Big changes

• Environment
  • Node.js 4 is no longer supported. Source Code was upgraded to a higher ecmascript version.
• Usage
  • 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
    • 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 built code, not in config)
    • There is a hidden none mode which disables everything
• Syntax
  • import() always returns a namespace object. CommonJS modules are wrapped into the default export
    • This probably breaks your code, if you used to import CommonJs with import()
• Configuration
  • You no longer need to use these plugins:
    • NoEmitOnErrorsPlugin -> optimization.noEmitOnErrors (on by default in production mode)
    • ModuleConcatenationPlugin -> optimization.concatenateModules (on by default in production mode)
    • NamedModulesPlugin -> optimization.namedModules (on by default in develoment mode)
  • CommonsChunkPlugin was removed -> optimization.splitChunks, optimization.runtimeChunk
• JSON
  • webpack now handles JSON natively
    • You may need to add type: "javascript/auto" when transforming JSON via loader to JS
    • Just using JSON without loader should still work
  • allows to import JSON via ESM syntax
    • unused exports elimination for JSON modules
• Optimization
  • Upgrade uglifyjs-webpack-plugin to v1
    • ES15 support

Big features

• Modules
  • 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
    • javascript/dynamic: Only CommonJS and, EcmaScript modules are not available
    • json: JSON data, it's available via require and import
    • webassembly/experimental: WebAssembly modules (currently experimental)
  • javascript/esm handles ESM more strictly compared to javascript/auto:
    • Imported names need to exist on imported module
    • Dynamic modules (non-esm, i. e. CommonJs) can only imported via default import, everything else (including namespace import) emit errors
  • In .mjs modules are javascript/esm by default
  • WebAssembly modules
    • can import other modules (JS and WASM)
    • Exports from WebAssembly modules are validated by ESM import
      • You'll get a warning/error when trying to import a non-existing export from WASM
    • can only be used in async chunks. They doesn't work in initial chunks (would be bad for web performance)
      • Import modules using WASM via import()
    • This is an experimental feature and subject of change
• Optimization
  • sideEffects: false is now supported in package.json
    • sideEffects in package.json also supports glob expressions and arrays of glob expressions
  • Instead of a JSONP function a JSONP array is used -> async script tag support, order no longer matter
  • New optimization.splitChunks option was introduced
    Details: gist.github.com/sokra/1522d…
  • Dead branches are now removed by webpack itself
    • Before: Uglify removed the dead code
    • Now: webpack removes the dead code (in some cases)
    • This prevents crashing when import() occur in a dead branch
• Syntax
  • webpackInclude and webpackExclude are supported by the magic comment for import(). They allow to filter files when using a dynamic expression.
  • Using System.import() now emits a warning
    • You can disable the warning with Rule.parser.system: true
    • You can disable System.import with Rule.parser.system: false
• Configuration
  • Resolving can now be configured with module.rules[].resolve. It's merged with the global configuration.
  • optimization.minimize has been added to switch minimizing on/off
    • By default: on in production mode, off in development mode
  • optimization.minimizer has been added to configurate minimizers and options
• Usage
  • Some Plugin options are now validated
  • CLI has been move to webpack-cli, you need to install webpack-cli to use the CLI
  • The ProgressPlugin (--progress) now displays plugin names
    • At least for plugins migrated to the new plugin system
• Performance
  • UglifyJs now caches and parallizes by default
  • Multiple performance improvements, especially for faster incremental rebuilds
  • performance improvement for RemoveParentModulesPlugin
• Stats
  • Stats can display modules nested in concatenated modules

Features

• Configuration
  • Module type is automatically choosen for mjs, json and wasm extensions. Other extensions need to be configured via module.rules[].type
  • Incorrect options.dependencies configurations now throw error
  • sideEffects can be overriden via module.rules
  • output.hashFunction can now be a Constructor to a custom hash function
    • You can provide a non-cryto hash function for performance reasons
  • add output.globalObject config option to allow to choose the global object reference in runtime exitCode
• Runtime
  • Error for chunk loading now includes more information and two new properties type and request.
• Devtool
  • remove comment footer from SourceMaps and eval
  • add support for include test and exclude to the eval source map devtool plugin
• Performance
  • webpacks AST can be passed directly from loader to webpack to avoid extra parsing
  • Unused modules are no longer unnecessarly concatenated
  • Add a ProfilingPlugin which write a (Chrome) profile file which includes timings of plugins
  • Migrate to using for of instead of forEach
  • Migrate to using Map and Set instead of Objects
  • Migrate to using includes instead of indexOf
  • Replaced some RegExp with string methods
  • Queue don't enqueues the same job twice
  • Use faster md4 hash for hashing by default
• Optimization
  • When using more than 25 exports mangled export names are shorter.
  • script tags are no longer text/javascript and async as this are the default values (saves a few bytes)
  • The concatenated module now generates a bit less code
  • constant replacements now don't need __webpack_require__ and argument is omitted
• Defaults
  • webpack now looks for the .wasm, .mjs, .js and .json extensions in this order
  • output.pathinfo is now on by default in develoment mode
  • in-memory caching is now off by default in production
  • entry defaults to ./src
  • output.path defaults to ./dist
  • Use production defaults when omiting the mode option
• Usage
  • Add detailed progress reporting to SourceMapDevToolPlugin
  • removed plugins now give a useful error message
• Stats
  • Sizes are now shown in kiB instead of kB in Stats
  • entrypoints are now shows by default in Stats
  • chunks now display <{parents}> >{children}< and ={siblings}= in Stats
  • add buildAt time to stats
  • stats json now includes the output path
• Syntax
  • A resource query is supported in context
  • Referencing entry point name in import() now emits a error instead of a warning
  • Upgraded to acorn 5 and support ES 2018
• Plugins
  • done is now an async hook

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 output.chunkCallbackName to the schema to allow configurating WebWorker template
• Using module.id/loaded now correctly bails out of Module Concatentation (Scope Hoisting)
• OccurenceOrderPlugin now sorts modules in correct order (instead of reversed)
• timestamps for files are read from watcher when calling Watching.invalidate
• fix incorrect -! behavior with post loaders
• add run and watchRun hooks for MultiCompiler
• this is now undefined in ESM
• VariableDeclaration are correctly identified as var, const or let
• Parser now parse the source code with the correct source type (module/script) when the module type javascript/dynamic or javascript/module is used.
• don't crash on missing modules with buildMeta of null
• add original-fs module for electron targets
• HMRPlugin can be added to the Compiler outside of plugins

Internal changes

• Replaced plugin calls with tap calls (new plugin system)
• Migrated many deprecated plugins to new plugin system API
• added buildMeta.exportsType: "default" for json modules
• Remove unused methods from Parser (parserStringArray, parserCalculatedStringArray)
• Remove ability to clear BasicEvaluatedExpression and to have multiple types
• Buffer.from instead of new Buffer
• Avoid using forEach and use for of instead
• Use neo-async instead of async
• Update tapable and enhanced-resolve dependencies to new major versions
• Use prettier

Removed features

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

Breaking changes for plugins/loaders

• 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
• Methods on Template are now static
• A new RuntimeTemplate class has been added and outputOptions and requestShortener has been moved to this class
  • Many methods has been updated to use the RuntimeTemplate instead
  • We plan to move code which accesses the runtime to this new class
• Module.meta has been replaced with Module.buildMeta
• Module.buildInfo and Module.factoryMeta have been added
• Some properties of Module have been moved into the new objects
• added loaderContext.rootContext which points to the context options. Loaders may use it to make stuff relative to the application root.
• add this.hot flag to loader context when HMR is enabled
• buildMeta.harmony has been replaced with buildMeta.exportsType: "namespace
• The chunk graph has changed:
  • Before: Chunks were connected with parent-child-relationships.
  • Now: ChunkGroups are connected with parent-child-relationships. ChunkGroups contain Chunks in order.
  • Before: AsyncDependenciesBlocks reference a list of Chunks in order.
  • Now: AsyncDependenciesBlocks reference a single ChunkGroup.
• file/contextTimestamps are Maps now
• map/foreach Chunks/Modules/Parents methods are now deprecated/removed
• NormalModule accept options object in Constructor
• Added required generator argument for NormalModule
• Added createGenerator and generator hooks for NormalModuleFactory to customize code generation
• Allow to customize render manifest for Chunks via hooks