Spread and rest
- Node.js course
- Beginner
- Lesson 11/116
- 5 min
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.
Practice 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).
Starter code
const a = [1, 2];
const b = [3, 4];
// create combined with spread
// log it with console.logSample solution
const a = [1, 2];
const b = [3, 4];
const combined = [...a, ...b];
console.log(combined);Expected output
[ 1, 2, 3, 4 ]
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Spread/rest usages
| Term | Meaning |
|---|---|
[...a, ...b] |
Merge arrays |
{ ...obj } |
Copy an object |
...args |
Rest parameter |
f(...arr) |
Spread array as arguments |
2 Merge with spread
const combined = [____];const combined = [...a, ...b];Explanation: ...a, ...b opens and merges two arrays.
3 What does the rest parameter do?
- Collects remaining arguments into an array Correct answer
- Copies an array
- Takes the first argument
- Reverses an array
Explanation: Rest collects remaining arguments into one array.