Using webpack to set up polyfills in your site

Webpack's Provide Plugin allows you to set up variables that get injected into your code when they are used in your codebase.

This means that if I want to polyfill the new ES6 native promise for older browsers, I can do it at build time.

Two polyfills you'll want to have set up in your site is the fetch and promise polyfills.

bower install --save es6-promise fetch

or

npm install --save es6-promise whatwg-fetch

(notice the fetch npm package is named differently to the bower package, this is important for later on)

Fetch is the new browser api replacing XHR requests, or $.ajax() requests.

For the polyfills you'll need to install the imports-loader and the exports-loader.

npm install --save-dev imports-loader exports-loader

Now we have our loaders installed, we can go to our webpack config and down to the plugins section.

Here we'll create a new instance of webpack.ProvidePlugin and set up our variables to be injected into the module when (and only when) it's used.

plugins: [
    new webpack.ProvidePlugin({
          Promise: 'imports-loader?this=>global!exports-loader?global.Promise!es6-promise',
          fetch: 'imports-loader?this=>global!exports-loader?global.fetch!fetch'
     })
]

NOTE: The name of fetch at the end of the line corresponds to the version you're using, if you installed via npm, you should use the following config:

plugins: [
    new webpack.ProvidePlugin({
          Promise: 'imports-loader?this=>global!exports-loader?global.Promise!es6-promise',
          fetch: 'imports-loader?this=>global!exports-loader?global.fetch!whatwg-fetch'
     })
]

IMPORTANT: As mumu in the comments pointed out, if you're using Webpack 2, you need to include the -loader in the name of your loaders now. e.g. imports-loader

When you webpack up your build, if you're now using fetch or promise somewhere in your code, you'll see it included in your bundle.

A way to test this is to go to a modern browser, load up your site and type fetch or Promise.

You should see the following output:

function Promise() { [native code] }

function fetch() { [native code] }

Then go to an older browser that doesn't support these features and you'll see something like this:

> fetch
> function(input, init) {...

> Promise
> function lib$es6$promise$promise$$Promise(resolver) {...

You know the polyfills are working now as they didn't kick in when the browser supported the feature but they have kicked in when the browser doesn't support the features.

Remember that they only kick in if you use the feature within your codebase, I spent a while trying to test the polyfills without using the feature in my code before realising how ProvidePlugin works.