fn_undefined.js

function greet(message, username) {
    // undefined are FALSY values in JS
    // !undefined means !false -> true
    if(!username) { // if username is not yet defined
        username = "user"; // we are assinging it some value
    }
   var greetMessage = `Hello, ${username}! ${message}`;
   console.log(greetMessage);
}
greet("Good evening");
greet("Good evening", "arjun");