# Check if a Variable Exists or Is Defined in JavaScript

> Use typeof with strict inequality to check if a JavaScript variable is defined, avoiding false positives from null and false negatives from falsy values

**URL:** https://sentry.io/answers/javascript-check-if-variable-exists-is-defined-initialized/

---

## The Problem

How can I check whether a variable is defined in JavaScript? I am considering the following options:

```javascript
if (myVariable)
```

```javascript
if (myVariable != null)
```

```javascript
if (typeof myVariable !== "undefined")
```

Assuming the variable is of any type, which of these is the most accurate way of checking it is defined? Is there another, better way?

## The Solution

To check that a variable is defined, avoiding false positives and false negatives, we must check that its type is not [`undefined`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/undefined), using the [`typeof`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/typeof) operator and [strict inequality](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Strict_inequality):

```javascript
let myVariable;

if (typeof myVariable !== "undefined") {
  console.log("myVariable is defined!");
} else {
  console.log("myVariable is not defined!"); // this line will be printed
}

```

The expression `if (myVariable)` will produce false negatives, as it fails for [`falsy`](https://developer.mozilla.org/en-US/docs/Glossary/Falsy) values of `myVariable`. For example, if `myVariable` is defined as `0` or `false`, this check will fail.

```javascript
const myVariable = 0;

if (myVariable) {
  console.log("myVariable is defined!");
} else {
  console.log("myVariable is not defined!"); // this line will be printed, even though myVariable is defined above
}
```

The expression `if (myVariable != null)` will produce false negatives, as a variable can be defined with the value `null`.

```javascript
const myVariable = null;

if (myVariable !== null) {
  console.log("myVariable is defined!");
} else {
  console.log("myVariable is not defined!"); // this line will be printed, even though myVariable is defined above
}
```

`if (typeof myVariable !== 'undefined')` works best as it is highly specific and eliminates both false positives and false negatives.

For more about the difference between `undefined` and `null`, see [this answer](https://sentry.io/answers/undefined-versus-null-in-javascript/).

---

*Source: [sentry.io/answers/javascript-check-if-variable-exists-is-defined-initialized/](https://sentry.io/answers/javascript-check-if-variable-exists-is-defined-initialized/)*