Class Fields (Stage 3)

[babel-bot]

Original issue submitted by @babel-bot in babel/babel#4408

Champions: @jeffmo (public class fields) @littledan (private + combined)
Spec Repo: https://github.com/tc39/proposal-class-fields
Spec Text: https://tc39.github.io/proposal-class-fields/
Slides: https://drive.google.com/file/d/0B-TAClBGyqSxWHpyYmg2UnRHc28/view

Moved to Stage 3 at the July 2017 meeting: https://github.com/tc39/agendas/blob/master/2017/07.md (https://twitter.com/lbljeffmo/status/890679542007738368)

Examples

class C {
  static x = 1, #y, [a];
  z, #w = 2, [b];
  
  a() {
    this.z;
    this.#w++;
    #w; // #w is this.#w
  }
}

Parsing/ESTree

• AST handled already in current classProperties and classPrivateProperties.
• May need to rename though (new plugin name).
• make sure no "private computed" parse
• need to support comma separated properties.

Transform

• combine the class-properties plugin + tbd private properties one

Contacts

• #proposal-class-fields on Slack!
• @littledan
• @diervo (dval on Slack)
• @Qantas94Heavy

[WebReflection]

dunno how much of an help this is, but I've been experimenting already with this and my conclusion is that, to represent the current proposal, each class needs it's own WeakMap.

Example

source

class A {
  #value = 1;
  valueOf() {
    return this.#value;
  }
}

class B extends A {
  #value = 2;
  toString() {
    return String(this.#value);
  }
}

pseudo target

const privatesA = new WeakMap;
function A() {
  privatesA.set(this, {__proto__: null, value: 1});
}
Object.defineProperties(
  A.prototype,
  {
    valueOf: {
      configurable: true,
      writable: true,
      value: function () {
        return privatesA.get(this).value;
      }
    }
  }
);

const privatesB = new WeakMap;
function B() {
  A.call(this);
  privatesB.set(this, {__proto__: null, value: 2});
}
Object.defineProperties(
  Object.setPrototypeOf(
    Object.setPrototypeOf(B, A).prototype,
    A.prototype
  ),
  {
    constructor: {
      configurable: true,
      writable: true,
      value: B
    },
    toString: {
      configurable: true,
      writable: true,
      value: function () {
        return String(privatesB.get(this).value);
      }
    }
  }
);

in that way new B().valueOf() would be 1 and new B().toString() would be "2" + no internal property ever leaks through symbols.

[ljharb]

Isn't that only the case if "value" is transformed to a string? I'd think you could use an in-scope constant - like a symbol or an object literal - and then it'd be unique and you could share one WeakMap for the entire file.

[WebReflection]

I'm not sure I am following you ... the example has nothing to do with the toString or valueOf case, it's a proof of concept of the implementation details and nothing else: WeakMap is the answer to this problem. It scales, it's not the best for performance reasons, yet is the most reliable.

[ljharb]

I understand that you must use a WeakMap. I'm responding to "each class needs it's own WeakMap" - it seems like both class A and B could share the same WeakMap, since each "value" is a distinct PrivateName.