What is the difference between shallow copy and deep copy?

Shallow copy creates a new object/array but copies references to nested objects — mutating a nested property affects both copies.

Deep copy recursively clones nested structures so the copies are fully independent.

const original = { a: 1, b: { c: 2 } }

// Shallow
const shallow = { ...original }
shallow.b.c = 99
original.b.c // 99 — shared reference

// Deep (simple — JSON trick)
const deep = structuredClone(original) // modern API
deep.b.c = 42
original.b.c // 99 — unchanged

Common shallow copy methods

• Spread: { ...obj }, [...arr]
• Object.assign({}, obj)
• Array.prototype.slice()

Deep copy approaches

• structuredClone(obj) — built-in, handles most types (not functions, symbols, prototypes)
• JSON.parse(JSON.stringify(obj)) — loses undefined, functions, Date, Map, circular refs
• Libraries (Lodash cloneDeep) for complex graphs

Interview trap: { ...obj } is not a deep clone. Nested objects remain aliased.