Consider that 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 CutomerName(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 CutomerName(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.