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JavaScript - Rest Parameter

Rest Parameter

The rest parameter in JavaScript allows a function to accept a variable number of arguments as an array. When the number of arguments that need to pass to the function is not fixed, you can use the rest parameters.

The JavaScript rest parameters allow you to collect all the remaining arguments in a single array. The rest parameter is represented with three dots (...) followed by a parameter name. This parameter name is the array that contains all the remaining arguments.

Rest Parameter Syntax

The rest parameter in JavaScript involves using three dots (...) followed by a parameter name in the function declaration.

function functionName(para1, para2, ...theArgs){
   // function body;
}

Here para1, and para2 are ordinary parameters while theArgs is a rest parameter. The rest parameter collects the rest of arguments (here, arguments other than the corresponding to the parameters – para1 and para1) and assigns to an array named theArgs.

We can write the rest parameter in function expression also same as in the function declaration.

The rest parameter should always be the last parameter in the function definition.

function funcName(...para1, para2, para2){}
// SyntaxError: Invalid or unexpected token

The function definition can have only one rest parameter.

function funcName(para1, ...para2, ...para3){}
//SyntaxError: Rest parameter must be last formal parameter

Example: Variable Length Parameter List

The rest parameters are very useful when you want to define a function that can handle a variable number of arguments. Let’s take the following example −

<html>
<body>
  <div> Rest parameter allows function to accept nay number of arguments.</div>
  <div id = "demo"> </div>
  <script>
    function sum(...nums) {
      let totalSum = 0;
      for (let num of nums) {
        totalSum += num;
      }
      return totalSum;
    }
    document.getElementById("demo").innerHTML = 
	 sum(10, 20, 30, 40) + "<br>" +
    sum(10, 20) + "<br>" +
    sum();
  </script>
</body>
</html>

Output

Rest parameter allows function to accept nay number of arguments.
100
30
0

Here, the rest parameter nums allows the function to accept any number of number arguments.

Example: Finding the Maximum Number

JavaScript rest parameter simplifies the process of finding the max number among a set of given numbers.

In this example, we use rest parameter to numbers to collect all arguments passed to the function. The spread operator is used to pass the individual values to the Math.max() function.

<html>
<body>
   <div> Finding the maximum number</div>
   <div id = "demo"> </div>
   <script>
      function getMax(...args){ 
         return Math.max(...args); 
      } 
      document.getElementById("demo").innerHTML = 
		getMax(10,20,30,40) + "<br>" +
      getMax(10,20,30);
  </script>
</body>
</html>

Output

Finding the maximum number
40
30

Here the rest parameter args allows the function getMax to accept any number of arguments.

Spread Operator and Rest Parameters

The spread operator (...) is closely related to rest parameters and is often used in conjunction with them. While the rest parameter collects function arguments into an array, the spread operator performs the opposite operation, spreading the elements of an array into individual arguments.

In the above example of finding the maximum number, we used both rest parameter and spread operator.

function getMax(...args){ // here ...args as rest parameter
    return Math.max(...args); // here ... works as spread operator
}

Example

In this example, the spread operator ... is used to pass the elements of the numbers array as individual arguments to the multiply function.

<html>
<body>
   <div> Spread operator in JavaScript<div>
   <div id="demo"></div>
   <script>
      function multiply(a, b, c) {
         return a * b * c;
      }
      const numbers = [2, 3, 4];
      document.getElementById("demo").innerHTML = multiply(...numbers);
   </script>
</body>
</html>

Output

Spread operator in JavaScript
24

Rest Parameter vs. Arguments Object

The introduction of rest parameters has implications for how we handle variable-length parameter lists compared to using the arguments object. Let's compare the two approaches:

Rest Parameters

<html>
<body>
  <div> Sum using rest parameter in JavaScript:</div>
  <div id = "demo"> </div>
  <script>
    function sum(...numbers) {
      return numbers.reduce((total, num) => total + num, 0);
    }
    document.getElementById("demo").innerHTML = sum(1, 2, 3, 4, 5);
  </script>
</body>
</html>

Output

Sum using rest parameter in JavaScript:
15

Arguments Object

<html>
<body>
  <div> Sum using arguments object in JavaScript:</div>
  <div id = "demo"> </div>
  <script>
    function sum() {
      const argsArray = Array.from(arguments);
      return argsArray.reduce((total, num) => total + num, 0);
    }
    document.getElementById("demo").innerHTML = sum(1, 2, 3, 4, 5);
  </script>
</body>
</html>

Output

Sum using arguments object in JavaScript:
15

While both approaches achieve the same result, the rest parameter syntax is more concise and readable. It also behaves more consistently with other modern JavaScript features.

Destructuring with Rest Parameter

The destructuring assignment is introduced in ES6. It allows us to access the individual values of the array without using array indexing. We can use the destructuring assignment to extract the values from the array created by rest parameter.

Example

In the example below, the destructuring assignment extracts the first two elements from the numbers array.

<html>
<body>
  <div> Destructuring assignment with rest parameter</div>
  <div id = "demo"> </div>
  <script>
    function getFirstTwo(...numbers) {
      const [first, second] = numbers;
      return `First: ${first}, Second: ${second}`;
    } 
    document.getElementById("demo").innerHTML = getFirstTwo(1, 2, 3, 4, 5);
  </script>
</body>
</html>

Output

Destructuring assignment with rest parameter
First: 1, Second: 2
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