JavaScript Christmas

A short recap on polyfills

A 2 minute read written by
Fredrik Stenbro

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Maybe you’ve heard of Fetch? The promise-based Web API for making network requests in the browser, and a favourite amongst developers. If you paste fetch(‘’) into the Chrome DevTools console and run it you can take a closer look at how it works. Sadly, you can’t do the same in Internet Explorer. It won’t work. That is because Internet Explorer has not implemented support for Fetch. Or Promise for that matter.

As developers of things on the web we want, and are often required, to support all major browsers out there. More users often converts to more success. So, of course we want to invite everyone in. The problem arises when our users, many unknowingly I suppose, are using old or specific browsers that do not support the latest and greatest features - which we, as developers, are longing to use. Polyfills to the rescue!

What is a polyfill?

If the browser that is running your code do not support the features that you want to apply in your modern JavaScript, like Object.assign() or fetch(), a common practice is to provide the pieces of code that is missing yourself. These pieces of code are called polyfills. It can be as easy as finding the required implementation yourself and adding it to your source code.

Here is an example of MDNs suggested polyfill for Number.isInteger():

Number.isInteger = 
    Number.isInteger || function(value) {              
        return typeof value === 'number' && isFinite(value) && Math.floor(value) === value;   

Just add this to your Javascript, and make sure its declared and on a reachable scope before it is used, and you’re good to go.

There are also many polyfills that can be found on NPM, like the popular polyfill for Fetch which removes the burden of you having to find the perfect implementation yourself.

But how to know if a feature is supported or not? I would recommend using the MDN Web Docs from Mozilla Developer Network. In addition to showing browser compatibility, it also contains great documentation and often suggested polyfills. CanIUse is another great alternative, offering a quick and easy way for looking up compatibility for a lot of different features in both Javascript, CSS and more.

If your code base requires a lot of polyfilling, maybe also some transpiling, adding tools like Babel.js to your build setup enables an easy way of securing that required polyfills are available when your code is running.

Polyfilling is often confused with transpiling. These two techniques solve different problems. While polyfilling adds a piece of missing code, transpiling transforms your code and enables you to write syntax that may be unsupported in certain browsers, like arrow functions and class declarations.

When can we stop worrying about polyfills?

As long as there are multiple major browsers from different companies doing things their own way, there is no way to guarantee that they are up to speed on the latest standards like ECMAScript and WHATWG.

Nonetheless, the need for polyfilling recent years have mostly been related to unsupported features in Internet Explorer. The market share of Internet Explorer is dropping slowly but steady, but is said to be supported throughout the lifespan of Windows 10. The good news is that Microsoft plans to release the brand new Edge Chromium, which is based on Google Chrome's open sourced engine, 15th of January next year. Let's hope Windows users adopt this version of Edge faster than the last, so that the era of error messages like X doesn't support property or method 'y' might come to an end.

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