JavaScript Rest parameter

Last Updated : 16 Jan, 2026

The rest parameter (...) allows a function to accept any number of arguments and collect them into a single array, making functions more flexible and dynamic.

  • Represented using three dots (...) followed by a parameter name
  • Collects multiple arguments into an array
  • Must always be the last parameter in a function
  • Useful for handling variable or unknown numbers of arguments
  • Simplifies function definitions compared to the arguments object
JavaScript 
function addNumbers(...numbers) {
  let sum = 0;

  for (let num of numbers) {
    sum += num;
  }

  return sum;
}

console.log(addNumbers(1, 2));        
console.log(addNumbers(1, 2, 3, 4));

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: 

Understanding How the Rest Parameter Works

[Example 1]: Without Using the Rest Parameter

Javascript code demonstrating the passing arguments more than the parameters without using rest parameter.

JavaScript 
function fun(a, b){
        return a + b;
    }
    console.log(fun(1, 2)); // 3
    console.log(fun(1, 2, 3, 4, 5)); // 3
  • Extra arguments beyond parameters are ignored without error.
  • Use ... to collect additional arguments into an array.
  • Rest parameters allow flexible argument handling for functions.

[Example 2]: Using the Rest Parameter

JavaScript code demonstrating the addition of numbers using the rest parameter. 

JavaScript 
// 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

We can calculate the sum of all arguments passed to the fun() function by iterating over them as an array using the for..of loop.

  • The rest parameter collects all arguments into an array.
  • The for..of loop traverses each element and adds it to the sum.

Note: The rest parameter must be the last argument because it gathers all remaining values into an array; placing any parameter after it causes a syntax error.

[Example 3]: Using Rest Parameter with Other Parameters

In this example, we are using the rest parameter with some other arguments inside a function.

javascript 
// 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');
  • The rest parameter collects extra arguments into an array, handling more than the defined parameters.
  • In the function, c[0] returns 'Lionel' from the rest parameter array.
  • The rest parameter is an array, allowing use of standard array methods for further manipulation.
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