The WebAssembly.Memory() constructor creates a new Memory object which is a resizable ArrayBuffer 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.


var myMemory = new WebAssembly.Memory(memoryDescriptor);


An object that can contain the following members:
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. In general, most WebAssembly modules shouldn't need to set a maximum.

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


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

Memory instances

All Memory instances inherit from the Memory() constructor's prototype object — this can be modified to affect all Memory instances.

Instance properties

Returns the function that created this object's instance. By default this is the WebAssembly.Memory() constructor.
An accessor property that returns the buffer contained in the memory.

Instance methods

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 example creates a new WebAssembly Memory instance with an initial size of 10 pages (640KiB), and a maximum size of 100 pages (6.4MiB).

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 our fetchAndInstantiate() utility function, 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.

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


Specification Status Comment
WebAssembly JavaScript API
The definition of 'Memory' in that specification.
Draft Initial draft definition.

Browser compatibility

Feature Chrome Edge Firefox Internet Explorer Opera Safari
Basic support 57 16 522 No 44 11
buffer 57 16 522 No 44 11
grow 57 16 522 No 44 11
prototype 57 16 522 No 44 11
Feature Android webview Chrome for Android Edge mobile Firefox for Android IE mobile Opera Android iOS Safari
Basic support 57 57 Yes1 522 No ? 11
buffer 57 57 Yes1 522 No ? 11
grow 57 57 Yes1 522 No ? 11
prototype 57 57 Yes1 522 No ? 11

1. This feature is behind the Experimental JavaScript Features preference.

2. Disabled in the Firefox 52 Extended Support Release (ESR).

See also

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