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What is a polyfill?

dmahely profile image Doaa Mahely ・3 min read

If you're like me, chances are you heard the word polyfill used before in a context where it wasn't easy to deduce a meaning. Thanks to my new tendency of writing down things I'd like to learn about and a subsequent discussion with @tryggvigy , I was able to get a better understanding of what polyfills are.

A polyfill is code that acts as a backup when certain properties, functions or syntax aren't supported in a browser or environment. It's not language-specific, although I've seen it mostly used with JavaScript and CSS. If that doesn't make sense, let's look at MDN's definition:

A polyfill is a piece of code (usually JavaScript on the Web) used to provide modern functionality on older browsers that do not natively support it.

New features are never supported on all browsers out of the box. Imagine wanting to refactor your code to use new ES2019 features such as the nullish coalescing operator or Array.prototype.flatMap. In this case, your code will break because older browsers will not recognize it as valid JavaScript. A polyfill can provide a backup in this case to ensure new features are backward-compatible.

Let's take a closer look with the example code from MDN's page on the nullish coalescing operator:

const foo = null ?? 'default string';
console.log(foo);
// expected output: "default string"

const baz = 0 ?? 42;
console.log(baz);
// expected output: 0
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Now let's try running it on Safari 13.0 (currently not supported):

SyntaxError: Unexpected token '?'
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At the time of writing, the ?? operator is supported for 77.08% of global users. If you're using it in production, you're ignoring a very big slice of potential users. To work around this, you can search for a ?? polyfill and add it to your code so that the new syntax will work with supported browsers, and the polyfill will work for the older unsupported browsers.

Thankfully, you don't actually need to go out and search for polyfills and manually add them to your codebase. Babel is an example of a tool (also referred to as a compiler, transpiler or transcompiler) that enables us to write the newest and latest JavaScript syntax without having to worry about keeping track of polyfills.

With Babel installed in our codebase, we can write this:

var foo = object.foo ?? "default";
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and have it automatically be converted to:

var _object$foo;

var foo = (_object$foo = object.foo) !== null && _object$foo !== void 0 ? _object$foo : "default";
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Another interesting example is the polyfill for Array.prototype.map which can be found here.

To end with, it's worth mentioning that using polyfills exclusively, as opposed to browsers and environments working to natively support new features, is not a good idea for performance and functionality reasons.

Thank you for reading. Until next time 👋
Cover photo by T.H. Chia on Unsplash

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