Size	Support
~1 KB	> IE8 (see below)

What's the use?

Made for easier use of the Polyfill.io API to detect browser support, offer dynamic polyfills and save an extra HTTP request.

Write modern code like fetch, Promise or Array.prototype.includes without the need to transform it or locally load a polyfill.

Note: Does not polyfill syntactic sugar like Classes, enhanced Object literals and features like Arrow Functions or Template Strings. Use compilers like Babel for that.

How does it work?

The module tests the given functions in fills (see below) against the window object to check if the browser supports them.

If not, it creates a link to the API service (i.e.https://cdn.polyfill.io/v2/polyfill.js?features=fetch) to load only the needed polyfills.
The script tag is put at the bottom of the page with the async attribute.

How to use?

NPM: npm i dynamic-polyfill
Yarn: yarn add dynamic-polyfill

import polyfill from 'dynamic-polyfill';
// or use require:
// const polyfill = require('dynamic-polyfill')

polyfill({
  fills: ['fetch', 'Array.prototype.includes'],
  options: ['gated', 'always'],
  minify: false,
  rum: false,
  afterFill() {
    // callback
  }
});

Fills

['fetch', 'Array.prototype.includes'] (default: empty)
A list of what can be polyfilled.
Put them in an array of strings. If empty, as default marked features on the website are being used.

Options

['gated', 'always'] (default: empty)

always
Polyfill should be included regardless of whether it is required by the user-agent making the request. If there are multiple browser-specific variants of the polyfill, the default one will be used for browser that doesn't actually require the polyfill. In some cases where the only way of implementing a polyfill is to use browser-specific proprietary technology, the default variant may be empty. Use in combination with gated to avoid console errors.

gated
If the polyfill is included in the bundle, it will be accompanied by a feature detect, which will only execute the polyfill if the native API is not present.

Minify

true | false (default: true)
Set to false for deeper insight of what is being polyfilled.

Real User Monitoring

rum: false (default: true)
Allows the polyfill service to gather performance data about itself using the resource timing API and to perform feature detection to improve our browser targeting.

More info under rum.

The service will default it to true in the future hence the module default choice. Set rum: false if you want to opt out.

User Agent

agent: 'ie/11.0.0' (default: empty)
Used to override the User-Agent string.
Set to polyfill to return default polyfill variants of all qualifying features. Useful if the polyfill service is being used from the server-side, and in that scenario, this is preferable to setting an inaccurate User-Agent header (the User-Agent header should properly be set to a string identifying the client you are using to make the request - for server side requests that might be cURL, for example).

User Agent Fallback

agentFallback: 'polyfill (default: empty)
What to do when the user agent is not recognised.
Use caution when setting this argument to polyfill on large feature sets, since huge polyfill bundles may cause crashes or lockups in extremely old or underpowered user agents.

After Fill

Put your modern code in this callback to make sure the polyfills are loaded first.

Example:

polyfill({
  fills: ['fetch'],
  afterFill() {
    main();
  }
});

function main() {
  fetch(url)
    .then(res => res.json());
}

Usage in IE8

For usage in IE8, you need a polyfill for Array.prototype.reduce(). You may copy the polyfill code from MDN.
Run it before dynamic-polyfill.

It is not included in this package to keep it as small as possible.

Not supported Fills

• ~html5-elements
• ~viewport

Use always flag for these.
There might be more. I have not tested all of them.
If you find a not supported fill, please leave a quick issue message on Github.
All the most used fills are supported.