Sentry Answers>JavaScript>

How can I Check for "undefined" in JavaScript?

How can I Check for "undefined" in JavaScript?

Matthew C.

The ProblemJump To Solution

You want to check if a value is undefined. For example, you may want to check if the value returned from a fetch request is defined before trying to access its properties. How do you do this?

The Solution

There are two common ways to check if a value is undefined.

You can use the strict equality operator (===) to check if a value is undefined:

Click to Copy
value === undefined

You can also check if the type of a value is undefined using the typeof operator:

Click to Copy
typeof value === 'undefined'

An interesting thing to note is that undefined is not a reserved word in JavaScript. A reserved word is a keyword that can’t be used as an identifier for naming things such as variables, properties, or functions. Reserved words include: import, const, and return. You can use undefined as a variable name, as long as the variable is not in the global scope. As can be seen in the example code below, you can make the typeof undefined equal to string:

Click to Copy
function myFunction() { const undefined = "string value"; console.log(undefined); // string value console.log(typeof undefined); // string } myFunction();

You can name variables undefined, but you shouldn’t because it will create bugs.

  • ResourcesImprove Web Browser Performance - Find the JavaScript code causing slowdowns
  • ResourcesJavaScript Frontend Error Monitoring 101
  • Syntax.fm logo
    Listen to the Syntax Podcast

    Tasty treats for web developers brought to you by Sentry. Get tips and tricks from Wes Bos and Scott Tolinski.

    SEE EPISODES

Loved by over 4 million developers and more than 90,000 organizations worldwide, Sentry provides code-level observability to many of the world’s best-known companies like Disney, Peloton, Cloudflare, Eventbrite, Slack, Supercell, and Rockstar Games. Each month we process billions of exceptions from the most popular products on the internet.

© 2024 • Sentry is a registered Trademark
of Functional Software, Inc.