The difference between Rest and spread operator in JavaScript

In this tutorial, we will learn the difference between rest and spread operator in JavaScript.

Rest Parameters ( … )

Rest parameters help us to pass an infinite number of function arguments.

Example:

function sum(a,b,...remaining){
     console.log(a);
     console.log(b);
     console.log(remaining);
}

sum(1,2,3,4,5,6,7);

//a -> 1

//b - > 2

//remaining - > [3,4,5,6,7]

In the above code, the sum function accepts three arguments a, b and remaining.where the first two arguments values are 1, 2 everything we passed after is represented as an array [3,4,5,6,7].

Spread operator

Spread operator helps us to expand the strings or array literals or object literals.

• Splitting the strings

 let name = "JavaScript";

 let arrayOfStrings = [...name];

 console.log(arrayOfStrings);

 // Ouptut -> ["J", "a", "v", "a", "S", "c", "r", "i", "p", "t"]

In the above example, we are using ... spread operator to split the single string into an array of strings.

• Merging arrays

const group1 = [1,2,3];
const group2 = [4,5,6];

const allGroups = [...group1,...group2];

console.log(allGroups)

//output -> [1, 2, 3, 4, 5, 6]

In the above code, we combined group1 and group2 by using the spread operator.

• Merging Objects

const obj1 = {
      a: 1,
      b: 2
}

const obj2 = {
      c: 3,
      d: 4
}

const merge  = {...obj1, ...obj2};

console.log(merge); // {a:1, b:2, c:3, d:4}

Using Spread operator in Function calls

We can use the spread operator to pass an array of numbers as a individual function arguments.

function sum(a,b,c){
    return a+b+c;
}

const nums = [1,2,3];

//function calling

sum(...nums) // 6

Summary

• If you see (…) dots on the function call then it is a spread operator.
• If you see (…) dots on the function parameter then it is a rest parameter.
• Spread operator helps us to merge Arrays or Objects.