How to set a default parameter value for a JavaScript function ?

Let us first take a look at a problem and then we’ll take a look at how default values for function parameters address that problem.
Let’s first create a simple function.
Example 1:

<script> 
    /* This function is simply going to 
        multiply two numbers that are passed in*/
    var multiplyIt = function(num1, num2) { 
  
        // So we are returning num1 times num2 
        return (num1 * num2); 
    }; 
  
    /* Now lets see what happens if we call the  
    above function without passing anything in*/
    console.log(multiplyIt());  
</script> 

This statement would generate an error in some languages but in JavaScript, it is allowed. So basically, the function is going to multiply undefined by undefined.

Output:

NaN

Because the function is trying to multiply two things that are not a number.

Traditionally, you would pass values in the function but it is much simpler to establish a default value and you do it as a part of the parameters in the function.
Example 2:

<script> 
    
    //These values will only be taken if a value is not passed in 
    var multiplyIt = function(num1 = 2, num2 = 5)  
        { 
            return (num1 * num2); 
        }; 
    console.log(multiplyIt());  
</script> 

Output:

Example 3: Now try by putting in values 10 and 100.

<script> 
    
    //In this case, these values will not be considered 
    var multiplyIt = function(num1 = 2, num2 = 5)  
        { 
            return (num1 * num2); 
        }; 
    console.log(multiplyIt(10, 100)); 
  
</script> 

Output:

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