What is the difference between a shallow and a deep copy?

Question

Accepted Answer

Objects are assigned **by reference**, so `b = a` doesn't copy — both point to the same object. A **shallow** copy duplicates the top level but shares nested references; a **deep** copy duplicates everything, fully independent.

```js
const original = { name: "Ann", address: { city: "NY" } };

// shallow copy: spread / Object.assign
const shallow = { ...original };
shallow.name = "Bob";            // ✅ independent (top-level primitive)
shallow.address.city = "LA";     // ⚠️ ALSO changes original.address.city — shared!

original.address.city; // "LA"  ← mutated through the shallow copy
```

The nested `address` object is the **same reference** in both, so mutating it affects both.

## Making a deep copy

```js
const deep = structuredClone(original); // modern, built-in, handles most types
deep.address.city = "Chicago";          // ✅ original is untouched
```

Other options and their caveats:

```js
JSON.parse(JSON.stringify(obj)); // simple, but LOSES functions, Dates → strings, undefined
// lodash: _.cloneDeep(obj)       // robust, for complex cases
```

## Why it matters

This is a top source of "why did my state change unexpectedly?" bugs, especially in React/Redux where you must not mutate state.

Spread is fine when you only change top-level fields, but for nested updates you must copy the nested level too (`{ ...obj, address: { ...obj.address, city } }`) or do a deep clone.