The Polymer library is in maintenance mode. For new development, we recommend Lit.

You're viewing an older version of the Polymer library documentation. Please see Polymer 3.0 for the latest.

Polymer CLI is still pre-release. Some options may be subject to change.

  1. Install Git.

  2. Install an active LTS version of Node.js (4.x or 6.x). The current version (7.x) should work, but is not officially supported.

  3. Install the latest version of Bower.

    npm install -g bower
  4. Install Polymer CLI.

    npm install -g polymer-cli

You're all set. Run polymer help to view a list of commands.

Polymer CLI is a command-line interface for Polymer projects. It includes a build pipeline, a boilerplate generator for creating elements and apps, a linter, a development server, and a test runner.

Polymer CLI works with two types of projects:

  • Elements projects. In an element project, you expose a single element or group of related elements which you intend to use in other element or app projects, or distribute on a registry like Bower or NPM. Elements are reusable and organized to be used alongside other elements, so components are referenced outside the project.
  • Application projects. In an app project, you build an application, composed of Polymer elements, which you intend to deploy as a website. Applications are self-contained, organized with components inside the application.

Complete the Install section to learn how to install Polymer CLI and then proceed to the Build an element or Build an app section to get started on your first project.

This section shows you how to start an element project.

  1. Create a directory for your element project. It's best practice for the name of your project directory to match the name of your element.

    mkdir my-el && cd my-el

    Where my-el is the name of the element you're creating.

  2. Initialize your element. Polymer CLI asks you a few questions as it sets up your element project.

    polymer init
  3. Select element.

  4. Enter a name for your element.

    The custom elements specification requires the element name to contain a dash.

  5. Enter a description of the element.

At this point, Polymer CLI generates files and directories for your element, and installs your project's dependencies.

After the initialization process Polymer CLI generates the following files and directories.

  • bower.json. Configuration file for Bower.
  • bower_components/. Project dependencies. See Manage dependencies.
  • demo/index.html. Demo of my-el.html.
  • index.html. Automatically-generated API reference.
  • my-el.html. Element source code.
  • test/my-el_test.html. Unit tests for the element.


When importing Polymer elements via HTML imports, always use the relative URL ../package-name/element-name.html (e.g. ../polymer/polymer.html or ../paper-elements/paper-button.html).


Suppose that you ran Polymer CLI to generate an element project. Your element is named my-el. You look inside my-el.html and see that Polymer has been imported like so:

<link rel="import" href="../polymer/polymer.html">

This path doesn't make sense. Relative to my-el.html, Polymer is actually located at bower_components/polymer/polymer.html. Whereas the HTML import above is referencing a location outside of your element project. What's going on?

Bower installs dependencies in a flat dependency tree, like so:


This works well on the application-level. All elements are siblings, so they can all reliably import each other using relative paths like ../polymer/polymer.html. This is why Polymer CLI uses relative paths when initializing your element project.

However, one problem with this approach, as stated earlier, is that this structure does not actually match the layout in your element project. Your element project is actually laid out like so:


Polymer CLI handles this by remapping paths. When you run polymer serve, all elements in bower_components are remapped to appear to be in sibling directories relative to my-el. The current element is served from the made-up path of /components/bower name, where bower name is the name field from your element project's bower.json file.

Polymer CLI supports initializing a project folder with one of several application templates. The CLI comes with a basic template, which is the most basic starting point for a Polymer-based application, as well as others that introduce more complex layout and application patterns.

This chapter teaches you more about basic app projects. See Polymer App Toolbox templates for more details on other templates.

Polymer CLI is designed for apps that follow the app shell architecture.

There are fundamental concepts of the app shell architecture that you should understand before creating your app project with Polymer CLI: the entrypoint, the shell, and fragments. See App structure from the App Toolbox docs for an in-depth overview of these concepts.

Follow the steps below to get your basic app project set up.

  1. Create a directory for your app project.

    mkdir app
    cd app

    Where app is the name of your project directory.

  2. Initialize your app. Polymer CLI asks you a few questions as it sets up your app.

    polymer init
  3. Select application.

  4. Enter a name for your app. Defaults to the name of the current directory.

  5. Enter a name for the main element in your project. The main element is the top-most, application-level element of your app. Defaults to the name of the current directory, followed by -app.

    The code samples throughout this doc use the example app element name my-app. When creating your app you'll want to replace any instance of my-app with the name of your main element.

  6. Enter a description for your app.

At this point, Polymer CLI generates files and directories for your app, and installs your project's dependencies.

After the initialization process Polymer CLI generates the following files and directories.

  • bower.json. Configuration file for Bower.
  • bower_components/. Project dependencies. See Manage dependencies.
  • index.html. Entrypoint page of the app.
  • src/my-app/my-app.html. Source code for main element.
  • test/my-app/my-app_test.html. Tests for main element.

You may want to compose your main element out of smaller, application-specific elements. These app-specific elements should be defined in the src directory, at the same level as my-app.


Currently there is no Polymer CLI command to generate application-specific elements. You should do it by hand and should not create an element project within your app project.

This section explains various useful Polymer CLI commands that you'll want to incorporate into your development workflow while you build your element or app project.

The commands are intended for both element and app projects unless otherwise noted.

If you want to run tests on your element or app project, cd to the base directory of your project and run:

polymer test

Polymer CLI automatically runs all of the tests that it finds in the test directory. You'll see the results of the tests in your CLI.

If you create your own tests, they should also go in the test directory.

The underlying library that powers polymer test is called web-component-tester (wct). Learn more about creating unit tests with wct in Test your elements.

If you want to view a live demo of your element or app, run the local web server:

polymer serve

To view the demo, point your browser to one of the following URLs.

Element project demo:


Element project API reference:


App project demo:


This section shows examples of using various polymer serve options.

Serve from port 3000:

polymer serve --port 3000

If you have configured a custom hostname on your machine, Polymer CLI can serve it with the --hostname argument (for example, app project demo is available at http://test:8080):

polymer serve --hostname test

Open up a page other than the default index.html in a specific browser (Apple Safari, in this case):

polymer serve --open app.html --browser Safari

Analyze your project for syntax errors, missing imports, bad databinding expressions and more. polymer lint helps with identifying issues across your HTML, JS, and CSS based on an in-depth analysis of web components in source code. It does not reinvent the wheel though, it focuses on issues specific to web components and Polymer, so it is a good adjunct to other tools like eslint and htmlhint.

Use it like so:

polymer lint --rules=polymer-1

This will lint all of the code in your project with the polymer-1 ruleset, which is appropriate for projects using Polymer 1.0. If you're upgrading to Polymer 2.0, then the polymer-2-hybrid or polymer-2 are a better choice, as they'll warn you about use of deprecated Polymer 1.0 features.

You can pass flags to the linter like --rules but even better is to put the configuration in polymer.json so that all you need to do is run polymer lint. Putting your configuration in polymer.json also means that other tools, like IDE plugins can use the same lint configuration.

Here's what that looks like:

  "lint": {
      "rules": ["polymer-1"],
      "ignoreWarnings": []
  • rules: An array of lint rules and rule collections to run on your project. For most projects, one of polymer-1, polymer-2-hybrid, or polymer-2 is all that's needed here.
  • ignoreWarnings: An array of warning codes to ignore.

The output of polymer lint looks like this:


index.html(83,12) warning [undefined-elements] - The element iron-collapse is not defined

This means that on line 83 of index.html there's an <iron-collapse> tag, but the linter can't find the definition of the iron-collapse custom element. This probably means that there's a missing HTML import in index.html. To ignore this warning, add undefined-elements to the ignoreWarnings array in polymer.json.

This command is for app projects only.

Generates a production-ready build of your app. This process includes minifying the HTML, CSS, and JS of the application dependencies, and generating a service worker to pre-cache dependencies.

Polymer CLI's build process is designed for apps that follow the app shell architecture. To make sure your app builds properly, create a polymer.json file at the top-level of your project and store your build configurations there. The following properties can be used:

  • entrypoint: The main entrypoint into your application for all routes. Often times this is your index.html file. This file should import the app shell file specified in the shell option. It should be minimal since it's loaded and cached for each route.
  • shell: The app shell file containing common code for the app.
  • fragment: An array of any HTML files that are not synchronously loaded from the app shell, such as async imports or any imports loaded on-demand (e.g. by importHref).
  • sources: An optional array of globs matching your application source files. This will default to all files in your project src/ directory, but configuring your own list of sources can be useful when your source files live in other directories.
  • includeDependencies: An optional array of globs matching any additional dependencies¬†you'd like to include with your build. If your application loads any files dynamically they can be missed by the analyzer, but you can include them here to make sure that they are always added to your build. *Note: If you ever use polymer-build to define your own build process you can decide to handle sources & dependencies differently. But Polymer CLI currently treats additional files included in sources & includeDependencies the same, so place any additional files wherever you think makes the most sense.

For example, suppose you added an app shell (app-shell.html) and two views (view-one.html and view-two.html) for your basic app project, as well as a directory of images to display within your application. You'd specify them in your build with the following polymer.json configuration:

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

You can also pass these values via command-line flags. For example, in a newly created basic app project you could run the following command to generate a build:

polymer build --entrypoint index.html

This can be useful for building simple projects on your machine but you will need to include the flag every time you run the command. For most projects a polymer.json configuration file will be easier to work with and share across your team.

Polymer CLI will generate a service worker for your build using the sw-precache library. To customize your service worker, create a sw-precache-config.js file in your project directory that exports your configuration. See the sw-precache README for a list of all supported options.

Note that the sw-precache library uses a cache-first strategy for maximum speed and makes some other assumptions about how your service worker should behave. Read the "Considerations" section of the sw-precache README to make sure that this is suitable for your application.

Polymer CLI generates two build versions:

  • bundled. All fragments are bundled together to reduce the number of file requests. Optimal for sending to clients or serving from servers that are not HTTP/2 compatible.
  • unbundled. Fragments are unbundled. Optimal for HTTP/2-compatible servers and clients.

Polymer CLI uses Bower for dependency management.

Dependencies are stored in the bower_components directory. You should never manually alter the contents of this directory.

Use the Bower CLI to manage dependencies.

# downloads dependency to bower_components/
# --save flag saves new dependency to bower.json
bower install --save PolymerElements/iron-ajax
# removes dependency from bower_components/ and bower.json
bower uninstall PolymerElements/iron-ajax

You can see a list of global options by running polymer help. Most of them are self-explanatory.

The following commands are currently only supported for the polymer build command, with planned support for other commands in the future.

  • entry
  • shell
  • fragment

See Build app for more information on how to use these options.