July 2017

[hzoo]

Just summarizing the updates from: https://github.com/tc39/agendas/blob/master/2017/07.md

Can discuss changes as they pertain to Babel implementation (syntax).
babel-polyfill implementation is https://github.com/zloirock/core-js

EDIT: made some issues for Babel updates

New Proposals

• Binary AST 🙌 : Stage 1
• Optional Catch Binding try {} catch {} : Stage 3
  • Add optional catch binding to stage 3 preset babel#6032
• Throw Expressions throw new Error(''): Stage 1

Moving Forward

• BigInt 10n: Stage 2 to Stage 3
  • BigInt (Stage 3) [WIP] babel#6015 (we're not going to put in a stage preset given perf issues)
• Numeric Separators 1_000: Stage 1 to Stage 2
  • Add numeric separator to stage 2 preset babel#6071
• export-ns-from export * as ns from 'mod': Stage 1 to Stage 2 (also "consensus PR")
  • Split the export-extensions plugin into two babel#6075
• Class Fields class A { #a = 1; b = 2 }: Stage 2 to Stage 3
  • Update Class Fields to Stage 3 and change default behavior babel#6073

Needs work

• export-default-from export default from 'mod': Stayed at Stage 1
• Decorators @a class A{}: Stayed at Stage 2
• Optional Chaining a?.b : Stayed at Stage 1

[hzoo]

Maybe we add here (and/or in each issue) the changes we need to make in each transform? Obviously if it's a new proposal we may need to implement something, and for proposals that progressed change the preset it's in and maybe some updates.

[littledan]

Private methods were introduced.

Decorators stayed at Stage 2 but with significant changes to semantics, for integration with fields and private, which the committee seemed mostly happy with except for the private part.

Optional chaining also has new draft semantics, but the committee seemed very sceptical of both the old and the new semantics.

Export-ns-from will be pursued as a needs-consensus PR, rather than with stages. For Babel purposes, it might make sense to put at Stage 3.

[littledan]

I believe Promise.prototype.finally reached Stage 3 as well. The semantics have changed slightly recently.

For core-js, I think the big need is BigInt, a polyfill. The rest are Babel/Bablyon changes.

[sricc]

I've never contributed to this project before but am interested in learning and helping where I can. Is there something that would be better suited for me to take a look at?

[hzoo]

• Class fields: The transform should be moved from the Stage 2 preset to the Stage 3 preset. And we should make the current "spec" mode the normal mode, and the normal mode the "loose" option. This is since the current spec requires using defineProperty instead of a simple assignment, and that class A { a; } should compile into a = undefined; Update Class Fields to Stage 3 and change default behavior babel#6073

[jridgewell]

@sricc: You could check out any of the beginner-friendly labelled issues: https://github.com/babel/babel/issues?q=is%3Aopen+is%3Aissue+label%3Abeginner-friendly

[sricc]

@jridgewell Thanks, will check those out.

[Andarist]

current spec requires using defineProperty instead of a simple assignment

@hzoo why defineProperty is required now? Im trying to see how they differ, if all descriptor options are set to true

[hzoo]

@Andarist some links babel/babel#4544, babel/babel#5056, babel/babel#6076, babel/babel#6108, https://twitter.com/BrendanEich/status/778342783031521280

@littledan can explain better ^

[littledan]

We had a long debate about = vs defineProperty in TC39. The main way they differ, besides setting the descriptor options to true, is in calling setters: If a superclass defines a setter, = will call that setter. This can be good or bad, depending on what you expect for your code. Lots of people had different intuitions here, and we settled on one of the two, with the tiebreaker being that your code is more locally predictable and understandable if those setters aren't called.

[Andarist]

Totally makes sense and I believe you have chosen wisely ;) Always forgetting about getters/setters when thinking about the code. Thanks for the clarification.

[hzoo]

I think we finished everything (or is in progress) ^ as of https://github.com/babel/babel/releases/tag/v7.0.0-beta.0, and we made an issue for throw expressions.

WIP PRs

• BigInt (Stage 3) [WIP] babel#6015 bigint
• Class private properties babel#6120 private props
• Decorators 2 Transform [WIP] babel#6107 decorators

[hzoo]

Ok closing since we implemented everything here, and next meeting issue is up #28