Inspect existing tooling before changing anything:
- biome.json, biome.jsonc, .prettierrc*, eslint.config.*, .eslintrc*, xo.config.*, and package.json scripts.
- If XO or ESLint already exists, run biome migrate eslint --write only as a starting point, then tighten the config manually.
Install locally:
```
pnpm add -D @biomejs/biome
```

Add scripts:

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

Keep tsc --noEmit in CI for semantic TypeScript checks. Biome replaces many XO/ESLint/Prettier checks, but it is not a full type-aware TypeScript checker.
After edits, run pnpm lint:fix, then pnpm lint, then project tests.

For snippet generation, run:

bash /mnt/skills/user/biome-js/scripts/biome-js-bootstrap.sh strict

Formatter Sample

XO-like formatter settings:

{
  "$schema": "./node_modules/@biomejs/biome/configuration_schema.json",
  "formatter": {
    "enabled": true,
    "formatWithErrors": false,
    "indentStyle": "tab",
    "lineEnding": "lf",
    "lineWidth": 100,
    "bracketSpacing": false
  },
  "javascript": {
    "formatter": {
      "quoteStyle": "single",
      "jsxQuoteStyle": "single",
      "quoteProperties": "asNeeded",
      "trailingCommas": "all",
      "semicolons": "always",
      "arrowParentheses": "asNeeded",
      "operatorLinebreak": "before"
    }
  },
  "json": {
    "formatter": {
      "trailingCommas": "none"
    }
  }
}

Adjust only when the existing repo already has a deliberate style. Biome defaults differ from XO: Biome defaults to double quotes and 80-column formatting, while XO prefers single quotes and semicolons.

Strict Linter Sample

Start with recommended rules, then promote XO-equivalent and XO-adjacent rules explicitly:

{
  "linter": {
    "enabled": true,
    "rules": {
      "recommended": true,
      "complexity": {
        "noExcessiveCognitiveComplexity": {
          "level": "warn",
          "options": { "maxAllowedComplexity": 15 }
        },
        "noExcessiveLinesPerFunction": {
          "level": "warn",
          "options": { "maxLines": 80, "skipBlankLines": true }
        },
        "noForEach": "warn",
        "noStaticOnlyClass": "error",
        "noUselessCatch": "error",
        "noVoid": "error",
        "useArrowFunction": "error",
        "useOptionalChain": "error",
        "useRegexLiterals": "error"
      },
      "correctness": {
        "noChildrenProp": "error",
        "noInvalidUseBeforeDeclaration": "error",
        "noNodejsModules": "off",
        "noProcessGlobal": "error",
        "noUndeclaredDependencies": "error",
        "noUnusedFunctionParameters": "error",
        "noUnusedImports": "error",
        "noUnusedVariables": "error",
        "useImportExtensions": "off",
        "useJsonImportAttributes": "error",
        "useParseIntRadix": "error"
      },
      "security": {
        "noBlankTarget": "error",
        "noDangerouslySetInnerHtml": "error",
        "noGlobalEval": "error"
      },
      "style": {
        "noCommonJs": "error",
        "noDefaultExport": "off",
        "noDoneCallback": "error",
        "noInferrableTypes": "error",
        "noNamespace": "error",
        "noNonNullAssertion": "warn",
        "noParameterAssign": "error",
        "noRestrictedGlobals": {
          "level": "error",
          "options": {
            "deniedGlobals": {
              "event": "Use an explicit local variable instead.",
              "error": "Use an explicit local variable instead.",
              "atob": "Use Buffer, Uint8Array helpers, or a project-approved decoder.",
              "btoa": "Use Buffer, Uint8Array helpers, or a project-approved encoder."
            }
          }
        },
        "noRestrictedImports": {
          "level": "error",
          "options": {
            "paths": {
              "domain": "Deprecated Node.js module.",
              "freelist": "Deprecated Node.js module.",
              "punycode": "Deprecated Node.js module.",
              "querystring": "Use URLSearchParams.",
              "smalloc": "Deprecated Node.js module.",
              "sys": "Deprecated Node.js module.",
              "colors": "Use util.styleText() or a project-approved logger."
            }
          }
        },
        "useAsConstAssertion": "error",
        "useBlockStatements": "error",
        "useConsistentArrayType": "error",
        "useConst": "error",
        "useDefaultParameterLast": "error",
        "useEnumInitializers": "error",
        "useExportType": "error",
        "useFilenamingConvention": "warn",
        "useImportType": "error",
        "useNamingConvention": "error",
        "useNodejsImportProtocol": "error",
        "useSelfClosingElements": "error",
        "useShorthandFunctionType": "error",
        "useTemplate": "error"
      },
      "suspicious": {
        "noArrayIndexKey": "error",
        "noAssignInExpressions": "error",
        "noConsole": "warn",
        "noDebugger": "error",
        "noDoubleEquals": "error",
        "noExplicitAny": "error",
        "noFocusedTests": "error",
        "noImplicitAnyLet": "error",
        "noShadowRestrictedNames": "error",
        "noTsIgnore": "error",
        "noUnsafeDeclarationMerging": "error",
        "useDefaultSwitchClauseLast": "error"
      }
    }
  }
}

Tune frontend-only security/a11y rules for React, Vue, Qwik, and Solid projects. For Node-only packages, keep JSX-specific rules enabled if they never trigger; disable them only when they create false positives.

XO to Biome Porting

Use this map when replacing XO:

XO / ESLint intent	Biome equivalent
`eqeqeq`	`suspicious.noDoubleEquals`
`no-debugger`	`suspicious.noDebugger`
`no-unused-vars`, unused imports	`correctness.noUnusedVariables`, `correctness.noUnusedImports`, `correctness.noUnusedFunctionParameters`
`prefer-const`	`style.useConst`
`curly`	`style.useBlockStatements`
`no-process-exit` / Node globals discipline	`correctness.noProcessGlobal` plus explicit `import process from 'node:process'`
`unicorn/prefer-node-protocol`	`style.useNodejsImportProtocol`
`@typescript-eslint/no-explicit-any`	`suspicious.noExplicitAny`
`@typescript-eslint/consistent-type-imports`	`style.useImportType`, `style.useExportType`
`no-restricted-imports`	`style.noRestrictedImports`
`no-restricted-globals`	`style.noRestrictedGlobals`
`complexity`, `max-lines`, `max-params`	`complexity.noExcessiveCognitiveComplexity`, `complexity.noExcessiveLinesPerFunction`, `complexity.useMaxParams`
`sort-imports`, `import/order`, duplicate imports	`assist.source.organizeImports`
Prettier/XO stylistic formatting	Biome formatter options, not linter rules

Known gaps:

Biome does not fully replace type-aware rules such as no-floating-promises, no-misused-promises, or exhaustive switch checks. Keep tsc --noEmit; keep a small ESLint/XO layer only if the project truly needs those semantic rules.
Biome does not implement every Unicorn rule. Prefer explicit Biome rules plus project conventions instead of inventing broad suppressions.
Biome unsafe fixes may change behavior. Do not apply --unsafe automatically in save hooks or CI.

Assist and Import Sorting

Enable safe source actions:

{
  "assist": {
    "enabled": true,
    "actions": {
      "source": {
        "recommended": true,
        "organizeImports": "on"
      }
    }
  }
}

Use biome check --write . for safe format, lint fixes, and assist actions. Use biome lint --write --unsafe ./src only after reviewing the diff.

VS Code

Install the official extension:

biomejs.biome — first-party Biome language server, formatter, linter diagnostics, quick fixes, and import organization.

Use workspace settings:

{
  "editor.defaultFormatter": "biomejs.biome",
  "editor.formatOnSave": true,
  "editor.codeActionsOnSave": {
    "source.fixAll.biome": "explicit",
    "source.organizeImports.biome": "explicit"
  },
  "biome.enabled": true,
  "biome.requireConfiguration": true
}

If the repo still has ESLint or Prettier, set their save-time formatters/fixers off for Biome-owned file types to avoid dueling edits.

Present Results

When reporting a Biome migration:

Name what was replaced: formatter, linter, import sorting, safe fixes, CI command.
Call out any XO rules that remain unmatched and whether tsc or a small ESLint layer covers them.
Report exact validation commands and remaining diagnostics.

biome-js

Biome JS/TS

Baseline Workflow