw new Error("RTF missing table");
-		var range = ({s: {c:0, r:0}, e: {c:0, r:rows.length - 1}});
-		rows.forEach(function(rowtf, R) {
-			if(Array.isArray(ws)) ws[R] = [];
-			var rtfre = /\\[\w\-]+\b/g;
-			var last_index = 0;
-			var res;
-			var C = -1;
-			var payload = [];
-			while((res = rtfre.exec(rowtf))) {
-				var data = rowtf.slice(last_index, rtfre.lastIndex - res[0].length);
-				if(data.charCodeAt(0) == 0x20) data = data.slice(1);
-				if(data.length) payload.push(data);
-				switch(res[0]) {
-					case "\\cell":
-						++C;
-						if(payload.length) {
-							// TODO: value parsing, including codepage adjustments
-							var cell = {v: payload.join(""), t:"s"};
-							if(Array.isArray(ws)) ws[R][C] = cell;
-							else ws[encode_cell({r:R, c:C})] = cell;
-						}
-						payload = [];
-						break;
-					case "\\par": // NOTE: Excel serializes both "\r" and "\n" as "\\par"
-						payload.push("\n");
-						break;
-				}
-				last_index = rtfre.lastIndex;
-			}
-			if(C > range.e.c) range.e.c = C;
-		});
-		ws['!ref'] = encode_range(range);
-		return ws;
-	}
-
-	function rtf_to_workbook(d, opts) { return sheet_to_workbook(rtf_to_sheet(d, opts), opts); }
-
-	/* TODO: this is a stub */
-	function sheet_to_rtf(ws) {
-		var o = ["{\\rtf1\\ansi"];
-		var r = safe_decode_range(ws['!ref']), cell;
-		var dense = Array.isArray(ws);
-		for(var R = r.s.r; R <= r.e.r; ++R) {
-			o.push("\\trowd\\trautofit1");
-			for(var C = r.s.c; C <= r.e.c; ++C) o.push("\\cellx" + (C+1));
-			o.push("\\pard\\intbl");
-			for(C = r.s.c; C <= r.e.c; ++C) {
-				var coord = encode_cell({r:R,c:C});
-				cell = dense ? (ws[R]||[])[C]: ws[coord];
-				if(!cell || cell.v == null && (!cell.f || cell.F)) continue;
-				o.push(" " + (cell.w || (format_cell(cell), cell.w)).replace(/[\r\n]/g, "\\par "));
-				o.push("\\cell");
-			}
-			o.push("\\pard\\intbl\\row");
-		}
-		return o.join("") + "}";
-	}
-
-	return {
-		to_workbook: rtf_to_workbook,
-		to_sheet: rtf_to_sheet,
-		from_sheet: sheet_to_rtf
-	};
-})();
+function rtf_to_sheet(d, opts) {
+  switch (opts.type) {
+    case "base64":
+      return rtf_to_sheet_str(Base64_decode(d), opts);
+    case "binary":
+      return rtf_to_sheet_str(d, opts);
+    case "buffer":
+      return rtf_to_sheet_str(has_buf && Buffer.isBuffer(d) ? d.toString("binary") : a2s(d), opts);
+    case "array":
+      return rtf_to_sheet_str(cc2str(d), opts);
+  }
+  throw new Error("Unrecognized type " + opts.type);
+}
+function rtf_to_sheet_str(str, opts) {
+  var o = opts || {};
+  var ws = o.dense ? [] : {};
+  var rows = str.match(/\\trowd[\s\S]*?\\row\b/g);
+  if (!rows)
+    throw new Error("RTF missing table");
+  var range = { s: { c: 0, r: 0 }, e: { c: 0, r: rows.length - 1 } };
+  rows.forEach(function(rowtf, R) {
+    if (Array.isArray(ws))
+      ws[R] = [];
+    var rtfre = /\\[\w\-]+\b/g;
+    var last_index = 0;
+    var res;
+    var C = -1;
+    var payload = [];
+    while ((res = rtfre.exec(rowtf)) != null) {
+      var data = rowtf.slice(last_index, rtfre.lastIndex - res[0].length);
+      if (data.charCodeAt(0) == 32)
+        data = data.slice(1);
+      if (data.length)
+        payload.push(data);
+      switch (res[0]) {
+        case "\\cell":
+          ++C;
+          if (payload.length) {
+            var cell = { v: payload.join(""), t: "s" };
+            if (cell.v == "TRUE" || cell.v == "FALSE") {
+              cell.v = cell.v == "TRUE";
+              cell.t = "b";
+            } else if (!isNaN(fuzzynum(cell.v))) {
+              cell.t = "n";
+              if (o.cellText !== false)
+                cell.w = cell.v;
+              cell.v = fuzzynum(cell.v);
+            }
+            if (Array.isArray(ws))
+              ws[R][C] = cell;
+            else
+              ws[encode_cell({ r: R, c: C })] = cell;
+          }
+          payload = [];
+          break;
+        case "\\par":
+          payload.push("\n");
+          break;
+      }
+      last_index = rtfre.lastIndex;
+    }
+    if (C > range.e.c)
+      range.e.c = C;
+  });
+  ws["!ref"] = encode_range(range);
+  return ws;
+}
+function rtf_to_workbook(d, opts) {
+  var wb = sheet_to_workbook(rtf_to_sheet(d, opts), opts);
+  wb.bookType = "rtf";
+  return wb;
+}
+function sheet_to_rtf(ws, opts) {
+  var o = ["{\\rtf1\\ansi"];
+  if (!ws["!ref"])
+    return o[0] + "}";
+  var r = safe_decode_range(ws["!ref"]), cell;
+  var dense = Array.isArray(ws);
+  for (var R = r.s.r; R <= r.e.r; ++R) {
+    o.push("\\trowd\\trautofit1");
+    for (var C = r.s.c; C <= r.e.c; ++C)
+      o.push("\\cellx" + (C + 1));
+    o.push("\\pard\\intbl");
+    for (C = r.s.c; C <= r.e.c; ++C) {
+      var coord = encode_cell({ r: R, c: C });
+      cell = dense ? (ws[R] || [])[C] : ws[coord];
+      if (!cell || cell.v == null && (!cell.f || cell.F)) {
+        o.push(" \\cell");
+        continue;
+      }
+      o.push(" " + (cell.w || (format_cell(cell), cell.w) || "").replace(/[\r\n]/g, "\\par "));
+      o.push("\\cell");
+    }
+    o.push("\\pard\\intbl\\row");
+  }
+  return o.join("") + "}";
+}
 function hex2RGB(h) {
 	var o = h.slice(h[0]==="#"?1:0).slice(0,6);
 	return [parseInt(o.slice(0,2),16),parseInt(o.slice(2,4),16),parseInt(o.slice(4,6),16)];
@@ -18631,6 +18655,7 @@ Workbook.WBProps.date1904 = true;
 	out.SSF = dup(table_fmt);
 	out.Props = Props;
 	out.Custprops = Custprops;
+	out.bookType = "xlml";
 	return out;
 }
 
@@ -21513,9 +21538,14 @@ var HTML_END = '</body></html>';
 function html_to_workbook(str, opts) {
 	var mtch = str.match(/<table[\s\S]*?>[\s\S]*?<\/table>/gi);
 	if(!mtch || mtch.length == 0) throw new Error("Invalid HTML: could not find <table>");
-	if(mtch.length == 1) return sheet_to_workbook(html_to_sheet(mtch[0], opts), opts);
+	if(mtch.length == 1) {
+		var w = sheet_to_workbook(html_to_sheet(mtch[0], opts), opts);
+		w.bookType = "html";
+		return w;
+	}
 	var wb = book_new();
 	mtch.forEach(function(s, idx) { book_append_sheet(wb, html_to_sheet(s, opts), "Sheet" + (idx+1)); });
+	wb.bookType = "html";
 	return wb;
 }
 
@@ -21631,7 +21661,9 @@ function parse_dom_table(table, _opts) {
 }
 
 function table_to_book(table, opts) {
-	return sheet_to_workbook(parse_dom_table(table, opts), opts);
+	var o = sheet_to_workbook(parse_dom_table(table, opts), opts);
+	//o.bookType = "dom"; // TODO: define a type for this
+	return o;
 }
 
 function is_dom_element_hidden(element) {
@@ -22422,10 +22454,13 @@ function parse_ods(zip, opts) {
 	if(!content) throw new Error("Missing content.xml in ODS / UOF file");
 	var wb = parse_content_xml(utf8read(content), opts, Styles);
 	if(safegetzipfile(zip, 'meta.xml')) wb.Props = parse_core_props(getzipdata(zip, 'meta.xml'));
+	wb.bookType = "ods";
 	return wb;
 }
 function parse_fods(data, opts) {
-	return parse_content_xml(data, opts);
+	var wb = parse_content_xml(data, opts);
+	wb.bookType = "fods";
+	return wb;
 }
 
 /* OpenDocument */
@@ -23307,7 +23342,7 @@ function parse_old_storage(buf, sst, rsst, v) {
   var ret;
   switch (buf[2]) {
     case 0:
-      break;
+      return void 0;
     case 2:
       ret = { t: "n", v: ieee };
       break;
@@ -23367,7 +23402,7 @@ function parse_new_storage(buf, sst, rsst) {
   var ret;
   switch (buf[1]) {
     case 0:
-      break;
+      return void 0;
     case 2:
       ret = { t: "n", v: d128 };
       break;
@@ -23672,6 +23707,7 @@ function parse_TN_DocumentArchive(M, root) {
   });
   if (out.SheetNames.length == 0)
     throw new Error("Empty NUMBERS file");
+  out.bookType = "numbers";
   return out;
 }
 function parse_numbers_iwa(cfb) {
@@ -23872,6 +23908,8 @@ function write_numbers_iwa(wb, opts) {
     throw new Error("Too many messages");
   }
   var entry = CFB.find(cfb, dependents[1].location);
+  if (!entry)
+    throw "Could not find ".concat(dependents[1].location, " in Numbers template");
   var x = parse_iwa_file(decompress_iwa_file(entry.content));
   var docroot;
   for (var xi = 0; xi < x.length; ++xi) {
@@ -23879,8 +23917,12 @@ function write_numbers_iwa(wb, opts) {
     if (packet.id == 1)
       docroot = packet;
   }
+  if (docroot == null)
+    throw "Could not find message ".concat(1, " in Numbers template");
   var sheetrootref = parse_TSP_Reference(parse_shallow(docroot.messages[0].data)[1][0].data);
   entry = CFB.find(cfb, dependents[sheetrootref].location);
+  if (!entry)
+    throw "Could not find ".concat(dependents[sheetrootref].location, " in Numbers template");
   x = parse_iwa_file(decompress_iwa_file(entry.content));
   for (xi = 0; xi < x.length; ++xi) {
     packet = x[xi];
@@ -23896,6 +23938,8 @@ function write_numbers_iwa(wb, opts) {
   entry.size = entry.content.length;
   sheetrootref = parse_TSP_Reference(sheetref[2][0].data);
   entry = CFB.find(cfb, dependents[sheetrootref].location);
+  if (!entry)
+    throw "Could not find ".concat(dependents[sheetrootref].location, " in Numbers template");
   x = parse_iwa_file(decompress_iwa_file(entry.content));
   for (xi = 0; xi < x.length; ++xi) {
     packet = x[xi];
@@ -23904,6 +23948,8 @@ function write_numbers_iwa(wb, opts) {
   }
   sheetrootref = parse_TSP_Reference(parse_shallow(docroot.messages[0].data)[2][0].data);
   entry = CFB.find(cfb, dependents[sheetrootref].location);
+  if (!entry)
+    throw "Could not find ".concat(dependents[sheetrootref].location, " in Numbers template");
   x = parse_iwa_file(decompress_iwa_file(entry.content));
   for (xi = 0; xi < x.length; ++xi) {
     packet = x[xi];
@@ -23916,6 +23962,8 @@ function write_numbers_iwa(wb, opts) {
     pb[7][0].data = write_varint49(range.e.c + 1);
     var cruidsref = parse_TSP_Reference(pb[46][0].data);
     var oldbucket = CFB.find(cfb, dependents[cruidsref].location);
+    if (!oldbucket)
+      throw "Could not find ".concat(dependents[cruidsref].location, " in Numbers template");
     var _x = parse_iwa_file(decompress_iwa_file(oldbucket.content));
     {
       for (var j = 0; j < _x.length; ++j) {
@@ -23958,6 +24006,8 @@ function write_numbers_iwa(wb, opts) {
       var row_headers = parse_shallow(store[1][0].data);
       var row_header_ref = parse_TSP_Reference(row_headers[2][0].data);
       oldbucket = CFB.find(cfb, dependents[row_header_ref].location);
+      if (!oldbucket)
+        throw "Could not find ".concat(dependents[cruidsref].location, " in Numbers template");
       _x = parse_iwa_file(decompress_iwa_file(oldbucket.content));
       {
         if (_x[0].id != row_header_ref)
@@ -23976,6 +24026,8 @@ function write_numbers_iwa(wb, opts) {
       oldbucket.size = oldbucket.content.length;
       var col_header_ref = parse_TSP_Reference(store[2][0].data);
       oldbucket = CFB.find(cfb, dependents[col_header_ref].location);
+      if (!oldbucket)
+        throw "Could not find ".concat(dependents[cruidsref].location, " in Numbers template");
       _x = parse_iwa_file(decompress_iwa_file(oldbucket.content));
       {
         if (_x[0].id != col_header_ref)
@@ -24020,6 +24072,8 @@ function write_numbers_iwa(wb, opts) {
       var sstref = parse_TSP_Reference(store[4][0].data);
       (function() {
         var sentry = CFB.find(cfb, dependents[sstref].location);
+        if (!sentry)
+          throw "Could not find ".concat(dependents[sstref].location, " in Numbers template");
         var sx = parse_iwa_file(decompress_iwa_file(sentry.content));
         var sstroot;
         for (var sxi = 0; sxi < sx.length; ++sxi) {
@@ -24027,6 +24081,8 @@ function write_numbers_iwa(wb, opts) {
           if (packet2.id == sstref)
             sstroot = packet2;
         }
+        if (sstroot == null)
+          throw "Could not find message ".concat(sstref, " in Numbers template");
         var sstdata = parse_shallow(sstroot.messages[0].data);
         {
           sstdata[3] = [];
@@ -24052,6 +24108,8 @@ function write_numbers_iwa(wb, opts) {
           var tileref = parse_TSP_Reference(tl[2][0].data);
           (function() {
             var tentry = CFB.find(cfb, dependents[tileref].location);
+            if (!tentry)
+              throw "Could not find ".concat(dependents[tileref].location, " in Numbers template");
             var tx = parse_iwa_file(decompress_iwa_file(tentry.content));
             var tileroot;
             for (var sxi = 0; sxi < tx.length; ++sxi) {
@@ -24390,6 +24448,8 @@ function parse_zip(zip, opts) {
 		if(dir.vba.length > 0) out.vbaraw = getzipdata(zip,strip_front_slash(dir.vba[0]),true);
 		else if(dir.defaults && dir.defaults.bin === CT_VBA) out.vbaraw = getzipdata(zip, 'xl/vbaProject.bin',true);
 	}
+	// TODO: pass back content types metdata for xlsm/xlsx resolution
+	out.bookType = xlsb ? "xlsb" : "xlsx";
 	return out;
 }
 
@@ -24831,7 +24891,7 @@ function readSync(data, opts) {
 			}
 			break;
 		case 0x03: case 0x83: case 0x8B: case 0x8C: return DBF.to_workbook(d, o);
-		case 0x7B: if(n[1] === 0x5C && n[2] === 0x72 && n[3] === 0x74) return RTF.to_workbook(d, o); break;
+		case 0x7B: if(n[1] === 0x5C && n[2] === 0x72 && n[3] === 0x74) return rtf_to_workbook(d, o); break;
 		case 0x0A: case 0x0D: case 0x20: return read_plaintext_raw(d, o);
 		case 0x89: if(n[1] === 0x50 && n[2] === 0x4E && n[3] === 0x47) throw new Error("PNG Image File is not a spreadsheet"); break;
 		case 0x08: if(n[1] === 0xE7) throw new Error("Unsupported Multiplan 1.x file!"); break;
@@ -24991,7 +25051,7 @@ function writeSync(wb, opts) {
 		case 'dif': return write_string_type(DIF.from_sheet(wb.Sheets[wb.SheetNames[idx]], o), o);
 		case 'dbf': return write_binary_type(DBF.from_sheet(wb.Sheets[wb.SheetNames[idx]], o), o);
 		case 'prn': return write_string_type(PRN.from_sheet(wb.Sheets[wb.SheetNames[idx]], o), o);
-		case 'rtf': return write_string_type(RTF.from_sheet(wb.Sheets[wb.SheetNames[idx]], o), o);
+		case 'rtf': return write_string_type(sheet_to_rtf(wb.Sheets[wb.SheetNames[idx]], o), o);
 		case 'eth': return write_string_type(ETH.from_sheet(wb.Sheets[wb.SheetNames[idx]], o), o);
 		case 'fods': return write_string_type(write_ods(wb, o), o);
 		case 'wk1': return write_binary_type(WK_.sheet_to_wk1(wb.Sheets[wb.SheetNames[idx]], o), o);
diff --git a/xlsx.mjs b/xlsx.mjs
index a5bc4c41a..ec3efcdcf 100644
--- a/xlsx.mjs
+++ b/xlsx.mjs
@@ -7840,7 +7840,11 @@ function dbf_to_sheet(buf, opts)/*:Worksheet*/ {
 }
 
 function dbf_to_workbook(buf, opts)/*:Workbook*/ {
-	try { return sheet_to_workbook(dbf_to_sheet(buf, opts), opts); }
+	try {
+		var o = sheet_to_workbook(dbf_to_sheet(buf, opts), opts);
+		o.bookType = "dbf";
+		return o;
+	}
 	catch(e) { if(opts && opts.WTF) throw e; }
 	return ({SheetNames:[],Sheets:{}});
 }
@@ -8151,6 +8155,7 @@ var SYLK = /*#__PURE__*/(function() {
 		keys(ws).forEach(function(k) { o[k] = ws[k]; });
 		var outwb = sheet_to_workbook(o, opts);
 		keys(wb).forEach(function(k) { outwb[k] = wb[k]; });
+		outwb.bookType = "sylk";
 		return outwb;
 	}
 
@@ -8269,7 +8274,11 @@ var DIF = /*#__PURE__*/(function() {
 	}
 
 	function dif_to_sheet(str/*:string*/, opts)/*:Worksheet*/ { return aoa_to_sheet(dif_to_aoa(str, opts), opts); }
-	function dif_to_workbook(str/*:string*/, opts)/*:Workbook*/ { return sheet_to_workbook(dif_to_sheet(str, opts), opts); }
+	function dif_to_workbook(str/*:string*/, opts)/*:Workbook*/ {
+		var o = sheet_to_workbook(dif_to_sheet(str, opts), opts);
+		o.bookType = "dif";
+		return o;
+	}
 
 	var sheet_to_dif = /*#__PURE__*/(function() {
 		var push_field = function pf(o/*:Array<string>*/, topic/*:string*/, v/*:number*/, n/*:number*/, s/*:string*/) {
@@ -10317,83 +10326,102 @@ function parse_FilePass(blob, length/*:number*/, opts) {
 }
 
 
-function rtf_to_workbook(d/*:RawData*/, opts)/*:Workbook*/ {
-	switch(opts.type) {
-		case 'base64': return rtf_to_book_str(Base64_decode(d), opts);
-		case 'binary': return rtf_to_book_str(d, opts);
-		case 'buffer': return rtf_to_book_str(has_buf && Buffer.isBuffer(d) ? d.toString('binary') : a2s(d), opts);
-		case 'array':  return rtf_to_book_str(cc2str(d), opts);
-	}
-	throw new Error("Unrecognized type " + opts.type);
-}
-
-/* TODO: RTF technically can store multiple tables, even if Excel does not */
-function rtf_to_book_str(str/*:string*/, opts)/*:Workbook*/ {
-	var o = opts || {};
-	var sname = o.sheet || "Sheet1";
-	var ws/*:Worksheet*/ = o.dense ? ([]/*:any*/) : ({}/*:any*/);
-	var wb/*:Workbook*/ = { SheetNames: [ sname ], Sheets: {} };
-	wb.Sheets[sname] = ws;
-
-	var rows = str.match(/\\trowd[\s\S]*?\\row\b/g);
-	if(!rows.length) throw new Error("RTF missing table");
-	var range/*:Range*/ = ({s: {c:0, r:0}, e: {c:0, r:rows.length - 1}}/*:any*/);
-	rows.forEach(function(rowtf, R) {
-		if(Array.isArray(ws)) ws[R] = [];
-		var rtfre = /\\[\w\-]+\b/g;
-		var last_index = 0;
-		var res;
-		var C = -1;
-		var payload = [];
-		while((res = rtfre.exec(rowtf))) {
-			var data = rowtf.slice(last_index, rtfre.lastIndex - res[0].length);
-			if(data.charCodeAt(0) == 0x20) data = data.slice(1);
-			if(data.length) payload.push(data);
-			switch(res[0]) {
-				case "\\cell":
-					++C;
-					if(payload.length) {
-						// TODO: value parsing, including codepage adjustments
-						var cell = {v: payload.join(""), t:"s"};
-						if(cell.v == "TRUE" || cell.v == "FALSE") { cell.v = cell.v == "TRUE"; cell.t = "b"; }
-						else if(!isNaN(fuzzynum(cell.v))) { cell.t = 'n'; if(o.cellText !== false) cell.w = cell.v; cell.v = fuzzynum(cell.v); }
-
-						if(Array.isArray(ws)) ws[R][C] = cell;
-						else ws[encode_cell({r:R, c:C})] = cell;
-					}
-					payload = [];
-					break;
-				case "\\par": // NOTE: Excel serializes both "\r" and "\n" as "\\par"
-					payload.push("\n");
-					break;
-			}
-			last_index = rtfre.lastIndex;
-		}
-		if(C > range.e.c) range.e.c = C;
-	});
-	ws['!ref'] = encode_range(range);
-	return wb;
-}
-
-/* TODO: standardize sheet names as titles for tables */
-function sheet_to_rtf(ws/*:Worksheet*//*::, opts*/)/*:string*/ {
-	var o = ["{\\rtf1\\ansi"];
-	var r = safe_decode_range(ws['!ref']), cell/*:Cell*/;
-	var dense = Array.isArray(ws);
-	for(var R = r.s.r; R <= r.e.r; ++R) {
-		o.push("\\trowd\\trautofit1");
-		for(var C = r.s.c; C <= r.e.c; ++C) o.push("\\cellx" + (C+1));
-		o.push("\\pard\\intbl");
-		for(C = r.s.c; C <= r.e.c; ++C) {
-			var coord = encode_cell({r:R,c:C});
-			cell = dense ? (ws[R]||[])[C]: ws[coord];
-			if(!cell || cell.v == null && (!cell.f || cell.F)) continue;
-			o.push(" " + (cell.w || (format_cell(cell), cell.w)).replace(/[\r\n]/g, "\\par "));
-			o.push("\\cell");
-		}
-		o.push("\\pard\\intbl\\row");
-	}
-	return o.join("") + "}";
+function rtf_to_sheet(d, opts) {
+  switch (opts.type) {
+    case "base64":
+      return rtf_to_sheet_str(Base64_decode(d), opts);
+    case "binary":
+      return rtf_to_sheet_str(d, opts);
+    case "buffer":
+      return rtf_to_sheet_str(has_buf && Buffer.isBuffer(d) ? d.toString("binary") : a2s(d), opts);
+    case "array":
+      return rtf_to_sheet_str(cc2str(d), opts);
+  }
+  throw new Error("Unrecognized type " + opts.type);
+}
+function rtf_to_sheet_str(str, opts) {
+  var o = opts || {};
+  var ws = o.dense ? [] : {};
+  var rows = str.match(/\\trowd[\s\S]*?\\row\b/g);
+  if (!rows)
+    throw new Error("RTF missing table");
+  var range = { s: { c: 0, r: 0 }, e: { c: 0, r: rows.length - 1 } };
+  rows.forEach(function(rowtf, R) {
+    if (Array.isArray(ws))
+      ws[R] = [];
+    var rtfre = /\\[\w\-]+\b/g;
+    var last_index = 0;
+    var res;
+    var C = -1;
+    var payload = [];
+    while ((res = rtfre.exec(rowtf)) != null) {
+      var data = rowtf.slice(last_index, rtfre.lastIndex - res[0].length);
+      if (data.charCodeAt(0) == 32)
+        data = data.slice(1);
+      if (data.length)
+        payload.push(data);
+      switch (res[0]) {
+        case "\\cell":
+          ++C;
+          if (payload.length) {
+            var cell = { v: payload.join(""), t: "s" };
+            if (cell.v == "TRUE" || cell.v == "FALSE") {
+              cell.v = cell.v == "TRUE";
+              cell.t = "b";
+            } else if (!isNaN(fuzzynum(cell.v))) {
+              cell.t = "n";
+              if (o.cellText !== false)
+                cell.w = cell.v;
+              cell.v = fuzzynum(cell.v);
+            }
+            if (Array.isArray(ws))
+              ws[R][C] = cell;
+            else
+              ws[encode_cell({ r: R, c: C })] = cell;
+          }
+          payload = [];
+          break;
+        case "\\par":
+          payload.push("\n");
+          break;
+      }
+      last_index = rtfre.lastIndex;
+    }
+    if (C > range.e.c)
+      range.e.c = C;
+  });
+  ws["!ref"] = encode_range(range);
+  return ws;
+}
+function rtf_to_workbook(d, opts) {
+  var wb = sheet_to_workbook(rtf_to_sheet(d, opts), opts);
+  wb.bookType = "rtf";
+  return wb;
+}
+function sheet_to_rtf(ws, opts) {
+  var o = ["{\\rtf1\\ansi"];
+  if (!ws["!ref"])
+    return o[0] + "}";
+  var r = safe_decode_range(ws["!ref"]), cell;
+  var dense = Array.isArray(ws);
+  for (var R = r.s.r; R <= r.e.r; ++R) {
+    o.push("\\trowd\\trautofit1");
+    for (var C = r.s.c; C <= r.e.c; ++C)
+      o.push("\\cellx" + (C + 1));
+    o.push("\\pard\\intbl");
+    for (C = r.s.c; C <= r.e.c; ++C) {
+      var coord = encode_cell({ r: R, c: C });
+      cell = dense ? (ws[R] || [])[C] : ws[coord];
+      if (!cell || cell.v == null && (!cell.f || cell.F)) {
+        o.push(" \\cell");
+        continue;
+      }
+      o.push(" " + (cell.w || (format_cell(cell), cell.w) || "").replace(/[\r\n]/g, "\\par "));
+      o.push("\\cell");
+    }
+    o.push("\\pard\\intbl\\row");
+  }
+  return o.join("") + "}";
 }
 function hex2RGB(h) {
 	var o = h.slice(h[0]==="#"?1:0).slice(0,6);
@@ -18723,6 +18751,7 @@ function parse_xlml_xml(d, _opts)/*:Workbook*/ {
 	out.SSF = dup(table_fmt);
 	out.Props = Props;
 	out.Custprops = Custprops;
+	out.bookType = "xlml";
 	return out;
 }
 
@@ -21614,9 +21643,14 @@ var HTML_END = '</body></html>';
 function html_to_workbook(str/*:string*/, opts)/*:Workbook*/ {
 	var mtch = str.match(/<table[\s\S]*?>[\s\S]*?<\/table>/gi);
 	if(!mtch || mtch.length == 0) throw new Error("Invalid HTML: could not find <table>");
-	if(mtch.length == 1) return sheet_to_workbook(html_to_sheet(mtch[0], opts), opts);
+	if(mtch.length == 1) {
+		var w = sheet_to_workbook(html_to_sheet(mtch[0], opts), opts);
+		w.bookType = "html";
+		return w;
+	}
 	var wb = book_new();
 	mtch.forEach(function(s, idx) { book_append_sheet(wb, html_to_sheet(s, opts), "Sheet" + (idx+1)); });
+	wb.bookType = "html";
 	return wb;
 }
 
@@ -21732,7 +21766,9 @@ function parse_dom_table(table/*:HTMLElement*/, _opts/*:?any*/)/*:Worksheet*/ {
 }
 
 function table_to_book(table/*:HTMLElement*/, opts/*:?any*/)/*:Workbook*/ {
-	return sheet_to_workbook(parse_dom_table(table, opts), opts);
+	var o = sheet_to_workbook(parse_dom_table(table, opts), opts);
+	//o.bookType = "dom"; // TODO: define a type for this
+	return o;
 }
 
 function is_dom_element_hidden(element/*:HTMLElement*/)/*:boolean*/ {
@@ -22523,10 +22559,13 @@ function parse_ods(zip/*:ZIPFile*/, opts/*:?ParseOpts*/)/*:Workbook*/ {
 	if(!content) throw new Error("Missing content.xml in ODS / UOF file");
 	var wb = parse_content_xml(utf8read(content), opts, Styles);
 	if(safegetzipfile(zip, 'meta.xml')) wb.Props = parse_core_props(getzipdata(zip, 'meta.xml'));
+	wb.bookType = "ods";
 	return wb;
 }
 function parse_fods(data/*:string*/, opts/*:?ParseOpts*/)/*:Workbook*/ {
-	return parse_content_xml(data, opts);
+	var wb = parse_content_xml(data, opts);
+	wb.bookType = "fods";
+	return wb;
 }
 
 /* OpenDocument */
@@ -23408,7 +23447,7 @@ function parse_old_storage(buf, sst, rsst, v) {
   var ret;
   switch (buf[2]) {
     case 0:
-      break;
+      return void 0;
     case 2:
       ret = { t: "n", v: ieee };
       break;
@@ -23468,7 +23507,7 @@ function parse_new_storage(buf, sst, rsst) {
   var ret;
   switch (buf[1]) {
     case 0:
-      break;
+      return void 0;
     case 2:
       ret = { t: "n", v: d128 };
       break;
@@ -23773,6 +23812,7 @@ function parse_TN_DocumentArchive(M, root) {
   });
   if (out.SheetNames.length == 0)
     throw new Error("Empty NUMBERS file");
+  out.bookType = "numbers";
   return out;
 }
 function parse_numbers_iwa(cfb) {
@@ -23973,6 +24013,8 @@ function write_numbers_iwa(wb, opts) {
     throw new Error("Too many messages");
   }
   var entry = CFB.find(cfb, dependents[1].location);
+  if (!entry)
+    throw "Could not find ".concat(dependents[1].location, " in Numbers template");
   var x = parse_iwa_file(decompress_iwa_file(entry.content));
   var docroot;
   for (var xi = 0; xi < x.length; ++xi) {
@@ -23980,8 +24022,12 @@ function write_numbers_iwa(wb, opts) {
     if (packet.id == 1)
       docroot = packet;
   }
+  if (docroot == null)
+    throw "Could not find message ".concat(1, " in Numbers template");
   var sheetrootref = parse_TSP_Reference(parse_shallow(docroot.messages[0].data)[1][0].data);
   entry = CFB.find(cfb, dependents[sheetrootref].location);
+  if (!entry)
+    throw "Could not find ".concat(dependents[sheetrootref].location, " in Numbers template");
   x = parse_iwa_file(decompress_iwa_file(entry.content));
   for (xi = 0; xi < x.length; ++xi) {
     packet = x[xi];
@@ -23997,6 +24043,8 @@ function write_numbers_iwa(wb, opts) {
   entry.size = entry.content.length;
   sheetrootref = parse_TSP_Reference(sheetref[2][0].data);
   entry = CFB.find(cfb, dependents[sheetrootref].location);
+  if (!entry)
+    throw "Could not find ".concat(dependents[sheetrootref].location, " in Numbers template");
   x = parse_iwa_file(decompress_iwa_file(entry.content));
   for (xi = 0; xi < x.length; ++xi) {
     packet = x[xi];
@@ -24005,6 +24053,8 @@ function write_numbers_iwa(wb, opts) {
   }
   sheetrootref = parse_TSP_Reference(parse_shallow(docroot.messages[0].data)[2][0].data);
   entry = CFB.find(cfb, dependents[sheetrootref].location);
+  if (!entry)
+    throw "Could not find ".concat(dependents[sheetrootref].location, " in Numbers template");
   x = parse_iwa_file(decompress_iwa_file(entry.content));
   for (xi = 0; xi < x.length; ++xi) {
     packet = x[xi];
@@ -24017,6 +24067,8 @@ function write_numbers_iwa(wb, opts) {
     pb[7][0].data = write_varint49(range.e.c + 1);
     var cruidsref = parse_TSP_Reference(pb[46][0].data);
     var oldbucket = CFB.find(cfb, dependents[cruidsref].location);
+    if (!oldbucket)
+      throw "Could not find ".concat(dependents[cruidsref].location, " in Numbers template");
     var _x = parse_iwa_file(decompress_iwa_file(oldbucket.content));
     {
       for (var j = 0; j < _x.length; ++j) {
@@ -24059,6 +24111,8 @@ function write_numbers_iwa(wb, opts) {
       var row_headers = parse_shallow(store[1][0].data);
       var row_header_ref = parse_TSP_Reference(row_headers[2][0].data);
       oldbucket = CFB.find(cfb, dependents[row_header_ref].location);
+      if (!oldbucket)
+        throw "Could not find ".concat(dependents[cruidsref].location, " in Numbers template");
       _x = parse_iwa_file(decompress_iwa_file(oldbucket.content));
       {
         if (_x[0].id != row_header_ref)
@@ -24077,6 +24131,8 @@ function write_numbers_iwa(wb, opts) {
       oldbucket.size = oldbucket.content.length;
       var col_header_ref = parse_TSP_Reference(store[2][0].data);
       oldbucket = CFB.find(cfb, dependents[col_header_ref].location);
+      if (!oldbucket)
+        throw "Could not find ".concat(dependents[cruidsref].location, " in Numbers template");
       _x = parse_iwa_file(decompress_iwa_file(oldbucket.content));
       {
         if (_x[0].id != col_header_ref)
@@ -24121,6 +24177,8 @@ function write_numbers_iwa(wb, opts) {
       var sstref = parse_TSP_Reference(store[4][0].data);
       (function() {
         var sentry = CFB.find(cfb, dependents[sstref].location);
+        if (!sentry)
+          throw "Could not find ".concat(dependents[sstref].location, " in Numbers template");
         var sx = parse_iwa_file(decompress_iwa_file(sentry.content));
         var sstroot;
         for (var sxi = 0; sxi < sx.length; ++sxi) {
@@ -24128,6 +24186,8 @@ function write_numbers_iwa(wb, opts) {
           if (packet2.id == sstref)
             sstroot = packet2;
         }
+        if (sstroot == null)
+          throw "Could not find message ".concat(sstref, " in Numbers template");
         var sstdata = parse_shallow(sstroot.messages[0].data);
         {
           sstdata[3] = [];
@@ -24153,6 +24213,8 @@ function write_numbers_iwa(wb, opts) {
           var tileref = parse_TSP_Reference(tl[2][0].data);
           (function() {
             var tentry = CFB.find(cfb, dependents[tileref].location);
+            if (!tentry)
+              throw "Could not find ".concat(dependents[tileref].location, " in Numbers template");
             var tx = parse_iwa_file(decompress_iwa_file(tentry.content));
             var tileroot;
             for (var sxi = 0; sxi < tx.length; ++sxi) {
@@ -24491,6 +24553,8 @@ function parse_zip(zip/*:ZIP*/, opts/*:?ParseOpts*/)/*:Workbook*/ {
 		if(dir.vba.length > 0) out.vbaraw = getzipdata(zip,strip_front_slash(dir.vba[0]),true);
 		else if(dir.defaults && dir.defaults.bin === CT_VBA) out.vbaraw = getzipdata(zip, 'xl/vbaProject.bin',true);
 	}
+	// TODO: pass back content types metdata for xlsm/xlsx resolution
+	out.bookType = xlsb ? "xlsb" : "xlsx";
 	return out;
 }