What is it

Objects that own resources that can be transferred from one context to another, ensuring that the resources are only available in one context at a time.

Following a transfer, the original object is no longer usable

it no longer points to the transferred resource
any attempt to read or write the object will throw an exception

Transferable objects are commonly used to share resources that can only be safely exposed to a single JavaScript thread at a time.

// Create an 8MB "file" and fill it. 8MB = 1024 * 1024 * 8 B
const uInt8Array = new Uint8Array(1024 * 1024 * 8).map((v, i) => i);
console.log(uInt8Array.byteLength); // 8388608

// Transfer the underlying buffer to a worker
worker.postMessage(uInt8Array, [uInt8Array.buffer]);
console.log(uInt8Array.byteLength); // 0

For example, an ArrayBuffer is a transferable object that owns a block of memory.

When such a buffer is transferred between threads, the associated memory resource is detached from the original buffer and attached to the buffer object created in the new thread.

The buffer object in the original thread is no longer usable because it no longer owns a memory resource.

Typed arrays like Int32Array and Uint8Array, are serializable, but NOT transferable.

However their underlying buffer is an ArrayBuffer, which is a transferable object.

We could have sent uInt8Array.buffer in the data parameter, but not uInt8Array in the transfer array.

Transferable Object

What is it