---
url: /docs/guide/usage/linter/rules/eslint/valid-typeof.md
---

### What it does

Enforce comparing `typeof` expressions against valid strings.

### Why is this bad?

For a vast majority of use cases, the result of the `typeof` operator is one of the
following string literals: `"undefined"`, `"object"`, `"boolean"`, `"number"`, `"string"`,
`"function"`, `"symbol"`, and `"bigint"`. It is usually a typing mistake to compare the
result of a `typeof` operator to other string literals.

### Examples

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

```js
typeof foo === "strnig";
typeof foo == "undefimed";
typeof bar != "nunber"; // spellchecker:disable-line
typeof bar !== "fucntion"; // spellchecker:disable-line
```

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

```js
typeof foo === "string";
typeof bar == "undefined";
typeof foo === baz;
typeof bar === typeof qux;
```

## Configuration

This rule accepts a configuration object with the following properties:

### requireStringLiterals

type: `boolean`

default: `false`

The `requireStringLiterals` option when set to `true`, allows the comparison of `typeof`
expressions with only string literals or other `typeof` expressions, and disallows
comparisons to any other value. Default is `false`.

With `requireStringLiterals` set to `true`, the following are examples of **incorrect** code:

```js
typeof foo === undefined;
typeof bar == Object;
typeof baz === "strnig";
typeof qux === "some invalid type";
typeof baz === anotherVariable;
typeof foo == 5;
```

With `requireStringLiterals` set to `true`, the following are examples of **correct** code:

```js
typeof foo === "undefined";
typeof bar == "object";
typeof baz === "string";
typeof bar === typeof qux;
```

## How to use

## Version

This rule was added in v0.0.3.

## References