The correct way to check if something is undefined in JavaScript / TypeScript: Use the typeof operator! It returns a string showing the type of the unevaluated operand.

Let’s say you have an object user that looks like this:

const user = {
  email: "[email protected]",
}

If you use the typeof operator on the email and id property, you'll get this:

console.log(typeof user.email); // "string"
console.log(typeof user.id); // "undefined"

Thus, to check for an undefined property you can combine an if-statement and the typeof operator:

if (typeof user.id === "undefined") {
  //
}

Easy! Yet, there are other ways to check for undefined. However, those are sometimes quirky and will throw a ReferenceError. So, I suggest that you always use the typeof operator if you are checking for undefined.