ESNext Configuration

/**
 * Base ESNext ESLint configuration
 * Accessible as: "esnext" or "recommended/esnext"
 */
env:
  es6: true               # Enable ES6 globals and features
  commonjs: true          # Enable CommonJS global variables

parser: babel-eslint      # Use babel-eslint for parsing modern syntax

parserOptions:
  ecmaVersion: 2018       # Support up to ES2018 features
  sourceType: module      # Enable ES6 modules
  ecmaFeatures:
    impliedStrict: true   # Apply strict mode to all code
    modules: true         # Enable ES6 module features

extends:
  - eslint:recommended              # Base ESLint recommended rules
  - plugin:import/errors            # Import plugin error rules
  - plugin:import/warnings          # Import plugin warning rules

/**
 * ESNext-specific safety rules
 * These rules are added on top of eslint:recommended
 */
rules: {
  // Array and callback rules
  "array-callback-return": "error",        // Enforce return in array callbacks
  
  // Arrow function rules  
  "arrow-body-style": "error",             // Require braces around arrow function bodies as needed
  
  // Class rules
  "class-methods-use-this": "error",       // Disallow class methods that don't use this
  
  // Property access rules
  "dot-notation": "error",                 // Enforce dot notation for property access
  
  // Equality rules
  "eqeqeq": "error",                      // Require === and !==
  
  // Function rules
  "no-alert": "error",                     // Disallow alert, confirm, prompt
  "no-empty-function": "error",            # Disallow empty functions
  "no-else-return": "error",              // Disallow else after return in if
  "no-eval": "error",                     // Disallow eval()
  "no-extend-native": "error",            // Disallow extending native objects
  "no-extra-bind": "error",               // Disallow unnecessary bind()
  "no-global-assign": "error",            // Disallow assignment to global variables  
  "no-implicit-globals": "error",         // Disallow var and function declarations in global scope
  "no-implied-eval": "error",             // Disallow implied eval()
  "no-invalid-this": "error",             // Disallow this outside class/function context
  "no-lonely-if": "error",                // Disallow if as only statement in else block
  "no-loop-func": "error",                // Disallow functions in loops
  "no-new": "error",                      // Disallow new without assignment
  "no-new-func": "error",                 // Disallow Function constructor
  "no-new-wrappers": "error",             // Disallow primitive wrapper constructors
  "no-proto": "error",                    // Disallow __proto__ property
  "no-script-url": "error",               // Disallow javascript: URLs
  "no-self-compare": "error",             // Disallow self comparison
  "no-throw-literal": "error",            // Disallow throwing literal values
  "no-unmodified-loop-condition": "error", // Require loop condition to be modified
  "no-unneeded-ternary": ["error", { "defaultAssignment": false }], // Disallow unnecessary ternary
  "no-unsafe-negation": "error",          // Disallow negation of left operand of relational operators
  "no-unused-expressions": ["error", {   // Disallow unused expressions
    "allowTernary": true,                 // Allow ternary as shorthand for if/else
    "allowShortCircuit": true             // Allow && and || as shorthand for if
  }],
  "no-use-before-define": ["error", "nofunc"], // Disallow use before define (ignore functions)
  "no-useless-call": "error",             // Disallow unnecessary call() and apply()
  "no-useless-computed-key": "error",     // Disallow unnecessary computed property keys
  "no-useless-concat": "error",           // Disallow unnecessary string concatenation
  "no-useless-constructor": "error",      // Disallow unnecessary constructors
  "no-useless-rename": "error",           // Disallow renaming to same name
  "no-var": "error",                      // Require let/const instead of var
  "no-with": "error",                     // Disallow with statements
  
  // Object rules
  "object-shorthand": "error",            // Require object shorthand syntax
  
  // Operator rules  
  "operator-assignment": "error",         // Require assignment operator shorthand
  
  // ES6 preference rules
  "prefer-arrow-callback": "error",       // Prefer arrow functions as callbacks
  "prefer-const": "error",                // Prefer const for never reassigned variables
  "prefer-rest-params": "error",          // Prefer rest parameters over arguments
  "prefer-spread": "error",               // Prefer spread over apply()
  
  // Condition rules (override eslint:recommended)
  "no-constant-condition": ["error", { "checkLoops": false }] // Allow infinite generators
}

/**
 * Import plugin rules for module validation
 * Included via extends: plugin:import/errors and plugin:import/warnings
 */
rules: {
  // Module resolution (errors)
  "import/no-unresolved": "error",        // Ensure imports resolve to actual files
  "import/named": "error",                // Ensure named imports exist in target module
  "import/namespace": "error",            // Ensure namespace imports are valid
  "import/default": "error",              // Ensure default import exists when used
  "import/export": "error",               // Report invalid exports
  
  // Import best practices (warnings)  
  "import/no-named-as-default": "warn",   // Warn on using exported name as default identifier
  "import/no-named-as-default-member": "warn", // Warn on using export as property of default
  
  // ESNext-specific import rules (errors)
  "import/no-amd": "error",               // Disallow AMD require/define
  "import/no-commonjs": "error",          // Disallow CommonJS require/exports
  "import/no-duplicates": "error",        // Disallow duplicate imports
  "import/no-extraneous-dependencies": "error", // Disallow imports not in package.json
  "import/no-mutable-exports": "error",   // Disallow var/let exports
  "import/no-namespace": "error",         // Disallow namespace imports
  "import/no-nodejs-modules": "error",    // Disallow Node.js built-in modules
  "import/prefer-default-export": "error" // Prefer default export for single export
}

# .eslintrc.yaml
extends:
  - esnext

# Optional project-specific overrides
rules:
  no-console: warn

# .eslintrc.yaml  
extends:
  - esnext

rules:
  # Allow console in development
  no-console: off
  
  # Stricter import rules
  import/no-extraneous-dependencies: [error, { devDependencies: false }]
  
  # Allow some flexibility
  class-methods-use-this: warn

# .eslintrc.yaml
extends:
  - esnext

rules:
  # Libraries should avoid console
  no-console: error
  
  # Prefer named exports for libraries
  import/prefer-default-export: off
  
  # Allow CommonJS for Node.js compatibility  
  import/no-commonjs: off

// ❌ Error: Use of var
var name = 'John';
var items = [];

// ✅ Correct: Use let/const
const name = 'John';
let items = [];

// ❌ Error: Variable never reassigned
let config = { debug: true };
let API_URL = 'https://api.example.com';

// ✅ Correct: Use const for never-reassigned variables
const config = { debug: true };
const API_URL = 'https://api.example.com';

// ❌ Error: Duplicate imports
import { map } from 'lodash';
import { filter } from 'lodash';

// ✅ Correct: Single import statement
import { map, filter } from 'lodash';

// ❌ Error: CommonJS in ES6 module
const fs = require('fs');
module.exports = { helper };

// ✅ Correct: ES6 imports/exports  
import fs from 'fs';
export { helper };

// ❌ Error: Unnecessary block body
const double = x => { return x * 2; };

// ✅ Correct: Concise arrow function
const double = x => x * 2;

// ✅ Also correct: Block body when needed
const process = x => {
  console.log(`Processing ${x}`);
  return x * 2;
};

// ❌ Error: Function callback instead of arrow
items.map(function(item) {
  return item.name;
});

// ✅ Correct: Arrow function callback
items.map(item => item.name);

npm install --save-dev eslint-config-recommended

# .eslintrc.yaml
extends:
  - recommended/esnext

npm install --save-dev eslint-config-esnext

# .eslintrc.yaml
extends:
  - esnext