How to Check For Undefined in JavaScript / TypeScript
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: "kai@example.com",
}
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
.