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Polymer 3.0 tools update

The Polymer CLI gets improved module support

The latest pre-release version of the Polymer CLI is available, and it includes enhanced support for ES6 modules. Specifically, Polymer CLI version 1.7.0-pre.10 now supports:

  • Bundling ES6 modules.
  • Transforming ES6 modules to AMD modules. (This was previously supported in polymer serve, but not in polymer build.) Bundled modules can be output in either ES6 or AMD form.
  • Support for dynamic imports, including transforming dynamic imports for browsers that don't support ES6 modules.
  • Support for transforming references to import.meta.url so they work on browsers that don't support ES6 modules. This will be used in a future version of modulizer and Polymer 3.0 preview to provide backward compatibility for the 2.x importPath property.

To get the new version, install the latest version of the Polymer CLI:

npm install -g polymer-cli@next

If installation fails, remove the existing version of Polymer CLI and retry the installation:

npm uninstall -g polymer-cli
npm install -g polymer-cli@next

Read on for more details on what's supported now.

Serve and test

The polymer serve and test commands both use the same development server to serve and transform projects. The development server already supported transforming ES6 modules to AMD modules at runtime. This release adds support for transforming dynamic imports to AMD modules, as well.

By default, the development server tries to autodetect support for ES6 and ES6 modules based on the browser's user agent. Using the--compile=always flag causes the server to always transpile to ES5 and transform ES6 modules to AMD.


Build now supports bundling and transforming ES6 modules. When working on ES6 modules, build produces either ES6 modules or AMD modules as output. It can also transform dynamic imports into AMD modules.

To create a build with ES6 modules transformed to AMD, set the transformModulesToAmd option for that build in the polymer.json file.

  "npm": true,
  "module-resolution": "node",
  "builds": [ 
      "name": "bundled-amd", 
      "bundle": { 
        "inlineScripts": false 
      "js": { 
        "transformModulesToAmd": true 

You can also use the command line flag --js-transform-modules-to-amd instead of setting the option in polymer.json.

polymer build --npm --module-resolution=node --js-transform-modules-to-amd 

Note: Currently, you must set the inlineScripts flag to false when bundling Polymer 3.x applications, as shown in the example above.

Known issues

This prerelease version of the CLI has the following known issues:

  • polymer-build#358. Modules transformed to AMD may fail because of incomplete Babel helper scripts. This occurs if a module uses the import * syntax. Other forms of the import statement shouldn't have this problem.

  • polymer-bundler#653. Inline module scripts are not processed correctly when bundling. If an inline module script imports another module that's in a bundle, the import specifier isn't rewritten correctly.

    <script type="module">
    // importing a module that gets bundled elsewhere
    // this import specifier will not get rewritten properly
    import {PolymerElement} from '@polymer/polymer/polymer-element.js';
    export class FooEl extends PolymerElement { ... }

    For now, avoid these and use the inlineScripts: false option when bundling. External module scripts should work fine:

    <script type="module" src="./foo-el.js"></script>

What's Next

This release brings our tools story much closer to completion, but we still have a few items left in the home stretch:

  • Init templates for Polymer 3.x applications and elements.
  • Support for generating API documentation.
  • More testing and bug fixes.