Shallow copy vs deep copy

Model answer. A shallow copy creates a new top-level object, but nested objects are still shared with the original. A deep copy duplicates every level, so nothing is shared.

const original = { name: 'Asha', marks: { maths: 90 } };
const shallow = { ...original };         // or Object.assign({}, original)
shallow.marks.maths = 50;
original.marks.maths;                    // 50: nested object was shared

const deep = structuredClone(original);  // modern built-in deep copy
deep.marks.maths = 70;
original.marks.maths;                    // still 50

Ways and limits:

• Spread / `Object.assign`: shallow only.
• `structuredClone`: deep; handles dates, maps, sets and cycles, but not functions or DOM nodes.
• `JSON.parse(JSON.stringify(x))`: deep but lossy: drops functions and undefined, turns dates into strings, fails on cycles and BigInt.

In state management (React, Redux) you usually copy only the path you change, instead of deep-copying everything.