Road to webpack 4 (week 20–21)


Tobias Koppers
Published in
3 min readAug 28, 2017


It has been more than 2 weeks after I installed the auto-closing bot for the webpack/webpack repo. There was a warning period of 15 days. This means bot closed all inactive issue → issue count went from 700 to 500 open issues :)

The process worked really great. Many incorrectly flagged issues where revived in the warning period by watchers.

Reviving is pretty easy as you just have to comment on the issue. The bot will automatically remove the inactive label.

Currently issues with 10 or more 👍 are never flagged inactive.

I guess I will lower the time until inactive to get rid of more issues. You should collect 10 👍 to have long-living issues.

Currently the high number of issue hide the important ones…


The last weeks I did some planning and experimentation with items planned for webpack 4. While webpack 3 didn’t include many breaking changes, webpack 4 probably will.


These are the main driving goals for webpack 4:

  • WASM support
  • New CSS workflow
  • Support for async script tags
  • Performance improvements
  • Optimization for ESM without side-effects
  • Support typings for plugins/hooks

These goals require some breaking changes:

  • New syntax for hooking into plugin hooks
    There will be a new tapable version which also include other improvements.
  • Add a type to modules (JS, ESM, WASM, CSS)
    This requires a lot of refactoring in the Asset Generation.
  • Refactoring of the Runtime code
    This allows different runtime code for different module type
  • Pushing to array instead of pure JSONP
    This allows to load chunks in any order on bootstrap.
  • and more minor breaking changes


WASM Support

This allows to use WASM modules with webpack in a way that make them feel very like ESM.

  • WASM module type
  • Asset Generation for WASM
  • Runtime code for WASM
  • WASM Parser to extract imports/exports


This changes the default way for packaging CSS with webpack. Instead of embedding them into JS, webpack will generate CSS assets by default.

  • CSS module type
  • Asset Generation for CSS
  • Runtime code for CSS
  • CSS Parser to extract imports/exports

Async script tag

This allows to load webpack script with <script async> for better bootstrap performance.

  • Changes to chunk loading runtime code
    wpJSONP = wpJSONP || [];wpJSONP.push(...) instead of wpJSONP(...)
  • Block entry point execution until all required chunks are ready
  • Options to define position to runtime code as it can be in any entry chunk now

Performance improvements

This will improve build performance.

  • Merge outstanding performance PRs
  • Enforce best performance for plugin hooks
  • Refactor more Arrays to Sets

Optimization for ESM without side-effects

This will improve tree shaking of libraries without side-effects.

  • Refactoring for ESM handling to allow more optimizations
  • Add support for "side-effects": false in package.json
  • Reconnect dependencies when side-effect-free modules are involved
  • Remove dependencies to side-effect-free modules when they are not used

Support typings for plugins/hooks

This will allow to write types of plugin hook to improve plugin development.

  • Rewrite tapable to allow typing, promises and improve performance
  • Create backward-compatibility layer to support old code
  • Refactor webpack and internal plugins to use new API

Stable Version

It probably take a while until all these changes are implemented. In this time webpack 3 will be supported as stable version and continue to get bug fixes until webpack 4 is stable enough.


I guess these changes take at least 2 months.

There will be a RC at least 1 month before the Release. This should allow plugin authors to test/upgrade their plugins to the breaking changes.

< week 18–19 week 22–23 >

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