Answers by Sentry

Undefined versus null in JavaScript

David Y.

The Problem

What is the difference between the values undefined and null in JavaScript?

The Solution

When a variable has been declared but not yet assigned a value, it will have the type and value undefined. undefined is one of JavaScript’s primitive types.

let myVar;

console.log(myVar); // will print "undefined"
console.log(typeof myVar); // will also print "undefined"

When a variable has not been declared or assigned, it will also be of type undefined and throw a ReferenceError when accessed. Therefore, typeof will produce the same result for declared but undefined variables and undeclared variables.

console.log(undeclaredVar); // will throw a ReferenceError
console.log(typeof undeclaredVar); // will print "undefined"

In contrast, null is a value that represents nothing. Think of null as an empty container and undefined as the absence of a container. A variable will only have the value null when it is explicitly assigned, and will be of type object.

const myVar = null;

console.log(myVar); // will print "null"
console.log(typeof myVar); // will print "object"

The difference between undefined and null is important when using JavaScript’s equality operators. null and undefined are considered the same when using loose equality (==) but not when using strict equality (===).

console.log(undefined == null); // will print "true"
console.log(undefined === null); // will print "false"

Get Started With Sentry

Get actionable, code-level insights to resolve JavaScript performance bottlenecks and errors.

  1. Create a free Sentry account

  2. Create a JavaScript project and note your DSN

  3. Grab the Sentry JavaScript SDK

<script src="https://browser.sentry-cdn.com/7.110.1/bundle.min.js"></script>
  1. Configure your DSN
Sentry.init({ dsn: 'https://<key>@sentry.io/<project>' });

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