A command-line tool for Polymer projects
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A command-line tool for Polymer projects.

For detailed help, check out the Polymer CLI guide.


Polymer-CLI includes a number of tools for working with Polymer and Web Components:

  • init - Initializes a Polymer project from one of several templates
  • build - Builds an application-style project
  • lint - Lints the project
  • serve - Runs a development server
  • test - Runs tests with web-component-tester


Install via npm:

$ npm install -g polymer-cli

Then run via polymer <command>:

$ polymer help

Project Structure

Polymer-CLI is somewhat opinionated about project structure.

There are two type of projects:

  • Elements projects

    Element projects contain one or more reusable element definitions, intended to be used from other elements, applications or pages. Element definitions live at the top level of the project so they are easy to import. Elements import their dependencies with relative paths that reference sibling folders to the project folder.

  • Application projects

    Application projects are self-contained and intended to be deployed as a standalone website. Application projects contain elements in a src/ folder and import their dependencies with absolute paths, or relative paths that reference folders inside the project folder.

Application Styles

Polymer-CLI currently supports two styles of applications:

  • Monolithic applications, which have a single entrypoint (usually index.html) and eagerly import all dependencies.

  • "App shell" applications, which have a very lightweight entrypoint, an app-shell with startup and routing logic, and possibly lazy loaded fragments.

App-shell Structure

App-shell apps are currently the preferred style for Polymer CLI, and most commands are being optimized to support them. App-shell apps usually have client-side routing (see the app-route element), and lazy load parts of the UI on demand.

Polymer-CLI supports this style by understand these different types of files:

  • entrypoint - The first file served by the web server for every valid route (usually index.html). This file should be very small, since it may not cache well and must reference resources with absolute URLs, due to being served from many URLs.
  • shell - The actual app shell, which includes the top-level logic, routing, and so on.
  • fragments - lazy loaded parts of the application, typically views and other elements loaded on-demand.


The project files are specified either as global flags: --entrypoint, --shell and zero or more --fragment flags, or in a polymer.json configuration file.


You can specify the project files in polymer.json so that commands like polymer build work without flags:

  "entrypoint": "index.html",
  "shell": "src/my-app/my-app.html",
  "fragments": [
  "sources": [
  "includeDependencies": [

Dependency Variants

A number of commands use a concept called "dependency variants", which are different sets of dependencies that the project should work with.

Typically, when a project can work with a wide range of major versions of dependencies, tests are only run against the most recent versions of the dependencies. This leaves compatibility with older versions untested. With dependency variants you can specify a a number of narrower sets of versions to test against to ensure compatibility.

Variants are specified in a "variants" property of bower.json. "variants" is a map of variant name to a patch that's applied to the rest of bower.json. Each property in a variant overwrites a property in bower.json if the property is a simple value or an Array, otherwise if the property is an Object, it's merged.

The result of patching a variant into the bower.json is used to perform a separate Bower install into a directory named bower_components-{variant_name}.


  "name": "example",
  "dependencies": {
    "polymer": "Polymer/polymer#1.7 - 2.x",
    "foobar": "Foo/bar#^2.0.0"
  "variants": {
    "polymer-1": {
      "dependencies": {
        "polymer": "Polymer/polymer#^1.7.0"
    "polymer-2": {
      "dependencies": {
        "polymer": "Polymer/polymer#^2.0.0"

When using polymer install --variants, this results in two additional folders, bower_components-polymer-1 and bower_components-polymer-2, installed as if they had their own bower.json files as follows:


  "name": "example",
  "dependencies": {
    "polymer": "Polymer/polymer#^1.7.0",
    "foobar": "Foo/bar#^2.0.0"


  "name": "example",
  "dependencies": {
    "polymer": "Polymer/polymer#^2.0.0",
    "foobar": "Foo/bar#^2.0.0"

Commands like install, serve and test recognize that variants are installed to offer additional support.

polymer install --variants installs dependency variants into the proper folders.

polymer serve will start up a development server for each variant on its own port so that you can easily switch between variants, viewing their demos and tests.

polymer test will run the test suite once for each variant, ensuring that you have test coverage against supported configurations.



Displays help on commands and options:

$ polymer help


Initializes a Polymer project from one of several templates.

Choose a template from a menu:

$ polymer init

Create a new project from the 'element' template:

$ polymer init element

You can download and run templates built by our community as well. Search npm for the full list.


Installs Bower dependencies, optionally installing multiple variants.

$ polymer install

This performs a Bower install of dependencies listed in bower.json, and is equivalent to bower install.

$ polymer install --variants

This performs a Bower install, and also installs any dependency variants specified in the "variants" property of bower.json. See Dependency Variants above.


With a polymer.json file:

$ polymer lint

Specifying a file to lint:

$ polymer lint index.html


Run test with web-component-tester:

$ polymer test


Specify project files as flags:

$ polymer build --entrypoint index.html --shell src/my-app/my-app.html

Use index.html as the entrypoint, or read from polymer.json:

$ polymer build

build is opinionated and defaults to a good build for app-shell apps. It writes the built output to build/bundled and build/unbundled folders. Both outputs have been run though HTML, JS and CSS optimizers, and have a Service Worker generated for them. The bundled folder contains the application files process by Vulcanize, Polymer's HTML bundler, for optimal loading via HTTP/1. The unbundled folder is optimized for HTTP/2 + Push.

While the build command should support most projects, some users will need greater control over their build pipeline. If that's you, check out the polymer-build library. Polymer-build can be called and customized programmatically, giving you much greater control than the CLI can provide. Visit the repo for usage information and examples.


Start the development server:

$ polymer serve

Start the development server, and open the default browser:

$ polymer serve -o

By default the server listens to localhost. To listen to a different address use the --hostname flag. For example:

$ polymer serve -o --hostname

Templates and Generators

Polymer-CLI initializes new projects with the init command, and includes a few built-in templates.

New templates can be distributed and installed via npm. Yeoman generators prefixed with generator-polymer-init will show up in the polymer init menu.

Compiling from Source

$ npm run build

You can compile and run the CLI from source by cloning the repo from Github and then running npm run build. But make sure you have already run npm install before building.

Supported node.js versions

Polymer CLI targets the current LTS version (4.x) of Node.js and later.