structuredClone

Learn to deep copy an object with structuredClone.

Deep copy of an object

If you copy an object with =, you only copy its reference; nested objects are shared. The structuredClone method produces a deep copy: every nested value is duplicated as a new object.

const original = { list: [1, 2] };
const copy = structuredClone(original);
copy.list.push(3);
// original.list is still [1, 2]

This lets you work on an independent copy without harming the original.

Circular references

If an object has a property that points back to itself, this is a circular reference. Copying with = keeps it, but classic deep-copy approaches can crash.

structuredClone handles circular references correctly; the structure is preserved.

const a = {};
a.self = a;
const copy = structuredClone(a);
// copy.self === copy

Exercise Make a deep copy

Make a deep copy of the template object and assign it to copy. Use the structuredClone method.

Show example solution

Call structuredClone with template.
Example solution:
const copy = structuredClone(template);

Test yourself

What does structuredClone produce?
```
structuredClone({a: {b: 1}})
```
- A deep copy
- A shallow copy
- A reference
- A string
Show answer

A deep copy — Correct! structuredClone makes a deep, independent copy.

Change the copy, keep the original

const copy = structuredClone(original);
copy.list.____(3);

Show answer

const copy = structuredClone(original);
copy.list.push(3);

Copying an object with a circular reference
```
const a = {}; a.self = a;
```
- structuredClone throws
- structuredClone copies correctly
- Only = works
- No method works
Show answer

structuredClone copies correctly — Correct! structuredClone supports circular references.

Next lesson: JSON limitations

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