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JavaScript import statements and package names

At the time of writing, web browsers can't load JavaScript modules by their package names. Since the LitElement library uses package names to import dependencies, you must use a build step to transform package names to URLs so that web browsers can find the files.

For example, the following import statement must be transformed to use a URL:

import { LitElement, html } from '@polymer/lit-element';

The transformed import statement might look something like this:

import { LitElement, html } from '../node_modules/@polymer/lit-element/lit-element.js';

If you're working with Polymer CLI, you can configure polymer serve and polymer build to take care of this transform for you.

Configure Polymer CLI to transform package names to URLs

Create a JSON configuration file called polymer.json in your top-level project folder. Set the following properties in polymer.json:

|Property|Value| |npm|true| |moduleResolution|"node"|

For example:


  "npm": true,
  "moduleResolution": "node",
  "entrypoint": "src/index.html",
  "shell": "src/my-project.js"

Build with Polymer CLI

  1. Configure Polymer CLI to build your project.

    Add a builds property to polymer.json. For example:

    json "builds": [{ "name": "mybuild", "bundle": true, "js": { "minify": true } }]

    polymer.json: Before

    json { "npm": true, "moduleResolution": "node", "entrypoint": "src/index.html", "shell": "src/my-project.js", }

    polymer.json: After

    json { "npm": true, "moduleResolution": "node", "entrypoint": "src/index.html", "shell": "src/my-project.js", "extraDependencies": [ "node_modules/@webcomponents/webcomponentsjs/**" ], "builds": [{ "name": "mybuild", "bundle": true, "js": { "minify": true } }] }

  2. From your top-level project folder, install npm components if you haven't already done so. Then, run polymer build:

    bash cd my-project npm install polymer build

    Polymer CLI builds your project. It adds a build folder to your top-level project folder, with sub-folders for each build you've configured. For example:

    Project folder structure: Before

    bash my-project/ node_modules/ @polymer/ ... @webcomponents/ ... src/ index.html my-project.js package.json polymer.json

    Project folder structure: After

    bash my-project/ node_modules/ @polymer/ ... @webcomponents/ ... src/ index.html my-project.js build/ mybuild/ node_modules/ @polymer/ ... @webcomponents/ ... src/ index.html my-project.js polymer.json package.json polymer.json

  3. Serve the built project locally.

    From your top-level project folder, run polymer serve build/buildname and open the applications URL. For example:

    ~/my-project > polymer serve build/mybuild info: [cli.command.serve] Files in this directory are available under the following URLs applications: reusable components:

    To view the served project in the example above, open