How to Enumerate Private JavaScript Class Fields

Question

How do we enumerate through private class fields?

class Person {
  #isFoo = true;
  #isBar = false;

  constructor(first, last) {
    this.firstName = first;
    this.lastName = last;
  }

  enumerateSelf() {
    console.log(this);
    // (pub/priv fields shown)

    // enumerate through instance fields
    for (let key in this) {
      console.log(key)
      // (only public fields shown)
    }

    // How do we enumerate/loop through private fields too?
  }
}

new Person('J', 'Doe').enumerateSelf();

Answer 1

It's not possible. They're private fields, and there is no enumeration method for them. Only the class declaration statically knows which ones were declared. They're not properties, there's not even a language value representing a private name, you cannot access them dynamically (like with bracket notation).

The best you'll get is

enumerateSelf() {
    console.log(this);
    for (let key in this) {
        console.log("public", key, this[key]);
    }
    console.log("private isFoo", this.#isFoo);
    console.log("private isBar", this.#isBar);
}

There was an open issue in the private-fields proposal about "Private field iteration", however one of the first comments by a TC39 member states "Private fields are not properties. You can't reflect on them by design.".

Answer 2

Maybe not an elegant solution but perhaps you can modify your structure to do something like:

class Person {
  #properties = {
      isFoo: true,
      isBar: false
  };

  constructor(first, last) {
    this.firstName = first;
    this.lastName = last;
  }

  enumeratePrivateSelf() {
    // enumerate through private fields
    for (let key in this.#properties) {
      console.log(key)
      // (only public fields shown)
    }
  }
}