function buildName(firstName: string, lastName = "Smith") {
    return firstName + " " + lastName;
}

let result1 = buildName("Bob");                //works correctly now, returns "Bob Smith"
let result2 = buildName("Bob", undefined);     //still works, also returns "Bob Smith"
let result3 = buildName("Bob", "Adams", "Sr.");//error, too many parameters
let result4 = buildName("Bob", "Adams");       //ah, just right