Major Differences between ESLint v9 and Previous Versions:

Feature	ESLint v9+ (Flat Config)	ESLint v8 and below (.eslintrc)
Config Format	JavaScript-only flat config (eslint.config.js)	JSON, YAML, or JS (.eslintrc)
File Structure	Flat array of config objects (more predictable merge order)	Hierarchical and deep merging of config
Config File Name	eslint.config.js	.eslintrc.js, .eslintrc.json, etc.
Plugins	Must be imported explicitly using import or require	Auto-loaded by name convention
Extends	No extends keyword — you spread config into the array manually	Uses extends: 'airbnb-base' etc.
Performance	Faster, deterministic resolution order	Slower with more complex merging
Scoping	Config applies only to matching files (using files/ignores)	Can be inherited by directory hierarchy
TypeScript Support	Better integration with @eslint/js and modern bundlers	Requires additional setup or plugin support

1. Config Format Flat Config (ESLint v9)

import js from '@eslint/js';
export default [
  js.configs.recommended,
  {
    rules: {
      'eqeqeq':'error',
    },
  },
];

Old Config

module.exports = {
  extends: ['eslint:recommended'],
  rules: {
   'eqeqeq':'error',
  },
};

1. File Structure: Flat vs Nested Flat Config (v9): No inheritance, flat array.Each config block is evaluated in order, top-down.

export default [
  {
    files: ['**/*.js'],
    rules: {
      'no-console': 'warn',
    },
  },
  {
    files: ['src/**/*.js'],
    rules: {
      'no-console': 'error',
    },
  },
];

Nested: Deep merge + inheritance. ESLint merges configs up the directory tree.

// root/.eslintrc.js
module.exports = {
  rules: {
    'no-console': 'warn',
  },
};

// root/src/.eslintrc.js
module.exports = {
  rules: {
    'no-console': 'error', // overrides parent
  },
};

1. Extends vs Import Flat Config (v9): No extends, uses direct imports

import js from '@eslint/js';
import airbnb from 'eslint-config-airbnb-base';

export default [
  js.configs.recommended,
  ...airbnb, // must be spread manually if it's an array
];

.eslintrc: Uses extends keyword

module.exports = {
  extends: ['eslint:recommended', 'airbnb-base'],
};

1. Plugins Usage: Flat Config: Plugins must be imported manually

import js from '@eslint/js';
import myPlugin from 'eslint-plugin-myplugin';

export default [
  js.configs.recommended,
  {
    plugins: {
      my: myPlugin,
    },
    rules: {
      'my/custom-rule': 'error',
    },
  },
];

.eslintrc: Plugins are auto-loaded by name

module.exports = {
  plugins: ['myplugin'],
  rules: {
    'myplugin/custom-rule': 'error',
  },
};