> ## Documentation Index
> Fetch the complete documentation index at: https://mintlify.com/eslint/eslint/llms.txt
> Use this file to discover all available pages before exploring further.

# Rule Categories

> Understanding ESLint rule categories and their purposes

# Rule Categories

ESLint rules are organized into three main categories based on their purpose. Understanding these categories helps you configure rules appropriately for your project.

## Problem Rules

Problem rules identify code that will cause errors or could lead to bugs. These rules help catch mistakes before runtime.

**Characteristics:**

* Prevent runtime errors
* Catch logical mistakes
* Identify code that doesn't work as intended
* Usually should be set to "error" level

**Examples:**

* [array-callback-return](/rules/array-callback-return) - Missing return statements in array methods can cause bugs
* [no-await-in-loop](/rules/no-await-in-loop) - Sequential awaits may indicate a parallelization opportunity
* [no-constant-binary-expression](/rules/no-constant-binary-expression) - Expressions that always evaluate the same way are usually mistakes
* [no-constructor-return](/rules/no-constructor-return) - Returning values from constructors is likely an error
* [no-unreachable-loop](/rules/no-unreachable-loop) - Loops that can't iterate more than once are bugs

<Warning>
  Problem rules should rarely be disabled as they catch real errors. If you need to disable one, add a comment explaining why.
</Warning>

## Suggestion Rules

Suggestion rules recommend better ways to write code. They don't prevent errors but encourage best practices and more maintainable code.

**Characteristics:**

* Enforce coding conventions
* Improve code quality and readability
* Prevent potential issues
* Can often be configured based on team preferences

**Examples:**

* [block-scoped-var](/rules/block-scoped-var) - Treat var as if it were block scoped
* [default-case](/rules/default-case) - Require default cases in switch statements
* [eqeqeq](/rules/eqeqeq) - Require === and !== instead of == and !=
* [no-eval](/rules/no-eval) - Disallow dangerous eval() function
* [prefer-promise-reject-errors](/rules/prefer-promise-reject-errors) - Require Error objects in promise rejections

<Tip>
  Suggestion rules are great for establishing team coding standards. Configure them based on your project's style guide.
</Tip>

## Layout Rules

Layout rules care about code formatting and visual consistency. They don't affect how code executes, only how it looks.

**Characteristics:**

* Enforce consistent formatting
* Improve code readability
* Prevent style inconsistencies
* Can often be auto-fixed
* Many are now deprecated in favor of Prettier

**Examples:**

* [array-bracket-spacing](/rules/array-bracket-spacing) - Spacing inside array brackets
* [brace-style](/rules/brace-style) - Consistent brace placement
* [comma-dangle](/rules/comma-dangle) - Trailing commas in object/array literals
* [indent](/rules/indent) - Consistent indentation
* [quotes](/rules/quotes) - Consistent quote style
* [semi](/rules/semi) - Semicolon usage

<Note>
  Many teams now use Prettier for formatting and disable layout rules. Consider using Prettier with eslint-config-prettier to avoid conflicts.
</Note>

## Choosing Rule Severity

How to set severity levels based on category:

### Problem Rules → Error

```js theme={null}
rules: {
  'no-unreachable-loop': 'error',
  'no-constructor-return': 'error'
}
```

### Suggestion Rules → Error or Warn

```js theme={null}
rules: {
  'eqeqeq': 'error',           // Enforce strictly
  'no-alert': 'warn',          // Allow but discourage
  'prefer-const': 'warn'       // Suggest but don't block
}
```

### Layout Rules → Error or Off

```js theme={null}
rules: {
  'semi': ['error', 'always'], // If not using Prettier
  'indent': 'off',             // If using Prettier
  'quotes': 'off'              // Let Prettier handle it
}
```

## Recommended Configurations

ESLint provides preset configurations:

* `eslint:recommended` - Core set of problem rules
* `eslint:all` - All available rules (not recommended for production)

```js theme={null}
module.exports = {
  extends: ['eslint:recommended'],
  rules: {
    // Override or add additional rules
  }
};
```