What are the spread and rest operators?

Question

Accepted Answer

Both use `...` but in **opposite directions**: spread **expands** a collection into individual items; rest **collects** items into a single array/object.

## Spread — expand

```js
const a = [1, 2], b = [3, 4];
const merged = [...a, ...b];          // [1, 2, 3, 4]
const copy = [...a];                   // shallow copy
const obj = { ...user, active: true }; // copy object + override/add a field
Math.max(...[1, 5, 3]);                // 5 — spread array into arguments
```

## Rest — collect

```js
function sum(...nums) {     // gather all args into an array
  return nums.reduce((a, n) => a + n, 0);
}
sum(1, 2, 3);               // 6

const [first, ...others] = [10, 20, 30]; // first=10, others=[20, 30]
const { id, ...rest } = user;            // pull out id, keep the rest
```

## A crucial detail: spread is a SHALLOW copy

```js
const original = { user: { name: "Ann" } };
const copy = { ...original };
copy.user.name = "Bob"; // ⚠️ also changes original.user.name — nested object is SHARED
```

Spread copies only the top level; nested objects are still shared references. For state updates you usually only change the top level, which is fine, but be aware for deep structures.

## Why it matters

Spread is the standard way to do **immutable updates** (copy + change) in React/Redux, and rest makes variadic functions and "grab the leftovers" destructuring clean.