cannot write to private class fields if TypeScript class is preloaded

[tve]

Build environment: any
Moddable SDK version: 3.9.2
Target device: any

Description
Trying to write to a private field of a preloaded & frozen class fails

Steps to Reproduce

1. Run the main & manifest provided below using mcconfig -d -m -p lin
2. Observe run-time error

/home/src/moddable/repro-weakmap/main.ts (2) # Exception: set: WeakMap instance is read-only!

3. Remove the preloading in manifest.json, re-run, the error is gone
4. Alternatively, change #foo to foo, re-run, the error is gone too

Other information
I suspect this is just the way it is due to the way private fields are implemented. If so, it would be good to document this limitation in moddable/tree/public/documentation/xs/preload.md

main.js:

class MySensor {
  foo

  constructor() {
    this.foo = 123
  }
}
Object.freeze(MySensor.prototype)

export default function main() {
  const sensor = new MySensor()
  trace("done\n")
}

manifest.json:

{
  "include": [
    "$(MODDABLE)/examples/manifest_base.json",
    "$(MODDABLE)/examples/manifest_typings.json",
    "$(MODDABLE)/modules/io/manifest.json"
  ],
  "modules": {
    "*": ["./*.ts"]
  },
  "preload": ["main"]
}

[phoddie]

XS supports private fields in a preloaded class. You can confirm that by changing the externs of your main.ts to ".js" and doing a clean build.

What's going on? The error reported is about an immutable WeakMaps. XS does not implement private fields using WeakMaps. And your code doesn't use a WeakMap directly. But TypeScript emulates private fields using them. Here's the code output by the TypeScript compiler for your project:

var __classPrivateFieldSet = (this && this.__classPrivateFieldSet) || function (receiver, state, value, kind, f) {
    if (kind === "m") throw new TypeError("Private method is not writable");
    if (kind === "a" && !f) throw new TypeError("Private accessor was defined without a setter");
    if (typeof state === "function" ? receiver !== state || !f : !state.has(receiver)) throw new TypeError("Cannot write private member to an object whose class did not declare it");
    return (kind === "a" ? f.call(receiver, value) : f ? f.value = value : state.set(receiver, value)), value;
};
var _MySensor_foo;
class MySensor {
    constructor() {
        _MySensor_foo.set(this, 123);
        __classPrivateFieldSet(this, _MySensor_foo, 123, "f");
    }
}
_MySensor_foo = new WeakMap();
// Object.freeze(MySensor.prototype)
export default function main() {
    const sensor = new MySensor();
    trace("done\n");
}
//# sourceMappingURL=main.js.map

The solution is to get TypeScript to use native engine support for private fields. I'm sure that's some adjustment to the tsconfig.json. I tried bumping ahead to ES2022, but that didn't help. Maybe you know how?

[tve]

Thanks for checking this out!

[phoddie]

Good news: it is enough to update to ES2022. I just did it wrong the first time.

Try updating mcmanifest.js:

	generate(tool) {
		let json = {
			compilerOptions: {
				baseUrl: "./",
				forceConsistentCasingInFileNames: true,
				module: "es2022",
				outDir: tool.modulesPath,
				paths: {
				},
				lib: ["es2022"],
				sourceMap: true,
				target: "ES2022",
				...tool.typescript.tsconfig?.compilerOptions
			},

And rebuild tools so that is used. I'm not sure if ES2023 is supported yet by tsc.

[tve]

yup, thanks! just came to the same conclusion...
thoughts about updating mcmanifest.js in the release?

[phoddie]

thoughts about updating mcmanifest.js in the release

Already committed. Will roll out later in the week.