Explain the difference between shallow copy and deep copy

A shallow copy duplicates the top-level properties of an object, but nested objects are still referenced. A deep copy duplicates all levels of an object, creating entirely new instances of nested objects. Object.assign() and the spread operator (...) create shallow copies. structuredClone() is the modern built-in for deep copies. JSON.parse(JSON.stringify()) and Lodash's _.cloneDeep are other common approaches, each with different tradeoffs around which values they can faithfully clone.

// Shallow copy - nested object is shared
let obj1 = { a: 1, b: { c: 2 } };
let shallowCopy = { ...obj1 };
shallowCopy.b.c = 3;
console.log(obj1.b.c); // 3 - original mutated too

// Deep copy - fully independent
let obj2 = { a: 1, b: { c: 2 } };
let deepCopy = structuredClone(obj2);
deepCopy.b.c = 4;
console.log(obj2.b.c); // 2 - original unchanged