structuredClone() global function

Baseline 2022

Newly available

Since March 2022, this feature works across the latest devices and browser versions. This feature might not work in older devices or browsers.

Note: This feature is available in Web Workers

The global structuredClone() method creates a deep clone of a given value using the structured clone algorithm.

The method also allows transferable objects in the original value to be transferred rather than cloned to the new object. Transferred objects are detached from the original object and attached to the new object; they are no longer accessible in the original object.

Syntax

js
structuredClone(value)
structuredClone(value, options)

Parameters

value

The object to be cloned. This can be any structured-cloneable type.

options Optional

An object with the following properties:

transfer

An array of transferable objects that will be moved rather than cloned to the returned object.

Return value

The returned value is a deep copy of the original value.

Exceptions

DataCloneError DOMException

Thrown if any part of the input value is not serializable.

Description

This function can be used to deep copy JavaScript values. It also supports circular references, as shown below:

js
// Create an object with a value and a circular reference to itself.
const original = { name: "MDN" };
original.itself = original;

// Clone it
const clone = structuredClone(original);

console.assert(clone !== original); // the objects are not the same (not same identity)
console.assert(clone.name === "MDN"); // they do have the same values
console.assert(clone.itself === clone); // and the circular reference is preserved

Transferring values

Transferable objects (only) can be transferred rather than duplicated in the cloned object, using the transfer property of the options parameter. Transferring makes the original object unusable.

Note: A scenario where this might be useful is when asynchronously validating some data in a buffer before saving it. To avoid the buffer being modified before the data is saved, you can clone the buffer and validate that data. If you also transfer the data, any attempts to modify the original buffer will fail, preventing its accidental misuse.

The following code shows how to clone an array and transfer its underlying resources to the new object. On return, the original uInt8Array.buffer will be cleared.

js
// 16MB = 1024 * 1024 * 16
const uInt8Array = Uint8Array.from({ length: 1024 * 1024 * 16 }, (v, i) => i);

const transferred = structuredClone(uInt8Array, {
  transfer: [uInt8Array.buffer],
});
console.log(uInt8Array.byteLength); // 0

You can clone any number of objects and transfer any subset of those objects. For example, the code below would transfer arrayBuffer1 from the passed in value, but not arrayBuffer2.

js
const transferred = structuredClone(
  { x: { y: { z: arrayBuffer1, w: arrayBuffer2 } } },
  { transfer: [arrayBuffer1] },
);

Examples

Cloning an object

In this example, we clone an object with one member, which is an array. After cloning, changes to each object do not affect the other object.

js
const mushrooms1 = {
  amanita: ["muscaria", "virosa"],
};

const mushrooms2 = structuredClone(mushrooms1);

mushrooms2.amanita.push("pantherina");
mushrooms1.amanita.pop();

console.log(mushrooms2.amanita); // ["muscaria", "virosa", "pantherina"]
console.log(mushrooms1.amanita); // ["muscaria"]

Transferring an object

In this example we create an ArrayBuffer and then clone the object it is a member of, transferring the buffer. We can use the buffer in the cloned object, but if we try to use the original buffer we will get an exception.

js
// Create an ArrayBuffer with a size in bytes
const buffer1 = new ArrayBuffer(16);

const object1 = {
  buffer: buffer1,
};

// Clone the object containing the buffer, and transfer it
const object2 = structuredClone(object1, { transfer: [buffer1] });

// Create an array from the cloned buffer
const int32View2 = new Int32Array(object2.buffer);
int32View2[0] = 42;
console.log(int32View2[0]);

// Creating an array from the original buffer throws a TypeError
const int32View1 = new Int32Array(object1.buffer);

Specifications

Specification
HTML Standard
# dom-structuredclone

Browser compatibility

BCD tables only load in the browser

See also