WebAssembly is a new type of code that can be run in modern web browsers — it is a low-level assembly-like language with a compact binary format that runs with near-native performance and provides languages such as C/C++ with a compilation target so that they can run on the web. It is also designed to run alongside JavaScript, allowing both to work together.

In a Nutshell

WebAssembly has huge implications for the web platform — it provides a way to run code written in multiple languages on the web at near native speed, with client apps running on the web that previously couldn’t have done so.

WebAssembly is designed to complement and run alongside JavaScript — using the WebAssembly JavaScript APIs, you can load WebAssembly modules into a JavaScript app and share functionality between the two. This allows you to take advantage of WebAssembly's performance and power and JavaScript's expressiveness and flexibility in the same apps, even if you don't know how to write WebAssembly code.

And what's even better is that it is being developed as a web standard via the W3C WebAssembly Working Group and Community Group with active participation from all major browser vendors.


WebAssembly concepts
Get started by reading the high-level concepts behind WebAssembly — what it is, why it is so useful, how it fits into the web platform (and beyond), and how to use it.
Compiling from C/C++ to WebAssembly
When you’ve written code in C/C++, you can then compile it into .wasm using a tool like Emscripten. Let’s look at how it works.
Loading and running WebAssembly code
After you have a .wasm, this article covers how to fetch, compile and instantiate it, combining the WebAssembly JavaScript API with the Fetch or XHR APIs.
Caching compiled WebAssembly modules
Caching large WebAssembly modules on the client is useful for improving app startup performance. This article explains how to do this using IndexedDB.
Using the WebAssembly JavaScript API
Once you've loaded a .wasm module, you'll want to use it. In this article we show you how to use WebAssembly via the WebAssembly JavaScript API.
Exported WebAssembly functions
Exported WebAssembly functions are the JavaScript reflections of WebAssembly functions which allow calling WebAssembly code from JavaScript. This article describes what they are.
Understanding WebAssembly text format
This article explains the wasm text format. This is the low-level textual representation of a .wasm module shown in browser developer tools when debugging.
Converting WebAssembly text format to wasm
This article provides a guide on how to convert a WebAssembly module written in the text format into a .wasm binary.

API reference

This object acts as the namespace for all WebAssembly related functionality.
A WebAssembly.Module object contains stateless WebAssembly code that has already been compiled by the browser and can be efficiently shared with Workers, cached in IndexedDB, and instantiated multiple times.
A WebAssembly.Instance object is a stateful, executable instance of a ModuleInstance objects contain all the Exported WebAssembly functions that allow calling into WebAssembly code from JavaScript.
The WebAssembly.instantiateStreaming() function is the primary API for compiling and instantiating WebAssembly code, returning both a Module and its first Instance.
A WebAssembly.Memory object is a resizable ArrayBuffer that holds the raw bytes of memory accessed by an Instance.
A WebAssembly.Table object is a resizable typed array of opaque values, like function references, that are accessed by an Instance.
Creates a new WebAssembly CompileError object.
Creates a new WebAssembly LinkError object.
Creates a new WebAssembly RuntimeError object.



Specification Status Comment
WebAssembly JavaScript Interface Working Draft Initial draft definition of the JavaScript API.

Browser compatibility

FeatureChromeEdgeFirefoxInternet ExplorerOperaSafari
Basic support5716522 No4411
CompileError5716522 No4411
Instance5716522 No4411
LinkError5716522 No4411
Memory5716522 No4411
Module5716522 No4411
RuntimeError5716522 No4411
Table5716522 No4411
compile5716522 No4411
compileStreaming61 No58 No47 No
instantiate5716522 No4411
instantiateStreaming61 No58 No47 No
validate5716522 No4411
FeatureAndroid webviewChrome for AndroidEdge mobileFirefox for AndroidOpera AndroidiOS SafariSamsung Internet
Basic support5757 Yes1522 ?11 ?
CompileError5757 Yes1522 ?11 ?
Instance5757 Yes1522 ?11 ?
LinkError5757 Yes1522 ?11 ?
Memory5757 Yes1522 ?11 ?
Module5757 Yes1522 ?11 ?
RuntimeError5757 Yes1522 ?11 ?
Table5757 Yes1522 ?11 ?
compile5757 Yes1522 ?11 ?
compileStreaming6161 No58 ? No ?
instantiate5757 Yes1522 ?11 ?
instantiateStreaming6161 No58 ? No ?
validate5757 Yes1522 ?11 ?

1. This feature is behind the Experimental JavaScript Features preference.

2. Disabled in the Firefox 52 Extended Support Release (ESR).

See also

Document Tags and Contributors

 Last updated by: jpmedley,