JavaScript Rest parameter

The rest parameter is an improved way to handle function parameters, allowing us to more easily handle various inputs as parameters in a function. The rest parameter syntax allows us to represent an indefinite number of arguments as an array. With the help of a rest parameter, a function can be called with any number of arguments, no matter how it was defined. Rest parameter is added in ES2015 or ES6 which improved the ability to handle parameter.

Syntax:

//... is the rest parameter (triple dots)
function functionname(...parameters)
{
statement;
}

Note: When … is at the end of the function parameter, it is the rest parameter. It stores n number of parameters as an array. Let’s see how the rest parameter works: 

Example: Let’s see how the rest parameter works: 

Javascript

<script>
    // Without rest parameter
    function fun(a, b){
        return a + b;
    }
    console.log(fun(1, 2)); // 3
    console.log(fun(1, 2, 3, 4, 5)); // 3
</script>

Output:

3
3

In the above code, no error will be thrown even when we are passing arguments more than the parameters, but only the first two arguments will be evaluated. It’s different in the case of the rest parameter. With the use of the rest parameter, we can gather any number of arguments into an array and do what we want with them. 

Example: Javascript code demonstrating the addition of numbers using the rest parameter. 

Javascript

<script>
    // es6 rest parameter
    function fun(...input){
        let sum = 0;
        for(let i of input){
            sum+=i;
        }
        return sum;
    }
    console.log(fun(1,2)); //3
    console.log(fun(1,2,3)); //6
    console.log(fun(1,2,3,4,5)); //15
</script>

Output:

3
6
15

We were able to get the sum of all the elements that we enter in the argument when we call the fun() function. We get the sum of all the elements in the array by making use of the for..of loop which is used to traverse the iterable elements inside an array. 

Note: The rest parameter has to be the last argument, as its job is to collect all the remaining arguments into an array. So having a function definition like the code below doesn’t make any sense and will throw an error. 

javascript

<script>
    // non-sense code
    function fun(a,...b,c){
         //code
         return;
    }
</script>

Let’s make use of the rest parameter with some other arguments inside a function, like this: 

javascript

<script>
    // rest with function and other arguments
    function fun(a,b,...c){
        console.log(`${a} ${b}`); //Mukul Latiyan
        console.log(c);  //[ 'Lionel', 'Messi', 'Barcelona' ]
        console.log(c[0]); //Lionel
        console.log(c.length); //3
        console.log(c.indexOf('Lionel')); //0
    }
    fun('Mukul','Latiyan','Lionel','Messi','Barcelona');
</script>

Output:

Mukul Latiyan
['Lionel', 'Messi', 'Barcelona']
Lionel
3
0

In the above code sample, we passed the rest parameter as the third parameter, and then we basically called the function fun() with five arguments the first two were treated normally and the rest were all collected by the rest parameter hence we get ‘Lionel’ when we tried to access c[0] and it is also important to note that the rest parameter gives an array in return and we can make use of the array methods that the javascript provides us.

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