function buildName(firstName = "Will", lastName: string) { return firstName + " " + lastName; } let result1 = buildName("Bob"); // error, too few parameters let result2 = buildName("Bob", "Adams", "Sr."); // error, too many parameters let result3 = buildName("Bob", "Adams"); // okay and returns "Bob Adams" let result4 = buildName(undefined, "Adams"); // okay and returns "Will Adams"