WebAssembly.Memory()

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.

Syntax

var myMemory = 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.  In general, most WebAssembly modules shouldn't need to set a maximum.

Exceptions

• 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

Memory.prototype.constructor

Returns the function that created this object's instance. By default this is the WebAssembly.Memory() constructor.

Memory.prototype.buffer

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

Instance methods

Memory.prototype.grow()

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

Examples

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. It then exports a memory from the module, stores some values in it, then exports a function and uses it to sum some values.

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

Specifications

Browser compatibility

[1] WebAssembly is enabled in Firefox 52+, although disabled in the Firefox 52 Extended Support Release (ESR.)

[2] Currently supported behind the “Experimental JavaScript Features” flag. See this blog post for more details.

See also

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