---
url: /docs/guide/usage/linter/rules/eslint/no-undef.md
---

### What it does

Disallow the use of undeclared variables.

This rule can be disabled for TypeScript code, as the TypeScript compiler
enforces this check.

### Why is this bad?

It is most likely a potential ReferenceError caused by a misspelling
of a variable or parameter name.

### Examples

Examples of **incorrect** code for this rule:

```javascript
var foo = someFunction();
var bar = a + 1;
```

## Configuration

This rule accepts a configuration object with the following properties:

### typeof

type: `boolean`

default: `false`

When set to `true`, warns on undefined variables used in a `typeof` expression.

## How to use

## Version

This rule was added in v0.0.8.

## References