This is unpublished documentation of working with Rust and WebAssembly, the published documentation is available on the main Rust and WebAssembly documentation site . Features documented here may not be available in released versions of tooling for Rust and WebAssembly.

What is WebAssembly?

WebAssembly (wasm) is a simple machine model and executable format with an extensive specification. It is designed to be portable, compact, and execute at or near native speeds.

As a programming language, WebAssembly is comprised of two formats that represent the same structures, albeit in different ways:

  1. The .wat text format (called wat for "WebAssembly Text") uses S-expressions, and bears some resemblance to the Lisp family of languages like Scheme and Clojure.

  2. The .wasm binary format is lower-level and intended for consumption directly by wasm virtual machines. It is conceptually similar to ELF and Mach-O.

For reference, here is a factorial function in wat:

  (func $fac (param f64) (result f64)
    get_local 0
    f64.const 1
    if (result f64)
      f64.const 1
      get_local 0
      get_local 0
      f64.const 1
      call $fac
  (export "fac" (func $fac)))

If you're curious about how a wasm file looks like you can use the wat2wasm demo with the above code.

Linear Memory

WebAssembly has a very simple memory model. A wasm module has access to a single "linear memory", which is essentially a flat array of bytes. This memory can be grown by a multiple of the page size (64K). It cannot be shrunk.

Is WebAssembly Just for the Web?

Although it has currently gathered attention in the JavaScript and Web communities in general, wasm makes no assumptions about its host environment. Thus, it makes sense to speculate that wasm will become a "portable executable" format that is used in a variety of contexts in the future. As of today, however, wasm is mostly related to JavaScript (JS), which comes in many flavors (including both on the Web and Node.js).