This post was originally published at kais.blog.
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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
.
Let's move your learning forward together! Follow me on Twitter for your daily dose of developer tips. Thanks for reading my content!
This post was originally published at kais.blog.
Top comments (2)
Have a look at the new syntax sugar here: developer.mozilla.org/en-US/docs/W...
or
developer.mozilla.org/en-US/docs/W...
Thanks for this addition. Nullish coalescing operator (
??
) and optional chaining (?.
) are indeed very useful. Nevertheless, the real check forundefined
is the one I've described. If you want to check if the property isundefined
like in my example - a simpleif
-statement withtypeof
is the way to go.