Polymer 2.x uses EcmaScript 2015 (commonly known as ES6). The following browsers support all of the ES6 features required by Polymer:
- Chrome or Chromium version 49 or later.
- Opera 36 or later.
- Safari or Mobile Safari 10 or later.
- Edge 15.15063 or later.
- Firefox 51 or later.
For other browsers, you should compile your application to ES5.
Information on multiple builds and differential serving is available in the documentation on building for production.
Compiling ES6 to ES5
The Polymer CLI and
polymer-build library support compiling ES6 to ES5 at build time. In
polymer serve and
polymer test commands compile at runtime when required by the
Device emulation may cause errors. When using DevTools device emulation in Chrome,
polymer serve will serve compiled code when emulating iOS devices, and uncompiled code when
emulating Android devices. When switching between devices, the browser may end up with both
compiled and uncompiled code in its cache, resulting in errors. To avoid this problem, run
polymer serve with the
--compile never option when testing with device emulation.
For best performance, you should serve ES6 code to browsers that support it, and only serve ES5 code to browsers that don't support ES6.
If you need to statically host your code and serve a single version to all browsers, compile to ES5.
When you use the Polymer CLI to compile your app, the CLI automatically compiles the correct files
custom-elements-es5-adapter.js, a lightweight polyfill that lets compiled ES5 work
on browsers that support native custom elements.
If you're putting together a custom build:
- Polymer requires all ES6 features except modules. (If you're using Babel, you can use
modulesoption set to false.)
- You must compile all your elements, the Polymer library, and any third-party elements you're using, but not the polyfills.
- If you're serving compiled code to browsers that support native custom elements, inject the
custom-elements-es5-adapter.jsscript at build time. The
polymer-buildlibrary provides a addCustomElementsEs5Adapter method to inject the script. If that method doesn't work with your build system, see the webcomponentsjs README for details on using the script.