feat(importStar): Cache non‑module results

[ExE-Boss]

Babel has been doing this since babel/babel#10161, babel/babel#10574 and babel/babel#10585.

[rbuckton]

This could also be implemented such that you don't need to repeatedly call __getImportStarCache using something like this:

__importStar = function (mod) {
  var cache = typeof WeakMap === "function" ? new WeakMap() : null;
  __importStar = function (mod) {
    ...
  };
  return _importStar(mod);
};

Which is similar to what we do for extendsStatics.

[rbuckton]

Are you proposing this be implemented in TypeScript as well for inline helper emit? If so we'd need to introduce a checker error if you declare your own value named WeakMap at the top of a module.

[ExE-Boss]

The issue there is that this is supposed to be an optimisation when two different modules that use tslib import the same CommonJS module, the value returned by __importStar is the same value for both imports, e.g.:

// modA.js
Object.defineProperty(exports, "__esModule", { value: true });
var tslib = require("tslib");

exports.example = tslib.__importStar(require("example"));

// modB.js
Object.defineProperty(exports, "__esModule", { value: true });
var tslib = require("tslib");

exports.example = tslib.__importStar(require("example"));

// test.js
var assert = require("assert");

var modA = require("./modA.js");
var modB = require("./modA.js");

// The following passes regardless of whether `example` has `__esModule` or not:
assert.strictEqual(modA.example, modB.example);

---

It doesn’t make sense to include the module cache in inline helpers, as modA and modB would then each have their own cache, and every new local __imporStar call would then have a cache miss, since the cache wouldn’t be global.

[rbuckton]

nit: Parens aren't needed here, since && has higher precedence than ||.

Suggested change

	if (mod === null || (typeof mod !== "object" && typeof mod !== "function")) return { default: mod };
	if (mod === null || typeof mod !== "object" && typeof mod !== "function") return { default: mod };

[rbuckton]

nit: Parens aren't necessary here.

Suggested change

	if (mod === null || (typeof mod !== "object" && typeof mod !== "function")) return { default: mod };
	if (mod === null || typeof mod !== "object" && typeof mod !== "function") return { default: mod };