WebAssembly, bytecode for browser

This joint project between browser vendors has defined an intermediate language for the Web, with asm.js as a starting point.

Presented by Brendan Eich, creator of JavaScript and partly of Asm.js, WebAssembly or wasm, is a new intermediate representation language (IRL) compatible with all browsers. It will allow different high-level languages to run in browsers being compiled to wasm. The initial goal is to compile programs and libraries from C and C ++ to WebAssembly. Then other languages will follow.

In June 2015, the development has just begun. But it is facilitated by the use of Asm.js as a starting point. This subset of JavaScript is compatible with all browsers.

The goal was to maintain compatibility between Asm.js and wasm for a few years. Not only for the support in browsers, but also because qu'Asm.js is also in development, and new features which will be include into it will be indispensable also to wasm. Eg threads with shared memory, support for multi-core processors.

WebAssembly diagram

Later, when all browsers will support the new WebAssembly language, both languages ​​may begin to differ. The second will be changed without modifying Asm.js, so JavaScript.

Wasm in particular receives the support of Google with the involvement of the NaCl team, so we can therefore anticipate the abandonment of the latter whose goal was close enough: to run native applications in the browser. Furthermore the V8 team announced the integration of wasm in the JavaScript compiler. So the wasm engine can produce IR code to be interpreted by the JS JIT engine.

Experiments have shown that parsing time of the new code (not the speed of execution) is 20 time faster than that of Asm.js. The size of the binary code generated is three times more compact than in Asm.js.
According to Brendan Eich, wasm is in fact a "compressed AST encoding" and not a true bytecode. AST for abstract syntax tree.

Some other advantage of wasm:

And a disadvantage:

Although compiled languages ​​in WebAssembly become a possible alternative to JavaScript, it will remain supported and privileged by browsers. Wasm will be a part of the JS compiler.

Why a new intermediate language?

Why not just use LLVM, or the Java bytecode, or .NET?

The primary objective of wasm is to be able to run source code written in C and C ++ in web applications. An immense amount of functions has been written in these languages, being able to reuse them with comparable execution time is really appreciable. 3D games are mostly written in C++: they become portable with wasm.
Wasm wants to be to the applications in the browser what Vulkan is to OpenGL and DirectX: a universal and portable intermediate code with a speed of execution close to native.


The LLVM bitcode was not retained because it is not portable. It contains metadata and is designed to produce executable binaries, not to work directly on all systems.
Moreover Google has already tried to use LLVM in the browser with NaCL and this has not been adopted by other publishers.
However, we use Emscriptem to compile the intermediate language of LLVM in wasm, so we keep the benefit of its tool chain.


The .NET runtime has an intermediate code that can run with a virtual machine or be compiled. But it is not suitable for C and C++ languages. Microsoft uses C++/CLI which has difference with native C++.


The Java virtual machine is not an option either because, if many languages ​​have been ported on this platform, this is not the case for C and C ++. Without even counting Oracle's propensity to sue for a reason as futile as the use of its API. The entire Web could be faced with the army of surly lawyers of the firm, which would be disastrous. WebAssembly will allow you to develop in peace.

Quotes from Brendan Eich about WebAssembly

The continued evolution of ASM.js is wasm.

At first, WebAssembly starts out just like ASM.js, but with a compressed syntax, that’s a binary syntax. But once all the browsers support both wasm and ASM.js, and after a decent interval of browser updates, then wasm can start to grow extra semantics that need not be put into JavaScript.

There are lots of languages you might compile to wasm.

Assuming stasis on the web — it’s not a good assumption, I think that was the mistake that happened long ago with projects like Portable Native Client and Dart, too

How to create and use WebAssembly code

A tutorial is provided on the site: Developer's guide to WebAssembly.

Tools to generate wasm code and run it are available on GitHub.