To declare optional function parameters in JavaScript, there are two approaches:

• 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 on the parameter list.

  Syntax:

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

  Example 1: In the following program the optional parameter is ‘b’:

  
  
  
  
  
  

  
  
  

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

  Output:
  

• 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 on the parameter list.

  Syntax:

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

  Example 2: In the following program the optional parameter is ‘b’:

  
  

  
  
  

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

  Output: