JavaScript Spread Operator

The Spread operator allows an iterable to expand in places where 0+ arguments are expected. It is mostly used in the variable array where there is more than 1 value is expected. It allows us the privilege to obtain a list of parameters from an array. 

The syntax of the Spread operator is the same as the Rest parameter but it works completely opposite of it. 

Syntax:

let variablename1 = [...value]; 

In the above syntax, … is a spread operator which will target all values in a particular variable. When … occurs in the function call or alike, it’s called a spread operator. Spread operator can be used in many cases, like when we want to expand, copy, concat, with math object. Let’s look at each of them one by one: 

Note: In order to run the code in this article make use of the console provided by the browser.

Concat(): The concat() method provided by javascript helps in the concatenation of two or more strings(String concat() ) or is used to merge two or more arrays. In the case of arrays, this method does not change the existing arrays but instead returns a new array. 

Example: This example shows the above-explained approach.

javascript

// normal array concat() method
let arr = [1, 2, 3];
let arr2 = [4, 5];

arr = arr.concat(arr2);

console.log(arr); // [ 1, 2, 3, 4, 5 ]

Output:

[1, 2, 3, 4, 5]

 We can achieve the same output with the help of the spread operator, the code will look something like this: 

Example:

javascript

// spread operator doing the concat job
let arr = [1, 2, 3];
let arr2 = [4, 5];

arr = [...arr, ...arr2];
console.log(arr); // [ 1, 2, 3, 4, 5 ]

Output:

[1,2,3,4,5]

Note: Though we can achieve the same result, it is not recommended to use the spread in this particular case, as for a large data set it will work slower when compared to the native concat() method.

Copy(like splice method): In order to copy the content of an array to another we can do something like this: 

Example:

javascript

// copying without the spread operator
let arr = ['a', 'b', 'c'];
let arr2 = arr;

console.log(arr2); // [ 'a', 'b', 'c' ]

Output:

[a,b,c]

The above code works fine because we can copy the contents of one array to another, but under the hood, it’s very different as when we mutate a new array it will also affect the old array(the one which we copied). See the code below: 

Example:

javascript

// changed the original array
let arr = ['a', 'b', 'c'];
let arr2 = arr;

arr2.push('d');

console.log(arr2);
console.log(arr);
//even affected the original array(arr)

Output:

['a', 'b', 'c', 'd']
['a', 'b', 'c', 'd']

In the above code we can clearly see that when we tried to insert an element inside the array, the original array is also altered which we didn’t intend and is not recommended. We can make use of the spread operator in this case, like this: 

Example:

javascript

// spread operator for copying
let arr = ['a', 'b', 'c'];
let arr2 = [...arr];

console.log(arr);
// [ 'a', 'b', 'c' ]

arr2.push('d');
//inserting an element at the end of arr2

console.log(arr2);
// [ 'a', 'b', 'c', 'd' ]
console.log(arr);
 // [ 'a', 'b', 'c' ]

Output:

(3) ['a', 'b', 'c']
(4) ['a', 'b', 'c', 'd']
(3) ['a', 'b', 'c']

By using the spread operator we made sure that the original array is not affected whenever we alter the new array.

Expand: Whenever we want to expand an array into another we do something like this: 

Example:

javascript

// normally used expand method
let arr = ['a', 'b'];
let arr2 = [arr, 'c', 'd'];

console.log(arr2);
// [ [ 'a', 'b' ], 'c', 'd' ]

Output:

(3) [Array(2), 'c', 'd']
0: (2) ['a', 'b']
1: "c"
2: "d"
length: 3
[[Prototype]]: Array(0)

Even though we get the content on one array inside the other one, actually it is an array inside another array which is definitely what we didn’t want. If we want the content to be inside a single array we can make use of the spread operator. 

Example:

javascript

// expand using spread operator

let arr = ['a', 'b'];
let arr2 = [...arr, 'c', 'd'];

console.log(arr2);
// [ 'a', 'b', 'c', 'd' ]

Output:

(4) ['a', 'b', 'c', 'd']

Math: The Math object in javascript has different properties that we can make use of to do what we want like finding the minimum from a list of numbers, finding the maximum, etc. Consider the case that we want to find the minimum from a list of numbers, we will write something like this: 

Example:

javascript

console.log(Math.min(1,2,3,-1)); //-1

Output:

-1

Now consider that we have an array instead of a list, this above Math object method won’t work and will return NaN, like: 

Example:

javascript

// min in an array using Math.min()
let arr = [1,2,3,-1];
console.log(Math.min(arr)); //NaN

Output:

NaN

When …arr is used in the function call, it “expands” an iterable object arr into the list of arguments In order to avoid this NaN output, we make use of a spread operator, like: 

Example:

javascript

// with spread
let arr = [1,2,3,-1];

console.log(Math.min(...arr));
//-1

Output:

-1

Example of spread operator with objects: ES6 has added spread property to object literals in javascript. The spread operator (…) with objects is used to create copies of existing objects with new or updated values or to make a copy of an object with more properties. Let’s take an example of how to use the spread operator on an object, 

Example:

javascript

const user1 = {
    name: 'Jen',
    age: 22
};

const clonedUser = { ...user1 };
console.log(clonedUser);

Output:

{name: 'Jen', age: 22}

Here we are spreading the user1 object. All key-value pairs of the user1 object are copied into the clonedUser object. Let’s look at another example of merging two objects using the spread operator, 

Example:

javascript

const user1 = {
    name: 'Jen',
    age: 22,
};

const user2 = {
    name: "Andrew",
    location: "Philadelphia"
};

const mergedUsers = { ...user1, ...user2 };
console.log(mergedUsers);

Output:

{name: 'Andrew', age: 22, location: 'Philadelphia'}

The mergedUsers is a copy of user1 and user2. Actually, every enumerable property on the objects will be copied to the mergedUsers object. The spread operator is just a shorthand for the Object.assign() method but, there are some differences between the two.

We have a complete list of Javascript Operators, to check those please go through the Javascript Operators Complete Reference article.

JavaScript is best known for web page development but it is also used in a variety of non-browser environments. You can learn JavaScript from the ground up by following this JavaScript Tutorial and JavaScript Examples.