Readonly Sets attempt

library-consumer.mjs

import { languagesSet } from "./some-library.mjs";

// Allowed
languagesSet.keys();
languagesSet.entries();
languagesSet.forEach(l => console.log);
languagesSet.size;
languagesSet.values();

// Will throw TypeError
languagesSet.clear();
languagesSet.add("ja-JP");
languagesSet.delete("en-US");

// Will throw TypeError but I wish it didn't:
languagesSet.has("en-US");

readonly-set.mjs

export default class ReadonlySet extends Set {
  #mutating = false;
  
  constructor(executor) {
    executor(this.#modify);
    super(undefined, {
      toValue() {
        if (!this.#mutating) {
          throw new TypeError("Cannot modify a ReadonlyMap");
        }
      }
    });
  }
  
  #modify(fn) {
    this.#mutating = true;
    try {
      fn();
    } finally {
      this.#mutating = false;
    }
  }
}

some-library.mjs

import ReadonlySet from "./readonly-set.mjs";

// This library exposes the set which reflects the environment,
// but it doesn't allow mutating the set.

// For this example we'll reflect navigator.languages as a Set.
// Better examples welcome.

export const languagesSet = new ReadonlySet(modify => {
  navigator.addEventListener("languagechange", () => modify(() => {
    languages.clear();
    navigator.languages.forEach(l => languages.add(l));
  });
});