WebAssembly.Memory() constructor
Baseline
Widely available
This feature is well established and works across many devices and browser versions. It’s been available across browsers since October 2017.
The WebAssembly.Memory() constructor creates a new Memory object whose buffer property is a resizable ArrayBuffer or SharedArrayBuffer that holds the raw bytes of memory accessed by a WebAssembly.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 create Memory 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 of WebAssembly pages. When present, the
maximumparameter acts as a hint to the engine to reserve memory up front. However, the engine may ignore or clamp this reservation request. Unshared WebAssembly memories don't need to set amaximum, but shared memories do. -
A boolean value that defines whether the memory is a shared memory or not. If set to
true, it is a shared memory. The default isfalse.
Note: A WebAssembly page has a constant size of 65,536 bytes, i.e., 64KiB.
Exceptions
TypeError-
Thrown if at least one of these conditions is met:
memoryDescriptoris not an object.initialis not specified.sharedis present andtrue, yetmaximumis not specified.
RangeError-
Thrown if at least one of these conditions is met:
maximumis specified and is smaller thaninitial.initialexceeds 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 a WebAssembly.Memory object: construct it from JavaScript, or have it exported by a WebAssembly module.
The following example (see memory.html on GitHub, and view 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 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. The Memory object's buffer property will return an ArrayBuffer.
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 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.
Specifications
| Specification |
|---|
| WebAssembly JavaScript Interface # dom-memory-memory |
The shared attribute is only documented in the Threading proposal for WebAssembly and not part of the official specs.
Browser compatibility
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