How to declare the optional function parameters in JavaScript ?

In this article, we will see how to declare optional function parameters in JavaScript.

These are the following approaches to doing so:

Using the Logical OR operator (‘||’)

In this approach, the optional parameter is Logically OR is used 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’: 

Javascript

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

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’: 

Javascript

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

Output

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

Using argument variable

In this approach, we are checking the length of the argument and by using if-else we are returning results accordingly.

Example: This exanpmle shows the implementation of the above-explained appraoch.

Javascript

function gfg(a, b) {
    // No parameters are passed
    if (arguments.length == 0) {
        a = "hello";
        b = "geeks"
    }
    // Only one parameter is passed
    if (arguments.length == 1) {
        b = "geeks";
    }
    return `${a + b}`;
}

console.log(gfg("hey"));

Output

heygeeks