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A (webpack) plugin is a object that has an apply method with one parameter, the compiler. I. e. a function is a plugin, because the function prototype defines a apply method. But you also can define a “class” with the apply method.

Many objects in webpack extends the Tapable class, which mean they expose a plugin method, which plugins can use to bind custom stuff.

I. e. the Compiler expose the "compile" plugin interface, which is called when the Compiler compiles

compiler.plugin("compile", function(params) {
    // Just print a text

A plugin only gets a reference to the compiler object, so if it want to plugin stuff into other webpack objects it have to gain access to them. I. e. the Compilation object:

compiler.plugin("compilation", function(compilation) {
    compilation.plugin("optimize", function() {
        console.log("The compilation is now optimizing your stuff");

interface types

There are multiple types of plugin interfaces.

  • Timing

    • sync (default): As seen above. Use return.
    • async: Last parameter is a callback. Signature: function(err, result)
    • parallel: The handlers are invoked parallel (async).
  • Return value

    • not bailing (default): No return value.
    • bailing: The handlers are invoked in order until one handler return something.
    • parallel bailing: The handlers are invoked parallel (async). The first (by order) returned value is significant.
    • waterfall: Each handler get the result value of the last handler as argument.

Compiler instance

run(compiler: Compiler) async

The run method of the Compiler is used to start a compilation. This is not called in watch mode.

watch-run(watching: Watching) async

The watch method of the Compiler is used to start a watching compilation. This is not called in normal mode.

compilation(c: Compilation, params: Object)

A Compilation is created. A plugin can use this to obtain a reference to the Compilation object. The params object contains useful references.

normal-module-factory(nmf: NormalModuleFactory)

A NormalModuleFactory is created. A plugin can use this to obtain a reference to the NormalModuleFactory object.

context-module-factory(cmf: ContextModuleFactory)

A ContextModuleFactory is created. A plugin can use this to obtain a reference to the ContextModuleFactory object.


The Compiler starts compiling. This is used in normal and watch mode. Plugins can use this point to modify the params object (i. e. to decorate the factories).

make(c: Compilation) parallel

Plugins can use this point to add entries to the compilation or prefetch modules. They can do this by calling addEntry(context, entry, name, callback) or prefetch(context, dependency, callback) on the Compilation.

after-compile(c: Compilation) async

The compile process is finished and the modules are sealed. The next step is to emit the generated stuff. Here modules can use the results in some cool ways.

The handlers are not copied to child compilers.

emit(c: Compilation) async

The Compiler begins with emitting the generated assets. Here plugins have the last chance to add assets to the c.assets array.

after-emit(c: Compilation) async

The Compiler has emitted all assets.

done(stats: Stats)

All is done.

failed(err: Error)

The Compiler is in watch mode and a compilation has failed hard.


The Compiler is in watch mode and a file change is detected. The compilation will be begin shortly (options.watchDelay).


All plugins extracted from the options object are added to the compiler.


All plugins extracted from the options object are added to the resolvers.


before-resolve(data) async waterfall

Before the factory starts resolving. The data object has this properties:

  • context The absolute path of the directory for resolving.
  • request The request of expression.

Plugins are allowed to modify the object or pass a new similar object to the callback.

after-resolve(data) async waterfall

After the factory has resolved the request. The data object has this properties:

  • request The resolved request. It acts as identifier for the NormalModule.
  • userRequest The request the user entered. It’s resolved, but do not contain pre or post loaders.
  • rawRequest The unresolved request.
  • loaders A array of resolved loaders. This is passed to the NormalModule and they will be executed.
  • resource The resource. It will be loaded by the NormalModule.
  • parser The parser that will be used by the NormalModule.


before-resolve(data) async waterfall

after-resolve(data) async waterfall

alternatives(options: Array) async waterfall

Compilation instance


The sealing to the compilation has started.


Optimize the compilation.

optimize-tree(chunks, modules) async

Async optimize of the tree.

optimize-modules(modules: Module[])

Optimize the modules.

after-optimize-modules(modules: Module[])

Optimizing the modules has finished.

optimize-chunks(chunks: Chunk[])

Optimize the chunks.

after-optimize-chunks(chunks: Chunk[])

Optimizing the chunks has finished.

revive-modules(modules: Module[], records)

Restore module info from records.

optimize-module-order(modules: Module[])

Sort the modules in order of importance. The first is the most important module. It will get the smallest id.

optimize-module-ids(modules: Module[])

Optimize the module ids.

after-optimize-module-ids(modules: Module[])

Optimizing the module ids has finished.

record-modules(modules: Module[], records)

Store module info to the records.

revive-chunks(chunks: Chunk[], records)

Restore chunk info from records.

optimize-chunk-order(chunks: Chunk[])

Sort the chunks in order of importance. The first is the most important chunk. It will get the smallest id.

optimize-chunk-ids(chunks: Chunk[])

Optimize the chunk ids.

after-optimize-chunk-ids(chunks: Chunk[])

Optimizing the chunk ids has finished.

record-chunks(chunks: Chunk[], records)

Store chunk info to the records.


Before the compilation is hashed.


After the compilation is hashed.


Before creating the chunk assets.

additional-chunk-assets(chunks: Chunk[])

Create additional assets for the chunks.

record(compilation, records)

Store info about the compilation to the records

optimize-chunk-assets(chunks: Chunk[]) async

Optimize the assets for the chunks.

The assets are stored in this.assets, but not all of them are chunk assets. A Chunk has a property files which points to all files created by this chunk. The additional chunk assets are stored in this.additionalChunkAssets.

after-optimize-chunk-assets(chunks: Chunk[])

The chunk assets has been optimized.

optimize-assets(assets: Object{name: Source}) async

Optimize all assets.

The assets are stored in this.assets.

after-optimize-assets(assets: Object{name: Source})

The assets has been optimized.


Before a module build has started.


A module has been build successfully.


The module build has been failed.

module-asset(module, filename)

An assets from a module was added to the compilation.

chunk-asset(chunk, filename)

An assets from a chunk was added to the compilation.

Parser instance (compiler.parser)

program(ast) bailing

General purpose plugin interface for the while AST of a code fragment.

statement(statement: Statement) bailing

General purpose plugin interface for every statement of the code fragment.

call <identifier>(expr: Expression) bailing

abc(1) => call abc

a.b.c(1) => call a.b.c

expression <identifier>(expr: Expression) bailing

abc => expression abc

a.b.c => expression a.b.c

expression ?:(expr: Expression) bailing

(abc ? 1 : 2) => expression ?!

Return a boolean value to omit parsing of the wrong path.

typeof <identifier>(expr: Expression) bailing

typeof a.b.c => typeof a.b.c

statement if(statement: Statement) bailing

if(abc) {} => statement if

Return a boolean value to omit parsing of the wrong path.

label <labelname>(statement: Statement) bailing

xyz: abc => label xyz

var <name>(statement: Statement) bailing

var abc, def => var abc + var def

Return false to not add the variable to the known definitions.

evaluate <expression type>(expr: Expression) bailing

Evaluate an expression.

evaluate typeof <identifier>(expr: Expression) bailing

Evaluate the type of an identifier.

evaluate Identifier <identifier>(expr: Expression) bailing

Evaluate a identifier that is a free var.

evaluate defined Identifier <identifier>(expr: Expression) bailing

Evaluate a identifier that is a defined var.

evaluate CallExpression .<property>(expr: Expression) bailing

Evaluate a call to a member function of a successfully evaluated expression.


  • compiler.resolvers.normal Resolver for a normal module
  • compiler.resolvers.context Resolver for a context module
  • compiler.resolvers.loader Resolver for a loader

Any plugin should use this.fileSystem as fileSystem, as it’s cached. It only has async named functions, but they may behave sync, if the user uses a sync file system implementation (i. e. in enhanced-require).

To join paths any plugin should use this.join. It normalize the paths. There is a this.normalize too.

A bailing async forEach implementation is available on this.forEachBail(array, iterator, callback).

To pass the request to other resolving plugins use the this.doResolve(types: String|String[], request: Request, callback) method. types are multiple possible request types that are tested in preference of order.

interface Request {
    path: String // The current directory of the request
    request: String // The current request string
    query: String // The querystring of the request, if any
    module: boolean // request begins with a module
    directory: boolean // The request points to a directory
    file: boolean // The request points to a file
    resolved: boolean // The request is resolved/done
    // undefined means false for boolean fields

// Examples
// from /home/user/project/file.js: require("../test?charset=ascii")
    path: "/home/user/project",
    request: "../test",
    query: "?charset=ascii"
// from /home/user/project/file.js: require("test/test/")
    path: "/home/user/project",
    request: "test/test/",
    module: true,
    directory: true

resolve(context: String, request: String)

Before the resolving process starts.

resolve-step(types: String[], request: Request)

Before a single step in the resolving process starts.

module(request: Request) async waterfall

A module request is found and should be resolved

directory(request: Request) async waterfall

A directory request is found and should be resolved

file(request: Request) async waterfall

A file request is found and should be resolved

The plugins may offer more extensions points

Here is a list what the default plugins in webpack offer. They are all (request: Request) async waterfall

The process for normal modules and contexts is module -> module-module -> directory -> file

The process for loaders is module -> module-loader-module -> module-module -> directory -> file


A module should be looked up in a specified directory. path contains the directory.

module-loader-module (only for loaders)

Used before module templates are applied to the module name. The process continues with module-module.

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