What is the difference between shallow copy and deep copy in JavaScript?

const original = { name: "Robin", skills: ["HTML"] };

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

original.skills.push("CSS");

console.log(shallow.skills);
console.log(deep.skills);

Output:

The spread { ...original } copies only the top level, so skills is still the same array. structuredClone() copies everything. (JSON.parse(JSON.stringify(obj)) also works, but it loses functions and dates.)

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