/migration-patterns:eslint-to-biome

Migrate from ESLint (and optionally Prettier) to Biome — a fast, unified linter and formatter for JavaScript/TypeScript written in Rust. Migrates configuration, updates pre-commit hooks, and removes old dependencies.

When to Use This Skill

Important: Biome covers ~90% of common ESLint rules but not all plugins. Audit eslint-plugin-* dependencies before migrating. The most common unsupported plugins are accessibility (jsx-a11y) and framework hooks (react-hooks). You may need to keep ESLint alongside Biome for these.

Context

• ESLint configs: !find . -maxdepth 2 \( -name '.eslintrc' -o -name '.eslintrc.js' -o -name '.eslintrc.cjs' -o -name '.eslintrc.json' -o -name '.eslintrc.yaml' -o -name '.eslintrc.yml' -o -name 'eslint.config.js' -o -name 'eslint.config.mjs' -o -name 'eslint.config.ts' \)
• Prettier configs: !find . -maxdepth 2 \( -name '.prettierrc' -o -name '.prettierrc.js' -o -name '.prettierrc.json' -o -name 'prettier.config.js' \)
• Biome config: !find . -maxdepth 2 -name 'biome.json'
• package.json: !find . -maxdepth 1 -name 'package.json'
• Pre-commit config: !find . -maxdepth 1 -name '.pre-commit-config.yaml'

Parameters

Execution

Execute this ESLint-to-Biome migration:

Step 1: Audit current ESLint/Prettier configuration

1. Read all ESLint config files found in context.
2. Read Prettier config files if present.
3. Check package.json for eslint* and prettier* dependencies.
4. Report findings:

   ESLint → Biome migration audit
   ================================
   ESLint config:    .eslintrc.json (N rules, plugins: [...])
   Prettier config:  .prettierrc (printWidth=N, singleQuote=true)
   ESLint plugins:   eslint-plugin-import, @typescript-eslint, ...
   Biome support:    ✓ eslint-plugin-import → Biome handles this
                     ⚠ eslint-plugin-jsx-a11y → No Biome equivalent (keep ESLint for this)
   biome.json:       NOT PRESENT
   

   If --check-only, stop here.

Step 2: Check plugin compatibility

For each eslint-plugin-* found, determine Biome coverage:

If unsupported plugins are found, ask the user whether to proceed with a partial migration (keep ESLint only for those plugins). If the user declines, abort.

Step 3: Create biome.json

Generate biome.json preserving equivalent rules from the existing ESLint and Prettier configurations:

{
  "$schema": "https://biomejs.dev/schemas/1.9.0/schema.json",
  "organizeImports": {
    "enabled": true
  },
  "linter": {
    "enabled": true,
    "rules": {
      "recommended": true
    }
  },
  "formatter": {
    "enabled": true,
    "indentStyle": "space",
    "indentWidth": 2,
    "lineWidth": 80
  },
  "javascript": {
    "formatter": {
      "quoteStyle": "double",
      "trailingCommas": "all",
      "semicolons": "always"
    }
  }
}

Adjust these defaults to match the Prettier config found in Step 1:

• lineWidth → from printWidth
• quoteStyle → from singleQuote: true → "single"
• trailingCommas → from trailingComma: "es5" → "es5"
• semicolons → from semi: false → "asNeeded"
• indentStyle → from useTabs: true → "tab"

Step 4: Update package.json

1. Remove ESLint and Prettier dev dependencies (if doing full migration):

   "devDependencies": {
     remove: eslint, @typescript-eslint/parser, @typescript-eslint/eslint-plugin,
             eslint-plugin-import, prettier, eslint-config-prettier, eslint-plugin-prettier
     keep: eslint-plugin-jsx-a11y, eslint-plugin-react-hooks (if partial migration)
   }
   

2. Add Biome:

   "@biomejs/biome": "^1.9.0"
   

3. Update scripts in package.json:

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

Step 5: Update .pre-commit-config.yaml (if present)

Remove any ESLint/Prettier pre-commit hooks. Add Biome:

- repo: https://github.com/biomejs/pre-commit
  rev: v0.6.0  # use latest
  hooks:
    - id: biome-check
      additional_dependencies: ["@biomejs/biome@1.9.0"]

Step 6: Delete old config files

Remove ESLint and Prettier config files (unless keeping ESLint for unsupported plugins — in that case, keep .eslintrc but note Biome replaces most rules):

• .eslintrc* → delete (if full migration)
• eslint.config.* → delete (if full migration)
• .prettierrc* → delete
• .eslintignore → convert relevant patterns to biome.json ignore section

Step 7: Verify migration

Run Biome to confirm it works:

npx @biomejs/biome check .
# or with bun:
bunx @biomejs/biome check .

Address any errors that weren't caught by ESLint (Biome may surface additional issues). Errors on first run are expected — fix them or add to biome.json ignore rules.

Step 8: Report

Print a summary:

ESLint → Biome migration complete
===================================
biome.json                CREATED
package.json              UPDATED (added @biomejs/biome, removed eslint/prettier)
.eslintrc.json            DELETED / KEPT (partial migration)
.prettierrc               DELETED
.pre-commit-config.yaml   UPDATED (added biome-check hook)

Partial migration note (if applicable):
  Kept eslint-plugin-jsx-a11y and eslint-plugin-react-hooks — these plugins
  have no Biome equivalent. ESLint is still needed for these rules.

Files to stage:
  git add biome.json package.json .pre-commit-config.yaml
  git add <deleted config files>

Agentic Optimizations

Quick Reference

See Also

• /migration-patterns:black-to-ruff-format — Python formatter migration (analogous pattern)
• /configure:repo — End-to-end repo config driver