node package manager


A library of Gulp build tasks


polymer-build is a library for building Polymer projects.


npm install --save-dev polymer-build

Relationship to Polymer CLI

The Polymer CLI uses polymer-build under the hood, so you can think of the CLI's build command like running a pre-configured polymer-build pipeline. Setting this up for you makes the CLI easy to use, but as a command-line wrapper its customization options are more limited. polymer-build allows you to completely customize your build and combine additional streams and build tasks in any order.

Consider using polymer-build instead of the CLI if you:

  • Want to customize your build(s) without using the Polymer CLI
  • Need to run your source code through custom optimizers/processors before, after, or during your build
  • Need to hook additional work into any part of the build process


While polymer-build was built to work easily with Gulp, it can be used in any Node.js environment. polymer-build is built on Node.js streams, and the build pipeline that you create with it is not much more than a series of connected streams. Files are represented as Vinyl file objects, which means that polymer-build can interop with any existing Gulp/Vinyl streams.

Check out the custom-build generator for an example of how polymer-build can be used to build a project.


PolymerProject represents your project in the build pipeline. Once configured, it will give you access to a collection of streams and helpers for building your project.

To create a new instance of PolymerProject, you'll need to give it some information about your application. If you already have a polymer.json configuration file in your project, you can create a new PolymerProject instance by loading it directly:

const PolymerProject = require('polymer-build').PolymerProject;
const project = new PolymerProject(require('./polymer.json'));

Or, you can pass in configuration options directly to the PolymerProject constructor:

const PolymerProject = require('polymer-build').PolymerProject;
const project = new PolymerProject({
  entrypoint: 'index.html',
  shell: 'src/my-app.html',
  fragments: [


Returns a readable stream of your project's source files. By default, these are the files in your project's src/ directory, but if you have additional source files this can be configured via the sources property in ProjectOptions.


Returns a readable stream of your project's dependencies. This stream is automatically populated based on the files loaded inside of your project. You can include additional dependencies via the extraDependencies property in ProjectOptions (this can be useful when the analyzer fails to detect a necessary dependency.)

const gulp = require('gulp');
const mergeStream = require('merge-stream');
// Create a build pipeline to pipe both streams together to the 'build/' dir 
mergeStream(project.sources(), project.dependencies())

Bundling Files


A stream that combines the files in your application to reduce the number of frontend requests needed. This can be a great way to improve performance when HTTP2/Push is not available.

By default, the bundler will create one "shared-bundle.html" containing all shared dependencies. You can optimize even further by defining "fragments" in your project options. Fragments are lazy loaded parts of the application, typically views and other elements loaded on-demand. When fragments are defined, the bundler is able to create smaller bundles containing code that is only required for specific fragments.

const gulp = require('gulp');
const mergeStream = require('merge-stream');
// Create a build pipeline to bundle our application before writing to the 'build/' dir 
mergeStream(project.sources(), project.dependencies())

Extracting Inlined CSS/JS

project.splitHtml() & project.rejoinHtml()

Web components will sometimes include inlined CSS & JavaScript. This can be a problem for tools that weren't built to read HTML. To get around this, you can include the optional splitHtml() and rejoinHtml() streams.

project.splitHtml() returns a stream that extracts any inlined CSS & JS into individual files. This can be useful for running your files through additional tools that don't handle inline code very well.

Note that this should be a temporary part of your overall build pipeline. Split files should always be rejoined with project.rejoinHtml() as soon as possible in the pipeline.

const gulpif = require('gulp-if');
const uglify = require('gulp-uglify');
const cssSlam = require('css-slam').gulp;
const htmlMinifier = require('gulp-html-minifier');
const mergeStream = require('merge-stream');
const sourcesStream = polymerProject.sources()
  .pipe(gulpif(/\.js$/, uglify()))
  .pipe(gulpif(/\.css$/, cssSlam()))
  .pipe(gulpif(/\.html$/, htmlMinifier()))
// Create a build pipeline to write our dependencies & optimized sources streams to the 'build/' dir 
// (not shown: project.dependencies() can also be split & optimized) 
mergeStream(sourcesStream, project.dependencies())

Generating Service Workers


generateServiceWorker() will generate the service worker code based on your build. Unlike other parts of polymer-build, generateServiceWorker() returns a promise and not a stream. It can only be run after your build has finished writing to disk, so that it is able to analyze the entire build as it exists.

For bundled builds, be sure to set the bundled option to true. See AddServiceWorkerOptions for a list of all supported options.

const generateServiceWorker = require('polymer-build').generateServiceWorker;
  buildRoot: 'build/',
  project: polymerProject,
  bundled: true // set if `polymerProject.bundler` was used 
  swPrecacheConfig: {
    // See for all supported options 
    navigateFallback: '/index.html',
}).then(() => { // ... 

generateServiceWorker() is built on top of the sw-precache library. Any options it supports can be passed directly to that library via the swPrecacheConfig option. See sw-preache for a list of all supported options

In some cases you may need to whitelist 3rd party services with sw-precache, so the Service Worker doesn't intercept them. For instance, if you're hosting your app on Firebase, you'll want to add the navigateFallbackWhitelist: [/^(?!\/__)/] option to your swPrecacheConfig as Firebase owns the __ namespace, and intercepting it will cause things like OAuth to fail.


Like generateServiceWorker(), but writes the generated service worker to the file path you specify in the path option ("service-worker.js" by default).

const addServiceWorker = require('polymer-build').addServiceWorker;
  buildRoot: 'build/',
  project: polymerProject,
}).then(() => { // ... 

Multiple Builds


Sometimes you'll want to pipe a build to multiple destinations. forkStream() creates a new stream that copies the original stream, cloning all files that pass through it.

const mergeStream = require('merge-stream');
const forkStream = require('polymer-build').forkStream;
// Create a combined build stream of your application files 
const buildStream = mergeStream(project.sources(), project.dependencies());
// Fork your build stream to write directly to the 'build/unbundled' dir 
const unbundledBuildStream = forkStream(buildStream)
// Fork your build stream to bundle your application and write to the 'build/bundled' dir 
const bundledBuildStream = forkStream(buildStream)


  1. Fork it!
  2. Create your feature branch: git checkout -b my-new-feature
  3. Commit your changes: git commit -am 'Add some feature'
  4. Push to the branch: git push origin my-new-feature
  5. Submit a pull request :D

You can compile polymer-build from source by cloning the repo and then running npm run build. Make sure you have already run npm install before compiling.

Supported Node.js Versions

polymer-build officially supports the latest LTS (4.x) and stable (6.x) versions of Node.js.