node package manager



Webpack for large teams.

Allows for distinct versioned Webpack components to link to one another at runtime, using independent builds and release schedules.


  entry: 'file.js'


Circus projects are built around components that are versioned and compiled to shared locations.

These components should follow all of the semver rules, with the most important being breaking changes MUST increment the MAJOR version component.

Bower is used to link these packages together and enforce versioning checks. NPM may also be used, but it is generally preferred to use bower for Circus components as this will ensure there are not version incompatibilities between components.

Stored in each released package are the paths to the resources that are required for that component. This may be any arbitrary number of JavaScript or CSS files, but generally it's preferred to minimize the number of files. At build time the WebPack will resolve these paths based on the dependencies of the application and components, creating a single entry point module that is able to load all components needed for this application.

Components may be referenced using normal dependency loading with the following specific behaviors:

  1. require('component') will load the main entry point for the component named component
  2. require('component/src/lib/file') will load the module src/lib/file within component
  3. require('module') will load the generic module named module from any component that defines it. Generally this is intended for system-wide modules such as require('thorax'), etc.

All of the above exports will automatically be available from a given component. Components that do not wish to expose anything outside of the first and second options may specificy the options.output.hideInternals flag. This will optimize the output of the given component by omitting the linker tables and also provide further isolation for the component, should this be desired. This value may either be true to omit all child modules or a regular expression which will omit any matching module names.

Component projects are resolved using the require.moduleDependencies webpack configuration flag. Any child directory of these paths that contains a circus.json declaration file (automatically build by Circus) is a candidate for linking vs. being compiled into the current build.


Circus is able to build and link to projects that build from the same source, yet conditionally compile to different targets. By defining custom plugins, etc, this allows for builds targeting specific environments.

Permutations are defined by passing an array of webpack configurations to Circus.config. Two additional pieces of data are required when creating permutations, configId and pathPrefix. The former is used to link between different permutations and the later is used to output the build artifacts in a separate folder.

    entry: 'file.js',
    configId: 'browser',
    output: {
      pathPrefix: 'browser'
    plugins: [
      new webpack.DefinePlugin({
        '$isHybrid': true
    entry: 'file.js',
    configId: 'hybrid',
    output: {
      pathPrefix: 'hybrid'
    plugins: [
      new webpack.DefinePlugin({
        '$isHybrid': false

Would generate two different builds for the given project, one under browser/ which targets users visiting via web browsers and one under hybrid/ which targets users who are viewing the site via a hybrid native application.

When linking to a component that defines permutations, the configId value of the configuration will be used to select the permutation to link to. Should a match not be found, then the component will be ignored. When linking to a component that does not define permutations, everything will match by default.

AMD Compatibility Mode

Circus can be configured to generate builds that can be consumed by require.js and other AMD build environments. This is done by via the circus-amd plugin.

Bootstrap File

Circus will only generate one copy of it's bootstrap code for a given project. For components that are intended to be consumed by other components, a bootstrap.js file is generated with all of the necessary data to load the component within a test environment.

For top-level applications that users interact with directly, it's recommended that the output.bootstrap configuration option be set to true. This will inline the bootstrap code into the application entry point as well as optimize the output for this case. When in this mode, it is not possible for other components to import this component as a dependency.

This bootstrap logic allows for circus components to dynamically link to one another without the overhead of having multiple copies of the initialization logic or potentially loading different versions of the same component.

CSS Loading

Circus webpack builds will also generate a single CSS module for each output JS file, when CSS files are included via the require.css call.

var css = require.css('./home.css');

When required, the css file will automatically be inserted into the document. The require call returns a reference to the HTML element that the style will be loaded through.


Circus builds generate a circus.json file which defines all of the metadata that is associated with the current build.

By default this file contains:

  • files All files built from the component.
  • chunks Defines the chunk files for this component.
  • modules List of all modules built into this component.
  • routes Lists the routes that are served by this component, mapping them to the module that serves that particular route.
  • entry Entry point used to link to and load this component. This may be a local path or a shared URL.

Plugins that wish to modify this file prior to output may connect to the circus-json plugin point:

  compilation.plugin('circus-json', function(json) {
    json.isASpecialComponent = true;

Consumers of the circus.json file must accept any additional fields that plugins may define gracefully.


The core Circus library provides the publish(options) method to easily publish a built component to whatever public server/service desired.

Options are:

  • buildDir: Path to build directory
  • sourceMap: "local" to update the source map locally but not to publish. false to omit any source maps entierly. Defaults to publishing any defined source maps.
  • publish(file, content, callback): Called when a particular file should be published. callback is of the form callback(err, name) where name is the url of the published file. This will be used to populate the published map key in the manifest file.
  • filter(dir, callback): Optional method used to prevent publishing a specific permutation. callback(filter) expects a boolean as the first parameter. True responses will publish that permutation.
  • callback(err, published): Called when all files have been published

Tasks such as minimization should be done prior to publishing, if necessary. An example gulp flow is outlined below.

var buildDir = 'build/';
gulp.task('publish', ['build', 'minify-css', 'minify-js'], function(done) {
    buildDir: 'build/',
    sourceMap: 'local',
    publish: function(name, data, callback) {
      publishToMyAwesomeService(name, data, callback);
    callback: function(err, published) {
gulp.task('minify-css', ['test'], function() {
  return gulp.src('build/**/*.css')
gulp.task('minify-js', ['test'], function() {
  return gulp.src('build/**/*.js')
    .pipe(sourcemaps.init({loadMaps: true}))
Gulp.task('build', ['lint'], function(done) {
  var webpackConfig = require(Path.join(process.cwd(), 'webpack.config.js'));
  webpack(config, function(err, stats) {
    if (err) {

Once published to the production environment, the bulid should be published to a package manager to allow for linking to to build. The generator-release can be utilized to this end:

yo release:publish --host=$hostName components $componentName build/

Karma Adapter

When using Karma for tests, the Circus Karma adapter should be used to prevent test execution until all components have been loaded.

    frameworks: ['circus', ...],
    plugins: [

The circus config should also be set to inline the bootstrap code, otherwise errors due to having multiple entry points may occur.