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For ordinary data that the JavaScript structured clone algorithm supports, use structuredClone(value):
const copy = structuredClone(value);It creates a deep copy, including nested data, and can reproduce circular references. It does not preserve every JavaScript object’s behavior, so the right method depends on what the object contains.
Use structuredClone() for nested data
structuredClone() is the built-in option for copying many common JavaScript values without retaining references to the original nested objects. For example:
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user: { name: "Ada" },
tags: ["typescript", "javascript"],
};
const copy = structuredClone(original);
copy.user.name = "Grace";
// original.user.name remains "Ada"Changing the nested object in the copy does not change the corresponding nested object in the original. The method is part of the JavaScript runtime, not a TypeScript-only feature. See MDN’s documentation for the structuredClone() method.
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What it can clone
Structured-cloneable values include plain objects and arrays, as well as built-ins such as dates, maps, sets, array buffers, data views, typed arrays, regular expressions, and errors. Primitive values other than symbols are supported too. See MDN’s list of types handled by the structured clone algorithm.
Cloning circular references
A circular reference points back to an object already encountered—for example, an object whose self property refers to itself. structuredClone() supports cycles by tracking references it has already visited:
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const original: { label: string; self?: unknown } = { label: "node" };
original.self = original;
const copy = structuredClone(original);
// The cloned self-reference points back to copy.The copied cycle points within the copy rather than back to the original. A simple recursive copying function may fail on cycles unless it also tracks previously visited objects.
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A deep copy does not necessarily mean a behaviorally identical object. The structured clone algorithm copies supported data, but it does not preserve arbitrary object behavior or every property detail.
- Class behavior: The original prototype chain is not cloned, so a class instance does not return as an instance with its original methods. Private class elements are not preserved.
- Property details: Property descriptors and accessors—including getters and setters—are not preserved.
- Unsupported values: Functions and DOM nodes cannot be cloned and cause a
DataCloneError.
If a copy must retain a class’s invariants or methods, use an explicit constructor or factory, or implement a domain-specific clone method. Reconstructing the instance makes the intended behavior explicit rather than relying on a generic data-copy operation.
Choose the cloning approach that fits the data
| Approach | Use it when | Main limitation |
|---|---|---|
structuredClone(value) |
The value is structured-cloneable and the runtime provides the API. | It supports cycles and many built-ins, but does not retain functions, DOM nodes, custom prototype behavior, property descriptors, or private fields. |
JSON.parse(JSON.stringify(value)) |
The data is deliberately limited to JSON-serializable values. | JSON serialization cannot represent all JavaScript values or object details, so the round trip can omit information. |
| Explicit reconstruction or a type-specific clone method | Class identity, invariants, or custom semantics must be retained. | You must define the correct copying behavior for the types in your application. |
When JSON serialization is enough
For data intentionally restricted to JSON, a stringify/parse round trip can make a deep copy:
const copy = JSON.parse(JSON.stringify(value));Do not treat this as a general-purpose clone. JSON serialization may omit properties or other aspects of the source; use it only when the data shape and those losses are acceptable. MDN explains the limits of serializable objects and the meaning of a deep copy.
Check TypeScript declarations and runtime support
If TypeScript reports that structuredClone is unknown, check the built-in declarations available to the project. TypeScript’s declarations depend on configuration such as target, and the lib option controls which built-in APIs TypeScript knows about. The TypeScript Handbook explains type declarations and library declaration structures.
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Then check the actual JavaScript runtime where the code will run. A TypeScript declaration only affects type checking; it does not add or polyfill a missing runtime API. Verify support for your target environment rather than relying on a declaration alone.
Copying versus transferring
structuredClone() also accepts a transfer option for transferable resources. A transfer is not an ordinary copy: the resource moves to the clone and becomes unusable on the original. Use it only when that change to the original is intended; otherwise, clone without transferring.
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