TheWebAssembly.Memory() constructor creates a newMemory object whosebuffer property is a resizableArrayBuffer orSharedArrayBuffer that holds the raw bytes of memory accessed by aWebAssembly.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 createMemory 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.

maximumOptional

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

sharedOptional

A boolean value that defines whether the memory is a shared memory or not. Ifset totrue, it is a shared memory. The default isfalse.

Exceptions

TypeError

Thrown if at least one of these conditions is met:

• memoryDescriptor is not an object.
• initial is not specified.
• shared is present andtrue, yetmaximum is not specified.

RangeError

Thrown if at least one of these conditions is met:

• maximum is specified and is smaller thaninitial.
• initial exceeds 65,536 (2^16). 2^16 pages is 2^16 * 64KiB = 4GiB bytes, which is the maximum range that a Wasm module can address, as Wasm currently only allows 32-bit addressing.
• Allocation fails. This may occur due to attempting to allocate too much at once, or if the User Agent is otherwise out of memory.

Examples

Creating a new Memory instance

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

The following example (seememory.html on GitHub, andview 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 theWebAssembly.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. TheMemory object'sbuffer property will return anArrayBuffer.

const memory = new WebAssembly.Memory({  initial: 10,  maximum: 100,});WebAssembly.instantiateStreaming(fetch("memory.wasm"), {  js: { mem: memory },}).then((obj) => {  const summands = new DataView(memory.buffer);  for (let i = 0; i < 10; i++) {    summands.setUint32(i * 4, i, true); // WebAssembly is little endian  }  const sum = obj.instance.exports.accumulate(0, 10);  console.log(sum);});

Creating a shared memory

By default, WebAssembly memories are unshared.You can create ashared memoryfrom JavaScript by passingshared: true in the constructor's initialization object:

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

This memory'sbuffer property will return aSharedArrayBuffer.

Specifications

Theshared attribute is only documented inthe Threading proposal for WebAssembly and not part of the official specs.

Browser compatibility

See also

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