javascript deep clone an object with structuredClone()

structuredClone(value) returns a deep copy of a value: nested objects and arrays are copied too, so changing the copy never touches the original. It is a global function in every current browser and in Node.js 17+, needs no library, and also copies Date, Map, Set, typed arrays and circular references.

Shallow copy vs deep copy

cat > clone.mjs <<'EOF'
const original = {
  name: "config",
  created: new Date("2026-01-02T03:04:05Z"),
  tags: new Set(["a", "b"]),
  nested: { retries: 3 },
};
original.self = original; // circular reference

const shallow = { ...original };
const deep = structuredClone(original);

shallow.nested.retries = 10;
console.log("after shallow edit:", original.nested.retries);
deep.nested.retries = 99;
console.log("after deep edit:", original.nested.retries);
console.log("Date kept:", deep.created instanceof Date, deep.created.toISOString());
console.log("Set kept:", deep.tags instanceof Set, [...deep.tags]);
console.log("circular kept:", deep.self === deep);
EOF
node clone.mjs

after shallow edit: 10
after deep edit: 10
Date kept: true 2026-01-02T03:04:05.000Z
Set kept: true [ 'a', 'b' ]
circular kept: true

The spread copy shares nested with the original, so its edit leaks back; the structuredClone copy does not. The Date stays a Date, the Set stays a Set, and the circular reference points at the new object.

What cannot be cloned

Functions, DOM nodes and some host objects make structuredClone throw a DataCloneError. Class instances lose their prototype and become plain objects; property getters are copied as their current values.

cat > fail.mjs <<'EOF'
class Point { constructor(x) { this.x = x; } norm() { return Math.abs(this.x); } }
const copy = structuredClone(new Point(-2));
console.log("class instance:", copy instanceof Point, copy);
try {
  structuredClone({ onClick() {} });
} catch (e) {
  console.log(e.name + ":", e.message);
}
EOF
node fail.mjs

class instance: false { x: -2 }
DataCloneError: onClick() {} could not be cloned.

Ways to copy an object

Pitfalls

• Old runtimes. On Node.js 16 and older, structuredClone is missing and the call fails with ReferenceError: structuredClone is not defined (see x is not defined). Upgrade Node or use the JSON round trip for plain data.
• Lost methods. A cloned class instance has no methods; calling one fails with x is not a function. Re-create the instance from the copied data if you need behaviour.
• Hand-written recursive copies of deeply nested or circular data can overflow the stack; structuredClone handles cycles for you.

Confirm it worked

Change a nested field on the copy and print the original: if the original is unchanged, the copy is deep. copy !== original alone proves nothing, because a shallow copy is also a new object.

FAQ