ESLint Rules

Individual configurable linting checks that enforce code quality standards

Core Idea: ESLint rules are atomic code quality checks that can be individually enabled, disabled, or configured to enforce specific patterns and practices in JavaScript and TypeScript code.

Key Elements

Rule Structure:
- Each rule has a unique identifier (e.g., no-unused-vars)
- Rules can be set to "off" (0), "warn" (1), or "error" (2)
- Many rules accept configuration options via array syntax
- Rules can be disabled for specific lines using inline comments
- Rules can target syntax, potential bugs, or style conventions
Rule Categories:
- Possible Errors: Detect code likely to contain bugs
- Best Practices: Enforce better ways of doing things
- Variables: Ensure proper variable declaration and usage
- Stylistic Issues: Enforce consistent code formatting
- ECMAScript 6: Handle ES6/ES2015+ features
- Plugin-specific: Extended functionality from plugins

Configuration Methods:

// Different ways to configure rules
{
  rules: {
    // Basic on/off
    "no-console": "error",
    
    // With severity level
    "no-unused-vars": 2,
    
    // With options
    "quotes": ["error", "single", { "avoidEscape": true }],
    
    // Turning off a rule from an extended config
    "react/prop-types": "off"
  }
}


- **Common Core Rules**:
    
    - `no-unused-vars`: Prevent unused variable declarations
    - `no-undef`: Disallow undeclared variables
    - `eqeqeq`: Require === and !== over == and !=
    - `no-console`: Disallow console.log statements
    - `quotes`: Enforce consistent quote style
    - `semi`: Require or disallow semicolons
    - `max-len`: Enforce maximum line length
    - `indent`: Enforce consistent indentation
- **Rule Customization Example**:
    
    ```js
    // Advanced rule configuration
    {
      "rules": {
        "complexity": ["warn", 10],
        "max-depth": ["error", 4],
        "max-lines": ["warn", {"max": 300, "skipBlankLines": true}],
        "no-magic-numbers": ["error", { 
          "ignore": [-1, 0, 1, 2], 
          "ignoreArrayIndexes": true 
        }],
        "no-restricted-syntax": [
          "error", 
          {
            "selector": "ForInStatement",
            "message": "For..in loops are discouraged, use Object.keys instead."
          }
        ]
      }
    }
    ```
    

## Connections

- **Related Concepts**: ESLint (the tool that applies these rules), ESLint Configurations (collections of rule settings)
- **Broader Context**: Code Style Guides (formalized as rule configurations)
- **Applications**: Static Code Analysis (mechanism for automated code review)
- **Components**: ESLint Plugins (provide additional rules beyond core)

## References

1. ESLint Rules Documentation: https://eslint.org/docs/latest/rules/
2. "Understanding ESLint Rules" by Nicholas C. Zakas
3. "JavaScript Linting Essentials" by Cory House

#eslint #javascript #typescript #code-quality #linting-rules

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**Connections:**
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**Sources:**
- From: Syntax - Lint como un desarrollador senior con eslint + husky + lint staged + acciones de github