Unit test the config.js module

Create a unit test suite for the config.js module. To enable this I
refactored the code to take in a file system interface (which just so
happens to match the `node:fs` interface) to make it possible to test
the logic without having to actually use the file system. Namely, the
test includes some logic to create a mocked in-memory file system that
is used for unit testing.

The rest of the existing code only had to be updated to provide the
`node:fs` module as a dependency to the config.js module.

Signed-off-by: Eric Cornelissen <[email protected]>

src/cli.js

@@ -12,6 +12,7 @@
 // You should have received a copy of the GNU Affero General Public License
 // along with this program.  If not, see <https://www.gnu.org/licenses/>.
 
+import * as fs from "node:fs/promises";
 import { argv, exit } from "node:process";
 
 import { readConfig } from "./config.js";
@@ -39,7 +40,7 @@ to ignore npm deprecation warnings for your dependencies.
 
 try {
 	const [config, deprecations] = await Promise.all([
-		readConfig(),
+		readConfig(fs),
 		obtainDeprecations().then(obtainDependencyPaths),
 	]);
 

src/config.js

@@ -12,12 +12,14 @@
 // You should have received a copy of the GNU Affero General Public License
 // along with this program.  If not, see <https://www.gnu.org/licenses/>.
 
-import { readFile } from "node:fs/promises";
-
-export async function readConfig() {
+/**
+ * @param {FileSystem} fs
+ * @returns {Object}
+ */
+export async function readConfig(fs) {
 	let rawConfig;
 	try {
-		rawConfig = await readFile("./.ndmrc", { encoding: "utf-8" });
+		rawConfig = await fs.readFile("./.ndmrc");
 	} catch (_) {
 		throw new Error("Configuration file .ndmrc not found");
 	}
@@ -28,3 +30,12 @@ export async function readConfig() {
 		throw new Error(`Configuration file invalid (${error.message})`);
 	}
 }
+
+/**
+ * @typedef {function(string): Promise<string>} ReadFile
+ */
+
+/**
+ * @typedef FileSystem
+ * @property {ReadFile} readFile
+ */

src/config.test.js

@@ -0,0 +1,119 @@
+// Copyright (C) 2024  Eric Cornelissen
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Affero General Public License as published
+// by the Free Software Foundation, version 3 of the License only.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU Affero General Public License for more details.
+//
+// You should have received a copy of the GNU Affero General Public License
+// along with this program.  If not, see <https://www.gnu.org/licenses/>.
+
+import * as assert from "node:assert/strict";
+import { mock, test } from "node:test";
+
+import {
+	readConfig,
+} from "./config.js";
+
+test("config.js", async (t) => {
+	await t.test("readConfig", async (t) => {
+		const testCases = {
+			"empty config": {
+				config: {},
+			},
+			"sample config": {
+				config: {
+					"[email protected]": {
+						"#ignore": true,
+					},
+				},
+			},
+			"unexpected config": {
+				config: [
+					"even though it shouldn't be an array",
+					"in this part of the code that is not",
+					"considered a problem",
+				],
+			},
+		};
+
+		for (const [name, testCase] of Object.entries(testCases)) {
+			await t.test(name, async () => {
+				const fs = createFs({ "./.ndmrc": JSON.stringify(testCase.config) });
+
+				const got = await readConfig(fs);
+				const want = testCase.config;
+				assert.deepEqual(got, want);
+			});
+		}
+
+		await t.test("usage of fs.readFile", async () => {
+			const fs = createFs({ "./.ndmrc": JSON.stringify("{}") });
+
+			try {
+				await readConfig(fs);
+			} catch (_) { }
+
+			assert.equal(fs.readFile.mock.callCount(), 1);
+
+			const got = fs.readFile.mock.calls[0];
+			assert.ok(got.arguments.length >= 1);
+			assert.match(got.arguments[0], /.ndmrc$/);
+		});
+
+		await t.test("config not in JSON format", async () => {
+			const fs = createFs({ "./.ndmrc": "I'm not valid JSON" });
+
+			await assert.rejects(
+				async () => {
+					await readConfig(fs);
+				},
+				(error) => {
+					assert.ok(error instanceof Error);
+					assert.match(
+						error.message,
+						/^Configuration file invalid \(.+?\)$/,
+					);
+
+					return true;
+				},
+			);
+		});
+
+		await t.test("file not found", async () => {
+			const fs = createFs({});
+
+			await assert.rejects(
+				async () => {
+					await readConfig(fs);
+				},
+				(error) => {
+					assert.ok(error instanceof Error);
+					assert.equal(
+						error.message,
+						"Configuration file .ndmrc not found",
+					);
+
+					return true;
+				},
+			);
+		});
+	});
+});
+
+function createFs(files) {
+	return {
+		readFile: mock.fn((path, _options) => {
+			if (Object.hasOwn(files, path)) {
+				const content = files[path];
+				return Buffer.from(content);
+			} else {
+				throw new Error();
+			}
+		}),
+	};
+}