Spread and rest

Learn spread and rest parameters with the ... operator.

What is spread?

The spread operator is written with ... and 'spreads' an array or object: it opens its content item by item.

Array example: const a = [1, 2], const b = [3, 4]. Writing const combined = [...a, ...b] makes combined equal to [1, 2, 3, 4].

Object example: const copy = { ...user }. This copies all fields of user into a new object.

Rest parameter

The rest parameter is also written with ... but does the opposite: it collects the remaining arguments into a single array. It is used in function definitions.

Example: function sum(...nums). When called as sum(1, 2, 3), nums becomes [1, 2, 3]. You can pass as many arguments as you want.

So ... means rest (collects) in a function definition, and spread (opens) in a call or assignment.

Exercise Merge arrays with spread

Declare two arrays: a = [1, 2] and b = [3, 4]. Use the spread operator to create a single array named combined. Then call console.log(combined).

Show example solution

const a = [1, 2];
const b = [3, 4];
const combined = [...a, ...b];
console.log(combined);

Test yourself

Spread/rest usages

[...a, ...b]

Merge arrays
{ ...obj }

Copy an object
...args

Rest parameter
f(...arr)

Spread array as arguments

Merge with spread

const combined = [____];

Show answer

const combined = [...a, ...b];

What does the rest parameter do?
- Collects remaining arguments into an array
- Copies an array
- Takes the first argument
- Reverses an array
Show answer

Collects remaining arguments into an array — Rest collects remaining arguments into one array.

Next lesson: Summary - Variables

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