Polyfilling the Fetch API for Old Browsers and Node.js

First some definitions.

A polyfill will try to emulate certain APIs, so can use them as if they were already implemented.

A transpiler on the other hand will transform your code and replace the respective code section by other code, which can already be executed.

(Thanks Sirko for those!)

Examples of polyfilling and transpiling

By polyfilling the Fetch API we make it usable in browsers where it isn't available by default. Another example would be making it available in its original functionality in Node.js.

By transpiling Spread syntax (an ES6 feature) into ES5 compatible JavaScript, we end up with source code that is easier for us to write and deployable code that works in older browsers!

Here's our ES6 code example.

function speak() {
  console.log(...args);
}

speak(1, 2, 3);

Here is that same code transpiled into ES5 compatible code.

"use strict";

function speak() {
  var _console;

  (_console = console).log.apply(_console, args);
}

speak(1, 2, 3);

(Technically, this is the loose transpilation because otherwise the above snippet would be eleven times longer.)

Using fetch in old browsers

We love the Fetch API because it means we can avoid using XMLHttpRequest. Instead of providing a callback, we can use lovely Promises.

Here's a fetch call that prints out the status code of a request.

fetch('https://httpstat.us')
    .then(res => console.log(res.status))

If we tried running that in Internet Explorer 11 (or Edge 13, or Chrome 39, and so on) we would get an error.

window.fetch would likely evaluate to undefined. We might get an error that looks like Uncaught TypeError: window.fetch is not a function.

It's easy to check one-off functionality for a feature on Can I use — here's fetch https://caniuse.com/#search=fetch. It isn't viable to check every feature that your code might use, so that's why we use things like @babel/preset-env.

@babel/preset-env is a smart preset that allows you to use the latest JavaScript without needing to micromanage which syntax transforms (and optionally, browser polyfills) are needed by your target environment(s). This both makes your life easier and JavaScript bundles smaller!

An even cooler feature of Babel is that it allows us to specify which platforms we want to support.

Why not just support all platforms? Because then the JavaScript bundle we send to our users would get larger and larger every year and website performance would grind to a halt.

Babel's Browserslist Integration lets us forget about version numbers and instead use handy shortcuts. Let's say we wanted to support 99.75% of browsers and no dead browsers. We can add "browserslist": "> 0.25%, not dead" to our package.json file.

However, you might be reading this because you found out that @babel/preset-env doesn't include a fetch polyfill. This is an open issue on GitHub (#9160). It looks like it's going to stay this way.

Babel only adds polyfill for ECMAScript methods (defined at https://tc39.github.io/ecma262/). Since fetch is defined by a web specification (https://fetch.spec.whatwg.org/), you need to add the polyfill yourself.

That's okay because we can use github/fetch to polyfill it for us.

Either by replacing all instances of fetch.

import 'whatwg-fetch'

window.fetch(...)

Or on a case-by-case basis.

import { fetch as fetchPolyfill } from 'whatwg-fetch'

window.fetch(...)   // use native browser version
fetchPolyfill(...)  // use polyfill implementation

Using fetch in Node.js

The Fetch API is common and people are fluent with it. It's productive if they can use it in all the JavaScript they write. Many people think it's available in Node.js by default. It's not but there's a package for that (™).

node-fetch/node-fetch let's us use the API we're fluent with to make HTTP calls on the back-end. Underneath, it uses native Node.js functionality.

// CommonJS
const fetch = require('node-fetch');

// ES Module
import fetch from 'node-fetch';

If you're looking for an isomorphic solution (this means using the same code in the browser and in Node.js) then you'll want Jason Miller's isomorphic-unfetch (but not for React Native, see #125) or Leonardo Quixada's cross-fetch.

These solutions will figure out which environment you're in and choose the correct polyfill.

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