ESLint 9完全指南：新特性、重大变化与升级攻略

// eslint.config.js
export default [
  {
    name: 'my-configuration',
    files: ['**/*.js'],
    ignores: ['**/dist/**'],
    languageOptions: {
      ecmaVersion: 2022,
      sourceType: 'module',
      globals: {
        browser: true,
        node: true,
        es2022: true
      }
    },
    linterOptions: {
      noInlineConfig: false,
      reportUnusedDisableDirectives: 'warn'
    },
    plugins: {
      react: reactPlugin
    },
    rules: {
      'no-unused-vars': 'error',
      'react/react-in-jsx-scope': 'off'
    }
  }
];

// eslint.config.js
import js from '@eslint/js'

export default [
  js.configs.recommended,
  {
    rules: {
      'no-unused-vars': 'warn'
    }
  }
]

// eslint.config.js
import { FlatCompat } from '@eslint/eslintrc'
import path from 'path'
import { fileURLToPath } from 'url'

const __filename = fileURLToPath(import.meta.url)
const __dirname = path.dirname(__filename)

const compat = new FlatCompat({
  baseDirectory: __dirname
})

export default [
  ...compat.extends('standard', 'react'),
  ...compat.env({
    es2022: true,
    node: true
  }),
  ...compat.plugins('react'),
  {
    rules: {
      'semi': 'error'
    }
  }
]

let id = 1234;      // 1234 永远不会被使用
id = calculateId();  // 变量被重新赋值

import { loadESLint } from 'eslint'

const eslint = await loadESLint()
const lazyLinter = new eslint.LegacyESLint({ overrideConfigFile: true })

// ESLint 9中不再报错
if (true) {
  function foo() { }  // 合法
}

// ESLint 9中会报错
class Foo {
  [key] = 'value';
}

// 复杂度计算包含可选链
const a = obj?.b?.c?.d?.e;

// ESLint 9 - 正确写法
module.exports = {
  meta: { type: 'suggestion' },
  create(context) {
    return {
      // 规则逻辑
    }
  }
}

// ESLint 9 - 错误写法（不再支持）
module.exports = function(context) {
  return {
    // 规则逻辑
  }
}