Test Against test262

The parser already does this, but we don’t do it for transforms.

We should run against test262 (the JavaScript test suite). This is made easier with test262-harness. In addition, it would be great to have it run in test262-report, as announced by Bocoup.

This will give us a few things:

  • Better confidence in Babel’s output. More tests/coverage is better for catching regressions and finding spec bugs worth working on.
  • Can use these to help do a reverse test against any kind of "loose" mode. We can purposely fail a test when that option intends to not adhere to the spec.
  • Can use this new data instead of compat-table for @babel/preset-env.

Polyfill behavior

This is regarding https://github.com/babel/babel/tree/master/packages/babel-preset-env#usebuiltins-usage

  • Allow any substitute polyfill instead of core-js. You should be able to override anything (custom Promise, etc)
  • Make "usage" option the default after it is stable.

Build/publish workflow

Codemods for TC39 Proposals

Lebab/others are already used to convert from ES5 -> ESNext, so incorporate it into Babel itself.

  • Refactor Lebab as Babel transforms (can keep the cli since it’s a separate tool)
    • Usecase: ES3 -> ES6+ (on source code)
    • Usecase: Remove usage of dropped proposals
    • Usecase: Auto upgrade to the latest version of a proposal spec (if possible)
    • Can we somehow combine forces in: babel-codemod/jscodeshift/lebab, prettier/recast/babel-generator? I really don’t want to update all of these: new syntax equals re-writing the printer in all of these places separately/out of sync.

Increasing the quality of community plugins

  • Work with the community to create guides on how to write plugins or understand ASTs, etc
  • Analysis of API’s/syntax used (Google BigQuery)
  • Have #blessed/sanctioned/curated packages according to some standard
    • Can use for smoke tests
    • Official testing package
    • Certain level of coverage, downloads, etc
    • Create a scoped namespace on github/npm for these? like webpack-contrib
    • Can enforce linting rules on apis?
    • Makes ecosystem changes easier if can notify and upgrade these plugins beforehand
    • Create a set of standard tests to verify against (handles all syntax)
    • Documented/tested set of options


  • allow custom version of babel-standalone (same as REPL to allow per PR tests)
  • integrate with repl? (both are in react)
  • auto publish a plugin to npm?
  • create tests?
  • hook into codesandbox?
    • *

Js中文网 – 前端进阶资源教程 www.javascriptC.com ,Babel中文文档


Performance table for ES6+ (used in babel-preset-env under new option e.g perf)

Add a new option in preset-env: will continue to compile if the native support is slower. Can setup a threshold, may need to compare against the size difference.

  • Use six-speed repo as a base, needs to apply for ES6 and proposals
  • Need continued maintainence

Compiled Output Stats

#5340 Can show the code size before and after compiling to give a sense of compiled output. Could create suggestions like using "loose" mode or not compiling, etc.

  • The REPL just added a before/after code-size
  • Maybe difficult to do per transform

Async Transforms

Support having async plugins. Will require it to be opt-in and for all other plugins to be async.

Plugin Ordering

  • Add before/after keys for plugins so they can specify order.
  • Possibily implement related plugins in the same "plugin" but just expose a flag out to the end-user.

Babel Helpers (shared code)

Repurpose babel / create babel-init

We can re-use the babel package for a more simplified 0 config™ version

Not sure what this looks like but had this idea for a really long time and didn’t really get anywhere – the cli version could just be https://github.com/developit/microbundle for libs? Maybe webpack/parcel would have it covered for apps?


Smoke Tests

  • Babel itself https://github.com/babel/babel/issues/6134
  • Important community plugins (babel-plugin-instanbul, css-in-js)
  • babel-core integrations (wrappers like babel-loader, test frameworks, etc)
  • Libraries ((p)react, redux, vue)
  • Tools that use individual packages (debugger.html, prepack)
  • OSS Apps (nylas-mail)

Better Debuggability

  • Translations
  • Real documentation on APIs
  • Up to date babel-handbook/merge into rehauled website
  • Link to common errors pages
Better REPL
  • Dropdown examples/examples of syntax from github?
  • Import any package from npm (can give test examples for 3rd party plugins, debugging issues)
  • Run any plugin from npm
  • Create a plugin from the repl (can we merge it with ASTExplorer/codesandbox?), even publish, run from URL?
  • Import/Export a config file
  • Combine ^ with the ability to run the version of Babel in a PR/master.
  • Use plugin’s tests in the repo as "examples" for docs.


#43 Rewrite it with probot

A bot is really useful to do github/maintainer activities automatically.

We haven’t updated this in a long time due to Andrew being busy and not setting up the automatic infra on AWS. Switching will make updating actually real so we can add some new features which would save some headache.

References: https://github.com/eslint/eslint-github-bot, https://github.com/open-bot/open-bot

Expanded Maintainer Guide

Better onboarding/contributing guide

  • Guide to different aspects of contributing with real examples to issues/PRs


  • Promote Open Collective, talking with companies about office hours, sponsorship, contracting?
  • Mentoring: Google Summer of Code/Rails Girls Summer of Code were great but was hard to keep up with volunteeers and I felt like we could be doing a lot more with full time help.
  • Maybe doing local meetups on contributing, or livestreaming development/maintenance work?
  • Should focus on bringing in maintainers that will lower the burden, not increase it even if there is upfront work
    • *

Big Wishlist (might be out of scope/complex/ecosystem)

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