Add ECMAScript 2020 features?

@babel/plugin-proposal-nullish-coalescing-operator and @babel/plugin-proposal-optional-chaining were just added to @babel/preset-env with the release of 7.8.0 — should they also make an appearance here?

[nicolo-ribaudo]

Class fields are stage 3, and shipping in browsers, which is a more important milestone for Babel imo than the stage.

preset-env has the shippedProposals option, for proposals which are not part of the specification yet but are already shipped in at least one browser/node. We should probably add class-fields to that option.

@developit Looking at your plugin, I think that you misinterpreted the reason for using the optimized [[Set]] in the plugin. Object.defineProperty is unrelated to this.bar = 2 in the constructor: it’s needed when there is a setter on the prototype of the class (almost always coming from a superclass or a subclass, but could also come from Object.prototype or from the class itself) because the setter shouldn’t be triggered. This isn’t statically analyzable, because almost always the base and derived classes are in two different files.

Also, the descriptor you pass to defineProperty must have the enumerable: true, configurable: true, writable: true properties. If you replicate this descriptor a bunch of times (for different properties), then it’s longer than using the helper.

I don’t think that it’s possible to make a spec-compliant version of the plugin smaller than the current one, and if someone isn’t concerned with spec compliancy in the [[Set]] vs [[Define]] case they can already enable loose mode, which produces this output:

class A extends B {
  x = 2;
}

// ->

class A extends B {
  constructor(...args) {
    super(...args);
    this.x = 2;
  }
}

[ljharb]

Optional chaining and nullish coalescing are standard / stage 4.

Class fields are stage 3, and shipping in browsers, which is a more important milestone for Babel imo than the stage.