How to declare the optional function parameters in JavaScript ?

In this article, we will see how to declare optional function parameters in JavaScript. There are two approaches to do so:

• Using the Logical OR operator (‘||’)
• Using the Assignment operator (“=”)

Using the Logical OR operator (‘||’): In this approach, the optional parameter is Logically ORed with the default value within the body of the function. 

Note: The optional parameters should always come at the end of the parameter list. 

Syntax:

function myFunc(a,b) {
  b = b || 0;
  // b will be set either to b or to 0.
}

Example: In the following program the optional parameter is ‘b’: 

html

<script>
    function check(a, b) {
        b = b || 0;
        console.log("Value of a is: " + a +
                    " Value of b is: " + b);
    }
    check(5, 3);
    check(10);
</script>

Output:

Value of a is: 5 Value of b is: 3
Value of a is: 10 Value of b is: 0

Using the Assignment operator (“=”): In this approach the optional variable is assigned the default value in the declaration statement itself. Note: The optional parameters should always come at the end of the parameter list. 

Syntax:

function myFunc(a, b = 0) {
   // function body
}

Example: In the following program the optional parameter is ‘b’: 

html

<script>
    function check(a, b = 0) {
        console.log("Value of a is: " + a +
                    " Value of b is: " + b);
    }
    check(9, 10);
    check(1);
</script>

Output:

Value of a is: 9 Value of b is: 10
Value of a is: 1 Value of b is: 0