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 object created by JavaScript or in WebAssembly code will be accessible and mutable from both JavaScript and WebAssembly, provided that the code constructed the object, or has been given the object.

Both WebAssembly and JavaScript can create Memory objects. If you want to access the memory created in JS from Wasm or vice versa, you can pass a reference to the memory from one side to the other.

Syntax

new WebAssembly.Memory(memoryDescriptor)

Parameters

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.

Note: A WebAssembly page has a constant size of 65,536 bytes, i.e., 64KiB.

Exceptions

  • If memoryDescriptor is an object, a TypeError is thrown.
  • If initial is not specified, a TypeError is thrown.
  • If maximum is specified and is smaller than initial, a RangeError is thrown.
  • If shared is present and true, yet maximum is not specified, a TypeError is thrown.

Examples

Creating a new Memory instance

There are two ways to get a WebAssembly.Memory object: construct it from JavaScript, or have it exported by a WebAssembly module.

The following example (see memory.html on GitHub, and view it live also) creates a new WebAssembly Memory instance with an initial size of 10 pages (640KiB), and a maximum size of 100 pages (6.4MiB). The example fetches and instantiates the loaded memory.wasm bytecode using the WebAssembly.instantiateStreaming() function, while importing the memory created in the line above. It then stores some values in that memory, exports a function, and uses the exported function to sum those values. The Memory object's buffer property will return an ArrayBuffer.

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

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

Creating a shared memory

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

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

This memory's buffer property will return a SharedArrayBuffer.

Specifications

Specification
WebAssembly JavaScript Interface: Exception Handling
# dom-memory-memory

The shared attribute is only documented in the Threading proposal for WebAssembly and not part of the official specs.

Browser compatibility

BCD tables only load in the browser

See also