WebAssembly.memory

The Memory() constructor of the WebAssembly global object creates a new Memory object instance, which represents a WebAssembly memory. These can be thought of as resizeable array buffers, wrappers around WebAssembly memorys.

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 WebAssembly pages.

maximum Optional

The maximum size the WebAssembly Memory is allowed to grow to, in WebAssembly pages.

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 constructor methods

None.

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

The following example creates a new WebAssembly Memory instance with an initial size of 10 pages (640KB), and a maximum size of 100 pages (6.4MB).

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

Note that you can also represent a memory instance in JavaScript by exporting it from 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.

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

Specifications

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

Browser compatibility

[1] Experimental support can be enabled in Chrome 51+ and Opera 38+ by going to chrome://flags and enabling the Experimental WebAssembly flag.

[2] Experimental support can be enabled in Firefox 47+ by enabling the javascript.options.wasm flag in about:config.

See also

• Memory instance page
• WebAssembly landing page.
• WebAssembly concepts
• Using the WebAssembly JavaScript API