Hey there! I am shafeek makandar, student at iNeuron. I love learning new stuffs and while doing so I also wanted to share my knowledge through this blogs.

Today we will learn about Spread and Rest operators in javascript 😊

• spread : used to unpack the elements from array or object.
• rest : used to pack the elements from array or object.

note : both the operators denoted with ... dots, but their use cases are different.

Spread

Let's understand where we use spread operator 👇

1. To make copy or clone of an array or object.

const array = [ 1, 2, 3, 4, 5 ];

const newArray = [ ...array ];

console.log(newArray); // output : [ 1, 2, 3, 4, 5 ]

const user = {
  id : '1',
  name : 'john',
  city : 'newyork'
}


const newUser = {
  ...user,
}
console.log(newUser); // output: { id: '1', name: 'john', city: 'newyork' }

note: both user & newUser are two different objects having same element.

• point to be note that spread will only make shallow copy not deep clone, let's understand with example.

const products = {
 id : '1',
 name : [ 'mobile', 'tablet'];
}

const newProducts = {
  ...products,
}

products.name.push('laptop');
console.log(newProducts); //output : { id: '1', name: [ 'mobile', 'tablet', 'laptop' ] }

noticed in the above code, when we push "laptop" into the products objects name property, newProducts properties also updated.

Note : when there is nested elements present in object or array, spread operator only copy the reference of the property.

1. merge array or objects.

We can merge arrays or objects using spread operator

const array_1 = [ 1, 2, 3 ];

const array_2 = [ 4, 5, 6 ];

const newArray = [ ...array_1, ...array_2 ];

console.log(newArray); //output: [ 1, 2, 3, 4, 5, 6 ]

  const user = {
   name : 'hitesh',
   profession : 'CTO at iNeuron',
  } 

  const adress = {
   city : 'bangalore'
  }

  const newUsers = {
   ...user,
   ...adress,
  }
  console.log(newUsers); //output : { name: 'hitesh', profession: 'CTO at iNeuron', city: 'bangalore' }

1. converting string into array

const name = "iNeuron";

const array = [ ...name ];

console.log(array); //output: [ 'i', 'N', 'e', 'u', 'r', 'o', 'n' ];

Rest

• mainly we use rest operator in the functions parameters or arguments.

for example we have array, given that to calculate sum.

 function add(num1, num2, num3){
 return num1 + num2 + num3;
}
console.log(add(5, 3, 6)); // output : 14

• but by using rest operator also we can achive this

let sum = 0;
 function add(...array){
 console.log(array); // output : [ 5, 3, 6 ]
 for(let element of array){
 sum = sum + element;
}
return sum;
}
console.log(add(5, 3, 6)); // output : 14

Note : what rest operator does is, it packs the parameters into array. when we log the array we will get [ 5, 3, 6 ].