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# valid-typeof

[Added in v0.4.2](https://github.com/web-infra-dev/rslint/releases/tag/v0.4.2)
## Configuration

| Preset                   | Configured Value |
| ------------------------ | ---------------- |
| ✅ js.configs.recommended | `"error"`        |


```ts title=rslint.config.ts
import { defineConfig, js } from '@rslint/core';

export default defineConfig([
  js.configs.recommended,
  {
    rules: {
      'valid-typeof': 'error',
    },
  },
]);
```

## Rule Details

Enforces comparing `typeof` expressions against valid string literals. The `typeof` operator can only return one of the following strings: `"undefined"`, `"object"`, `"boolean"`, `"number"`, `"string"`, `"function"`, `"symbol"`, `"bigint"`. Comparing a `typeof` expression against any other value is almost certainly a bug.

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

```javascript
typeof foo === 'strnig';
typeof foo == 'undefned';
typeof bar != 'nunber';
typeof bar !== 'fucntion';
typeof foo === undefined;
```

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

```javascript
typeof foo === 'string';
typeof bar == 'undefined';
typeof baz === 'object';
typeof qux !== 'function';
typeof foo === typeof bar;
```

## Options

### `requireStringLiterals`

When set to `true`, requires that `typeof` expressions are only compared to string literals or other `typeof` expressions, and disallows comparisons to any other value.

Examples of additional **incorrect** code with `{ "requireStringLiterals": true }`:

```javascript
typeof foo === undefined;
typeof foo === Object;
typeof foo === someVariable;
```

## Original Documentation

- [ESLint: valid-typeof](https://eslint.org/docs/latest/rules/valid-typeof)
- [Source code](https://github.com/eslint/eslint/blob/v10.8.1/lib/rules/valid-typeof.js)