Why WebAssembly is a game changer for the web — and a source of pride for Mozilla and Firefox

David Bryant
Mozilla Tech
Published in
4 min readMar 7, 2017

With today’s release of Firefox, we are the first browser to support WebAssembly. If you haven’t yet heard of WebAssembly, it’s an emerging standard inspired by our research to enable near-native performance for web applications.

WebAssembly is one of the biggest advances to the Web Platform over the past decade.

This new standard will enable amazing video games and high-performance web apps for things like computer-aided design, video and image editing, and scientific visualization. Over time, many existing productivity apps (e.g. email, social networks, word processing) and JavaScript frameworks will likely use WebAssembly to significantly reduce load times while simultaneously improving performance while running. Unlike other approaches that have required plug-ins to achieve near-native performance in the browser, WebAssembly runs entirely within the Web Platform. This means that developers can integrate WebAssembly libraries for CPU-intensive calculations (e.g. compression, face detection, physics) into existing web apps that use JavaScript for less intensive work.

To get a quick understanding of WebAssembly, and to get an idea of how some companies are looking at using it, check out this video. You’ll hear from engineers at Mozilla, and partners such as Autodesk, Epic, and Unity.

It’s been a long, winding, and exciting road getting here.

JavaScript was originally intended as a lightweight language for fairly simple scripts. It needed to be easy for novice developers to code in. You know — for relatively simple things like making sure that you fill out a form correctly when you submit it.

A lot has changed since then. Modern web apps are complex computer programs, with client and server code, much of it written in JavaScript.

But, for all the advances in the JavaScript programming language and the engines that run it (including Mozilla’s SpiderMonkey engine), JavaScript still has inherent limitations that make it a poor fit for some scenarios. Most notably, when a browser actually executes JavaScript it typically can’t run the program as fast as the operating system can run a comparable native program written in other programming languages.

We’ve always been well aware of this at Mozilla but that has never limited our ambitions for the web. So a few years ago we embarked on a research project — to build a true virtual machine in the browser that would be capable of safely running both JavaScript and high-speed languages at near-native speeds. In particular we set a goal to allow modern video games to run in Firefox without plug-ins, knowing the Web Platform would then be able to run nearly any kind of application. Our first major step, after a great deal of experimentation, was to demonstrate that games built upon popular game engines could run in Firefox using an exploratory low-level subset of JavaScript called asm.js.

The asm.js sub-language worked impressively well, and we knew the approach could work even better as a first-class web standard. So, using asm.js as a proof of concept, we set out to collaborate with other browser makers to establish such a standard that could run as part of browsers. Together with expert engineers across browser makers, we established consensus on WebAssembly. We expect support for it will soon start shipping in other browsers.

Web apps written with WebAssembly can run at near-native speeds because, unlike JavaScript, all the code a programmer writes is parsed and compiled ahead of time before reaching the browser. The browser then just sees low-level, machine-ready instructions it can quickly validate, optimize, and run.

In some ways, WebAssembly changes what it means to be a web developer, as well as the fundamental abilities of the web. With WebAssembly and an accompanying set of tools, programs written in languages like C/C++ can be ported to the web so they run with near-native performance. We expect that, as WebAssembly continues to evolve, you’ll also be able to use it with programming languages often used for mobile apps, like Java, Swift, and C#.

If you’re interested in hearing more about the backstory of WebAssembly, check out this behind-the-scenes look.

WebAssembly is shipping today in Firefox on Windows, MacOS, Linux, and Android. We’re particularly excited about the potential on mobile — do all those apps really need to be native?

If you’d like to try out some applications that use WebAssembly, upgrade to Firefox 52, and check out this demo of Zen Garden by Epic. For your convenience, we’ve embedded a video of the demo below.

If you’re a developer interested in working with WebAssembly, check out WebAssembly documentation on MDN. You might also want to see this series of blog posts by Lin Clark that explain WebAssembly through some cool cartoons.

Here at Mozilla we’re focused on moving the web forward and on making Firefox the best browser, hands down. With WebAssembly shipping today and Project Quantum well underway, we’re more bullish about the web — and about Firefox — than ever.

Special thanks to Ryan Pollock for contributing to this post.



David Bryant
Mozilla Tech

Mozilla Fellow, lifelong engineer and builder of things. Spent time at Bell Labs, Sun Microsystems, and Nokia.