structuredClone — proper deep copy

The right way to deep-copy

JSON.parse(JSON.stringify(x)) is the classic deep-copy hack, but:

• Loses Date objects (become strings).
• Loses Map and Set (become empty objects).
• Loses undefined values (become missing keys).
• Throws on cycles.

structuredClone is the standard answer since 2022:

const original = {
  d: new Date(),
  set: new Set([1, 2, 3]),
  nested: { arr: [1, 2, 3] },
};

const copy = structuredClone(original);
copy.nested.arr.push(4);
original.nested.arr.length; // 3, untouched

What it handles

Dates, Maps, Sets, ArrayBuffers, typed arrays, Blobs, even circular references. Symbols and functions don't survive (they're not serializable — same as JSON.stringify).

When to reach for it

Anywhere you'd previously use the JSON.parse hack — Redux state, undo stacks, "snapshot then mutate" patterns. C9-09 uses it for an undo stack.