How can I Check for "undefined" in JavaScript?

The Problem

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:

value === undefined

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

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:

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.