WebAssembly.Memory.prototype.grow()

pageIncrease

The number of WebAssembly pages you want to grow the memory by (each one is 64KiB in size).

The previous size of the memory, in units of WebAssembly pages.

• RangeError: If the current size added with pageIncrease exceeds the Memory instance's maximum size capacity.

The following example creates a new WebAssembly Memory instance with an initial size of 1 page (64KiB), and a maximum size of 10 pages (640KiB).

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

We can then grow the instance by one page like so:

const bytesPerPage = 64 * 1024;
console.log(memory.buffer.byteLength / bytesPerPage); // "1"
console.log(memory.grow(1)); // "1"
console.log(memory.buffer.byteLength / bytesPerPage); // "2"

Note the return value of grow() here is the previous number of WebAssembly pages.

Every call to grow will detach any references to the old buffer, even for grow(0)! Detachment means that the ArrayBuffer's byteLength becomes zero, and it no longer has bytes accessible to JavaScript. Accessing the buffer property after calling grow, will yield an ArrayBuffer with the correct length.

const memory = new WebAssembly.Memory({
  initial: 1,
});
const oldMemoryView = new Uint8Array(memory.buffer);
memory.grow(1);
// the array is empty!
console.log(oldMemoryView); // Uint8Array []

const memory = new WebAssembly.Memory({
  initial: 1,
});
memory.grow(1);
const currentMemoryView = new Uint8Array(memory.buffer);
// the array is full of zeros
console.log(currentMemoryView); // Uint8Array(131072) [ 0, 0, 0, ... ]
// 131072 = 64KiB * 2

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