WebAssembly.Memory() constructor

The WebAssembly.Memory() constructor creates a new Memory object whose buffer property 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.

memoryDescriptor

An object that can contain the following members:

initial

The initial size of the WebAssembly Memory, in units of WebAssembly pages.

maximum Optional

The maximum size the WebAssembly Memory is allowed to grow to, in units of WebAssembly pages. When present, the maximum parameter acts as a hint to the engine to reserve memory up front. However, the engine may ignore or clamp this reservation request. Unshared WebAssembly memories don't need to set a maximum, but shared memories do.

shared Optional

A boolean value that defines whether the memory is a shared memory or not. If set to true, it is a shared memory. The default is false.

• If memoryDescriptor is not of type object, a TypeError is thrown.
• If maximum is specified and is smaller than initial, a RangeError is thrown.

There are two ways to get a WebAssembly.Memory object. The first way is to construct it from JavaScript. The following example 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 second way to get a WebAssembly.Memory object is to have it exported by a WebAssembly module. 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);
});

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