The WebAssembly.Memory object is a resizable ArrayBuffer or SharedArrayBuffer that holds raw bytes of memory accessed by a WebAssembly.Instance.

Both WebAssembly and JavaScript can create Memory objects. If you want to access the memory created in JS from WebAssembly, or vice versa, you can export the memory from the module to JavaScript or import memory from JavaScript to the module when it is instantiated.

Originally you could only perform memory operations on a single memory in the Wasm module, so while multiple Memory objects could be created, there wasn't any point in doing so. More recent implementations allow WebAssembly memory instructions to operate on a specified memory. For more information see Multiple memories in Understanding WebAssembly text format.

Constructor

WebAssembly.Memory()

Creates a new Memory object.

Instance properties

Memory.prototype.buffer Read only

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 64KiB in size). Detaches the previous buffer.

Examples

Creating a new Memory object

There are two ways to get a WebAssembly.Memory object. The first way is to construct it from JavaScript. The following snippet creates a new WebAssembly Memory instance with an initial size of 10 pages (640KiB), and a maximum size of 100 pages (6.4MiB). Its buffer property will return an ArrayBuffer.

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

The following example (see memory.html on GitHub, and view it live also) 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. Note the use of DataView to access the memory so we always use little-endian format.

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);
});

Another way to get a WebAssembly.Memory object is to have it exported by a WebAssembly module. This memory can be accessed in the exports property of the WebAssembly instance (after the memory is exported within the WebAssembly module). The following example imports a memory exported from WebAssembly with the name memory, and then prints out the first element of the memory, interpreted as a Uint32Array.

WebAssembly.instantiateStreaming(fetch("memory.wasm")).then((obj) => {
  const values = new DataView(obj.instance.exports.memory.buffer);
  console.log(values.getUint32(0, true));
});

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.

Using a 64-bit address

The following snippet creates a new WebAssembly Memory instance with a 64-bit address type. The initial and maximum values must be BigInt values:

const memory = new WebAssembly.Memory({
  address: "i64",
  initial: 1n,
  maximum: 10n,
});

Specifications

Browser compatibility

webassembly.api.Memory

webassembly.multiMemory

See also

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