npm install --save-dev @biomejs/biome

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{
  "scripts": {
    "check": "biome check .",
    "check:write": "biome check --write .",
    "format": "biome format --write .",
    "lint": "biome lint .",
    "lint:fix": "biome lint --write ."
  }
}

{
  "$schema": "https://biomejs.dev/schemas/1.9.4/schema.json",
  "vcs": {
    "enabled": true,
    "clientKind": "git",
    "useIgnoreFile": true
  },
  "files": {
    "ignoreUnknown": false,
    "ignore": ["node_modules", "dist", "build", ".next"]
  },
  "formatter": {
    "enabled": true,
    "indentStyle": "space",
    "indentWidth": 2,
    "lineWidth": 100
  },
  "linter": {
    "enabled": true,
    "rules": {
      "recommended": true
    }
  },
  "javascript": {
    "formatter": {
      "quoteStyle": "single",
      "semicolons": "asNeeded",
      "trailingCommas": "all"
    }
  }
}

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{
  "formatter": {
    "enabled": true,
    "formatWithErrors": false,
    "indentStyle": "space",
    "indentWidth": 2,
    "lineEnding": "lf",
    "lineWidth": 80,
    "attributePosition": "auto"
  },
  "javascript": {
    "formatter": {
      "quoteStyle": "single",
      "jsxQuoteStyle": "double",
      "quoteProperties": "asNeeded",
      "trailingCommas": "all",
      "semicolons": "asNeeded",
      "arrowParentheses": "always",
      "bracketSpacing": true,
      "bracketSameLine": false
    }
  },
  "json": {
    "formatter": {
      "trailingCommas": "none"
    }
  }
}

{
  "linter": {
    "enabled": true,
    "rules": {
      "recommended": true,
      "a11y": {
        "recommended": true,
        "noAutofocus": "error",
        "useKeyWithClickEvents": "warn"
      },
      "complexity": {
        "recommended": true,
        "noForEach": "off",
        "useLiteralKeys": "error"
      },
      "correctness": {
        "recommended": true,
        "noUnusedVariables": "error",
        "useExhaustiveDependencies": "warn"
      },
      "performance": {
        "recommended": true,
        "noAccumulatingSpread": "warn"
      },
      "security": {
        "recommended": true,
        "noDangerouslySetInnerHtml": "error"
      },
      "style": {
        "recommended": true,
        "noNonNullAssertion": "warn",
        "useConst": "error",
        "useTemplate": "warn"
      },
      "suspicious": {
        "recommended": true,
        "noExplicitAny": "warn",
        "noArrayIndexKey": "error"
      }
    }
  }
}

{
  "files": {
    "ignore": [
      "node_modules",
      "dist",
      "build",
      ".next",
      "coverage",
      "*.min.js",
      "public/assets/**"
    ],
    "ignoreUnknown": false,
    "include": ["src/**/*.ts", "src/**/*.tsx"]
  }
}

{
  "overrides": [
    {
      "include": ["**/*.test.ts", "**/*.spec.ts"],
      "linter": {
        "rules": {
          "suspicious": {
            "noExplicitAny": "off"
          }
        }
      }
    },
    {
      "include": ["scripts/**/*.js"],
      "formatter": {
        "lineWidth": 120
      }
    }
  ]
}

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{
  "[javascript]": {
    "editor.defaultFormatter": "biomejs.biome",
    "editor.formatOnSave": true,
    "editor.codeActionsOnSave": {
      "quickfix.biome": "explicit",
      "source.organizeImports.biome": "explicit"
    }
  },
  "[typescript]": {
    "editor.defaultFormatter": "biomejs.biome",
    "editor.formatOnSave": true,
    "editor.codeActionsOnSave": {
      "quickfix.biome": "explicit",
      "source.organizeImports.biome": "explicit"
    }
  },
  "[typescriptreact]": {
    "editor.defaultFormatter": "biomejs.biome",
    "editor.formatOnSave": true
  },
  "[json]": {
    "editor.defaultFormatter": "biomejs.biome",
    "editor.formatOnSave": true
  },
  "biome.lspBin": "./node_modules/@biomejs/biome/bin/biome"
}

{
  "editor.formatOnSave": true,
  "editor.formatOnPaste": true,
  "editor.defaultFormatter": "biomejs.biome",
  "biome.rename": true,
  "files.autoSave": "onFocusChange"
}

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// .prettierrc (old)
{
  "semi": false,
  "singleQuote": true,
  "trailingComma": "all",
  "printWidth": 100
}

// biome.json (new)
{
  "formatter": {
    "lineWidth": 100
  },
  "javascript": {
    "formatter": {
      "semicolons": "asNeeded",
      "quoteStyle": "single",
      "trailingCommas": "all"
    }
  }
}

// .eslintrc.json (old)
{
  "rules": {
    "no-unused-vars": "error",
    "prefer-const": "warn"
  }
}

// biome.json (new)
{
  "linter": {
    "rules": {
      "correctness": {
        "noUnusedVariables": "error"
      },
      "style": {
        "useConst": "warn"
      }
    }
  }
}

{
  "scripts": {
    "lint": "biome lint .",
    "lint:fix": "biome lint --write .",
    "format": "biome format --write .",
    "check": "biome check --write ."
  }
}

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**`.husky/pre-commit`**:
```bash
#!/usr/bin/env sh
. "$(dirname -- "$0")/_/husky.sh"

npx lint-staged

{
  "lint-staged": {
    "*.{js,ts,jsx,tsx,json}": [
      "biome check --write --no-errors-on-unmatched"
    ]
  }
}

pre-commit:
  commands:
    lint:
      glob: "*.{js,ts,jsx,tsx,json}"
      run: biome check --write --no-errors-on-unmatched {staged_files}

#!/bin/bash

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name: Biome CI

on:
  push:
    branches: [main, develop]
  pull_request:
    branches: [main]

jobs:
  biome:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v4

      - name: Setup Node.js
        uses: actions/setup-node@v4
        with:
          node-version: '20'
          cache: 'npm'

      - name: Install dependencies
        run: npm ci

      - name: Run Biome CI
        run: npx @biomejs/biome ci .

      - name: Check formatting
        run: npx @biomejs/biome format .

      - name: Lint
        run: npx @biomejs/biome lint .

biome:
  image: node:20-alpine
  stage: test
  script:
    - npm ci
    - npx @biomejs/biome ci .
  cache:
    paths:
      - node_modules/
  only:
    - merge_requests
    - main

version: 2.1

jobs:
  biome:
    docker:
      - image: cimg/node:20.11
    steps:
      - checkout
      - restore_cache:
          keys:
            - deps-{{ checksum "package-lock.json" }}
      - run: npm ci
      - save_cache:
          paths:
            - node_modules
          key: deps-{{ checksum "package-lock.json" }}
      - run: npx @biomejs/biome ci .

workflows:
  test:
    jobs:
      - biome

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**Configuration**:
```json
{
  "organizeImports": {
    "enabled": true
  }
}

// Before
import { useState } from 'react';
import axios from 'axios';
import { Button } from './components/Button';
import type { User } from './types';
import './styles.css';

// After (sorted)
import type { User } from './types';

import axios from 'axios';
import { useState } from 'react';

import { Button } from './components/Button';

import './styles.css';

{
  "linter": {
    "rules": {
      "suspicious": {
        "noExplicitAny": "warn",
        "noUnsafeDeclarationMerging": "error"
      },
      "correctness": {
        "noUnusedVariables": "error"
      },
      "style": {
        "useImportType": "error",
        "useExportType": "error"
      }
    }
  }
}

// Biome detects unused variables
const unused = 123; // ❌ Error

// Enforces type imports
import { User } from './types'; // ❌ Error
import type { User } from './types'; // ✅ Correct

// Detects unsafe type assertions
const num = "123" as any as number; // ⚠️ Warning

my-monorepo/
├── biome.json (root config)
├── packages/
│   ├── web/
│   │   └── biome.json (extends root)
│   ├── api/
│   │   └── biome.json
│   └── shared/
│       └── biome.json

{
  "$schema": "https://biomejs.dev/schemas/1.9.4/schema.json",
  "extends": [],
  "formatter": {
    "enabled": true,
    "indentStyle": "space",
    "indentWidth": 2
  },
  "linter": {
    "enabled": true,
    "rules": {
      "recommended": true
    }
  }
}

{
  "extends": ["../../biome.json"],
  "formatter": {
    "lineWidth": 100
  },
  "linter": {
    "rules": {
      "style": {
        "noNonNullAssertion": "off"
      }
    }
  }
}

{
  "scripts": {
    "check": "biome check .",
    "check:packages": "biome check packages/*",
    "format": "biome format --write .",
    "lint": "biome lint packages/*"
  }
}

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{
  "linter": {
    "rules": {
      "a11y": {
        "recommended": true,
        "useButtonType": "error",
        "useKeyWithClickEvents": "error"
      },
      "correctness": {
        "useExhaustiveDependencies": "warn",
        "useHookAtTopLevel": "error"
      },
      "suspicious": {
        "noArrayIndexKey": "error"
      }
    }
  },
  "javascript": {
    "formatter": {
      "jsxQuoteStyle": "double"
    }
  }
}

{
  "files": {
    "ignore": [".next", "out", "node_modules"]
  },
  "overrides": [
    {
      "include": ["app/**/*.tsx", "pages/**/*.tsx"],
      "linter": {
        "rules": {
          "a11y": {
            "recommended": true
          }
        }
      }
    }
  ]
}

{
  "linter": {
    "rules": {
      "security": {
        "recommended": true,
        "noGlobalEval": "error"
      },
      "correctness": {
        "noUnusedVariables": "error"
      }
    }
  },
  "javascript": {
    "formatter": {
      "semicolons": "always"
    }
  }
}

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// Ensure correct category
{
  "linter": {
    "rules": {
      "correctness": {  // Category name matters
        "noUnusedVariables": "error"
      }
    }
  }
}

{
  "lint": "biome check src/ --diagnostic-level=error",
  "lint:fix": "biome check src/ --write",
  "format": "biome format src/ --write"
}