WebAssembly.Memory

The WebAssembly.Memory object is a resizable ArrayBuffer or SharedArrayBuffer that holds the raw bytes of memory accessed by a WebAssembly Instance.

A memory created by JavaScript or in WebAssembly code will be accessible and mutable from both JavaScript and WebAssembly.

WebAssembly.Memory()

Creates a new Memory object.

Memory.prototype.buffer

An accessor property that returns the buffer contained in the memory.

Memory.prototype.grow()

Increases the size of the memory instance by a specified number of WebAssembly pages (each one is 64KB in size).

There are two ways to get a WebAssembly.Memory object. The first way is to construct it from JavaScript. The following snippet creates a new WebAssembly Memory instance with an initial size of 10 pages (640KiB), and a maximum size of 100 pages (6.4MiB). Its buffer property will return an ArrayBuffer.

var memory = new WebAssembly.Memory({initial:10, maximum:100});

The following example (see memory.html on GitHub, and view it live also) fetches and instantiates the loaded memory.wasm byte code using the WebAssembly.instantiateStreaming() method, while importing the memory created in the line above. It then stores some values in that memory, then exports a function and uses it to sum some values.

WebAssembly.instantiateStreaming(fetch('memory.wasm'), { js: { mem: memory } })
.then(obj => {
  var i32 = new Uint32Array(memory.buffer);
  for (var i = 0; i < 10; i++) {
    i32[i] = i;
  }
  var sum = obj.instance.exports.accumulate(0, 10);
  console.log(sum);
});

The second way to get a WebAssembly.Memory object is to have it exported by a WebAssembly module. This memory can be accessed in the exports property of the Web Assembly instance (after the memory is exported within the Web Assembly module). The following snippet imports a memory exported from WebAssembly with the name memory, and then prints out the first element of the memory, interpreted as an Uint32Array.

WebAssembly.instantiateStreaming(fetch('memory.wasm'))
.then(obj => {
   var i32 = new Uint32Array(obj.instance.exports.memory.buffer);
   console.log(i32[0]);
 });

By default, WebAssembly memories are unshared. You can create a shared memory by passing shared: true in the constructor's initialization object:

let memory = new WebAssembly.Memory({initial:10, maximum:100, shared: true});

This memory's buffer property will return a SharedArrayBuffer.

• WebAssembly overview page
• WebAssembly concepts
• Using the WebAssembly JavaScript API