# JavaScript: How to Check for Undefined Values

> Check if a value is undefined in JavaScript using strict equality or the typeof operator, and why undefined can be reassigned as a variable name

**URL:** https://sentry.io/answers/how-can-i-check-for-undefined-in-javascript/

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## 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 (`===`)](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Strict_equality) to check if a value is `undefined`:

```javascript
value === undefined
```

You can also check if the type of a value is `undefined` using the [`typeof`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/typeof) operator:

```javascript
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`:

```javascript
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.

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*Source: [sentry.io/answers/how-can-i-check-for-undefined-in-javascript/](https://sentry.io/answers/how-can-i-check-for-undefined-in-javascript/)*