simple-immutable.js

class Immutable {
  constructor(of) {
    Object.keys(of).forEach(key => Object.defineProperty(this, key, { enumerable: true, get: () => this.of[key] }));
    Object.defineProperty(this, "of", { enumerable: false, get: () => of });
  }

  set(key, value) {
    return new this.constructor({ ...this.of, [key]: value });
  }
}

class Foo extends Immutable {
  constructor(obj) {
    super(obj);

    this.fooClass = true;
  }

  setFoo(val) {
    return super.set("foo", val);
  }
}