Big changes

• Node.js 4 is no longer supported. Source Code was upgraded to a higher ecmascript version.
• You have to choose (mode or --mode) between two modes now: production or development
  • production enables all kind of optimizations to generate optimized bundles
  • development enables comments and hint for development and enables the eval devtool
    • WIP: addition hints in development mode
  • production doesn't support watching, development is optimized for fast incremental rebuilds
  • production also enables module concatenating (Scope Hoisting)
  • You can configure this in detail with the flags in optimization.* (build your custom mode)
  • process.env.NODE_ENV are set to production or development (only in build code, not in config)
  • There is a hidden none mode which disables everything
• import() returns always a namespace object. CommonJS modules are wrapped into the default export
  • This probably breaks your code, if you used to import CommonJs with import()
• You no longer need to use these plugins:
  • NoEmitOnErrorsPlugin -> optimize.noEmitOnErrors (on by default in production mode)
  • ModuleConcatenationPlugin -> optimize.concatenateModules (on by default in production mode)
• webpack now handles JSON different
  • You may need to add type: "javascript/auto" when transforming JSON via loader to JS
  • Just using JSON without loader should still work

Big features

• webpack now supports these module types:
  • javascript/auto: (The default one in webpack 3) Javascript module with all module systems enabled: CommonJS, AMD, ESM
  • javascript/esm: EcmaScript modules, all other module system are not available
  • json: JSON data, JSON data is passed through unchanged and isn't parsed
  • webassembly/experimental: WebAssembly modules (currently experimental)
• javascript/esm handles ESM more strict compared to javascript/auto:
  • Imported names need to exist on imported module
  • Non-ESM can only imported via default import, everything else (including namespace import) emit errors
• In .mjs modules
  • javascript/esm is used
  • imports need to have an extension. No extensions are tried.
• sideEffects: false is now supported in package.json
• Instead of a JSONP function a JSONP array is used -> async support
  • WIP: There is no way to move runtime to another chunk yet
• webpackInclude and webpackExclude are supported by the magic comment for import(). They allow to filter files when using a dynamic expression.
• Resolving can now be configured with module.rules[].resolve. It's merged with the global configuration.
• Sone Plugin options are now validated
  • WIP: Better output, no process exit, stack trace, more plugins
• Multiple performance improvements, especially for faster incremental rebuilds

Features

• Plugins added via CLI are prepended to take priority over config plugins
• Module type is automatically choosen for mjs, json and wasm extensions. Other extensions need to be configured via module.rules[].type
• added loaderContext.rootContext which points to the context options. Loaders may use it to make stuff relative to the application root.
• Error for chunk loading now includes more information and two new properties type and request.
• Incorrect options.dependencies configurations now throw error
• webpacks AST can be passed directly from loader to webpack to avoid extra parsing
• When using more than 25 exports mangled export names are shorter.
• webpack now looks for the .wasm, .mjs, .js and .json extensions in this order
• Sizes are now shown in kiB instead of kB in Stats
• A resource query is supported in context
• output.pathinfo is now on by default in develoment mode
• in-memory caching is now off by default in production
• script tags are no longer text/javascript and async as this are the default values (saves a few bytes)

Bugfixes

• Generated comments no longer break on */
• webpack no longer modifies the passed options object
• Compiler "watch-run" hook now has the Compiler as first parameter
• add chunkCallbackName to the schema to allow configurating WebWorker template

Removed features

• removed module.loaders
• removed loaderContext.options
• removed Compilation.notCacheable flag
• removed NoErrorsPlugin
• removed Dependency.isEqualResource
• removed NewWatchingPlugin

Breaking changes for plugins

• new plugin system
  • plugin method is backward-compatible
  • Plugins should use Compiler.hooks.xxx.tap(<plugin name>, fn) now
• New major version of enhanced-resolve
• Templates for chunks may now generate multiple assets
• Chunk.chunks/parents/blocks are no longer Arrays. A Set is used internally and there are methods to access it.
• Parser.scope.renames and Parser.scope.definitions are no longer Objects/Arrays, but Map/Sets.
• Parser uses StackedSetMap (LevelDB-like datastructure) instead of Arrays
• Compiler.options is no longer set while applying plugins
• Harmony Dependencies has changed because of refactoring
• Dependency.getReference() may now return a weak property. Dependency.weak is now used by the Dependency base class and returned in the base impl of getReference()
• Constructor arguments changed for all Modules
• Merged options into options object for ContextModule and resolveDependencies
• Changed and renamed dependencies for `import()
• Moved Compiler.resolvers into Compiler.resolverFactory accessible with plugins
• Dependency.isEqualResource has been replaced with Dependency.getResourceIdentifier

Incompatible plugins

• html-webpack-plugin: Plugin system upgrade (webpack v4 migration plan jantimon/html-webpack-plugin#816)

Please comment

Incompatible loaders

• file-loader -> Workaround A
• vue-loader -> Workaround A
• ejs-loader -> Workaround A
• eslint-loader -> Workaround A
• postcss-loader

Please comment

Broken features

Please comment

Workarounds:

Workround A

For loaders not migrated away from using this.options:

new LoaderOptionsPlugin({
  options: {
    context: process.cwd() // or the same value as `context`
  }
})

Don't expect this alpha release to be super stable yet...

Please comment if you find additional changes not in the changelog