How to determine if a variable is undefined or null in JavaScript?

Hello everyone! I just started learning JavaScript and encountered this problem. In my code, I often need to check whether a variable exists or not, but I get confused between `undefined` and `null`. For example, I have a function that takes a parameter `name`. If the user didn't pass a name, I want to display the message "Name not specified". But if they passed `null`, I also want to handle that as the absence of a name. Here's what I tried: ```javascript function greet(name) { if (name == undefined) { console.log("Name not specified"); } else { console.log("Hello, " + name); } } greet(); // outputs "Name not specified" — works greet(null); // outputs "Hello, null" — but should output "Name not specified"! ``` Why doesn't `name == undefined` catch `null`? I read that there is an op