How can you use the typeof operator to check if a variable is undefined in JavaScript?

In JavaScript, you can use the `typeof` operator to check if a variable is undefined. When `typeof` is applied to an undefined variable, it returns the string `"undefined"`. Here's an example:

let myVariable;  // Declaring a variable without assigning a value

if (typeof myVariable === "undefined") {

    console.log("myVariable is undefined");

} else {

    console.log("myVariable is defined");

}

In this example, `typeof myVariable` is used to check if `myVariable` is undefined. If the result is equal to the string `"undefined"`, it means the variable is indeed undefined, and the corresponding message will be logged.

It's important to note that when checking for the existence of a variable, you should use `typeof` specifically and compare the result against the string `"undefined"`. Avoid comparing directly with `undefined` to avoid potential issues in case someone reassigns the value of `undefined`. For example, consider this scenario:

let undefined = "I'm not really undefined!";

let myVariable;

if (typeof myVariable === "undefined") {

    console.log("myVariable is undefined");

} else {

    console.log("myVariable is defined");

}

In this case, using `typeof myVariable === "undefined"` is more robust and avoids potential conflicts with a variable named `undefined` that has a different value.

Modern JavaScript also allows using the `===` (strict equality) operator with `undefined` directly, but using `typeof` is a more established and consistent practice across different codebases.