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The Compilation instance extends from the compiler i.e. compiler.compilation. It is the literal compilation of all the objects in the require graph. This object has access to all the modules and their dependencies (most of which are circular references). In the compilation phase, modules are loaded, sealed, optimized, chunked, hashed and restored, etc. This would be the main lifecycle of any operations of the compilation.

compiler.plugin("compilation", function(compilation) {
  // the main compilation instance
  // all subsequent methods are derived from compilation.plugin


The normal module loader, is the function that actually loads all the modules in the module graph (one-by-one).

compilation.plugin('normal-module-loader', function(loaderContext, module) {
  // this is where all the modules are loaded
  // one by one, no dependencies are created yet


The sealing of the compilation has started.

compilation.plugin('seal', function() {
  // you are not accepting any more modules
  // no arguments


Optimize the compilation.

compilation.plugin('optimize', function() {
  // webpack is begining the optimization phase
  // no arguments

optimize-tree(chunks, modules) async

Async optimization of the tree.

compilation.plugin('optimize-tree', function(chunks, modules) {


optimize-modules(modules: Module[])

Optimize the modules.

compilation.plugin('optimize-modules', function(modules) {
  // handle to the modules array during tree optimization

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

Optimizing the modules has finished.

optimize-chunks(chunks: Chunk[])

Optimize the chunks.

//optimize chunks may be run several times in a compilation

compilation.plugin('optimize-chunks', function(chunks) {
    // Unless you've specified multiple entries in your config
    // there's only one chunk at this point
    chunks.forEach(function (chunk) {
        // Chunks have circular references to their modules
        chunk.modules.forEach(function (module){
            // module.loaders, module.rawRequest, module.dependencies, etc.

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

additional-assets async

Create additional assets for the compilation

Here's an example that downloads an image.

compiler.plugin('compilation', function(compilation) {
  compilation.plugin('additional-assets', function(callback) {
    download('', function(resp) {
      if(resp.status === 200) {
        compilation.assets['webpack-version.svg'] = toAsset(resp);
      } else {
        callback(new Error('[webpack-example-plugin] Unable to download the image'));

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.

Here's an example that simply adds a banner to each chunk.

compilation.plugin("optimize-chunk-assets", function(chunks, callback) {
  chunks.forEach(function(chunk) {
    chunk.files.forEach(function(file) {
      compilation.assets[file] = new ConcatSource("\/**Sweet Banner**\/", "\n", compilation.assets[file]);

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

The chunk assets have been optimized. Here's an example plugin from @boopathi that outputs exactly what went into each chunk.

var PrintChunksPlugin = function() {};

PrintChunksPlugin.prototype.apply = function(compiler) {
  compiler.plugin('compilation', function(compilation, params) {
    compilation.plugin('after-optimize-chunk-assets', function(chunks) {
      console.log( {
        return {
          includes: {
            return m.request;

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.

compilation.plugin('build-module', function(module){
  console.log('About to build: ', module);


A module has been built successfully.

compilation.plugin('succeed-module', function(module){
  console.log('Successfully built: ', module);


The module build has failed.

compilation.plugin('failed-module', function(module){
  console.log('Failed to build: ', module);

module-asset(module, filename)

An asset from a module was added to the compilation.

chunk-asset(chunk, filename)

An asset from a chunk was added to the compilation.