Optional and default parameters

Say, for example, we have a function with three parameters, and sometimes we may only pass values for the first two parameters in the function. In TypeScript, we can handle such scenarios using the optional parameter. We can define the first two parameters as normal and the third parameter as optional, as given in the following code snippet:

function CustomerName(firstName: string, lastName: string, middleName?: string) { 
    if (middleName) 
        return firstName + " " + middleName + " " + lastName; 
    else 
        return firstName + " " + lastName; 
} 
//ignored optional parameter middleName 
var customer1 = customerName("Rajesh", "Gunasundaram"); 
//error, supplied too many parameters 
var customer2 = customerName("Scott", "Tiger", "Lion", "King");  
//supplied values for all 
var customer3 = customerName("Scott", "Tiger", "Lion");  

Here, middleName is the optional parameter, and it can be ignored when calling the function.

Now, let's see how to set default parameters in a function. If a value is not supplied to a parameter in the function, we can define it to take the default value that is configured:

function CustomerName(firstName: string, lastName: string, middleName: 
     string = 'No Middle Name') { 
    if (middleName) 
        return firstName + " " + middleName + " " + lastName; 
    else 
        return firstName + " " + lastName; 
} 

Here, middleName is the default parameter that will have No Middle Name by default if the value is not supplied by the caller.