WebAssembly.Memory

WebAssembly.Memory()

Creates a new Memory object.

Memory.prototype.buffer Read only

Returns the buffer contained in the memory.

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.

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 an Uint32Array.

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

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.

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