Migrate from ESLint and Prettier

Biome provides dedicated commands to ease the migration from ESLint and Prettier.

If you don’t want to know the details, just run the following commands:

biome migrate eslint --write
biome migrate prettier --write

Migrate from ESLint

Many Biome linter rules are inspired by or identical to the ESLint rules or the rules of an ESLint plugin. We handle some ESLint plugins such as TypeScript ESLint, ESLint JSX A11y, ESLint React, and ESLint Unicorn. However, Biome has its own naming convention for its rules. Biome uses camelCaseRuleName while ESLint uses kebab-case-rule-name. Moreover, Biome has often chosen to use different names to better convey the intent of its rules. The source of a rule can be found on the page describing the rule. You can also find the equivalent Biome rule from an ESLint rule using the dedicated page.

To ease the migration, Biome provides the biome migrate eslint subcommand. This subcommand will read your ESLint configuration and attempt to port its settings to Biome. The subcommand is able to handle both the legacy and the flat configuration files. It supports the extends field of the legacy configuration and loads both shared and plugin configurations. For flat configuration files, the subcommand will attempt to search for JavaScript extension only (js, cjs, mjs) to be loaded into Node.js. The subcommand needs Node.js to load and resolve all the plugins and extends configured in the ESLint configuration file. The subcommand also migrates .eslintignore.

Given the following ESLint configuration:

.eslintrc.json

{
  "extends": ["plugin:unicorn/recommended"],
  "plugins": ["unicorn"],
  "ignore_patterns": ["dist/**"],
  "globals": {
    "Global1": "readonly"
  },
  "rules": {
    "eqeqeq": "error"
  },
  "overrides": [
    {
      "files": ["tests/**"],
      "rules": {
        "eqeqeq": "off"
      }
    }
  ]
}

And the following Biome configuration:

biome.json

{
  "linter": {
    "enabled": true,
    "rules": {
      "recommended": true
    }
  }
}

Run the following command to migrate your ESLint configuration to Biome.

npm

npx @biomejs/biome migrate eslint --write

pnpm

pnpx @biomejs/biome migrate eslint --write

bun

bunx --bun @biomejs/biome migrate eslint --write

deno

deno run -A npm:@biomejs/biome migrate eslint --write

yarn

yarn exec biome -- migrate eslint --write

The subcommand overwrites your initial Biome configuration. For example, it disables recommended. This results in the following Biome configuration:

biome.json

{
  "organizeImports": { "enabled": true },
  "linter": {
    "enabled": true,
    "rules": {
      "recommended": false,
      "complexity": {
        "noForEach": "error",
        "noStaticOnlyClass": "error",
        "noUselessSwitchCase": "error",
        "useFlatMap": "error"
      },
      "style": {
        "noNegationElse": "off",
        "useForOf": "error",
        "useNodejsImportProtocol": "error",
        "useNumberNamespace": "error"
      },
      "suspicious": {
        "noDoubleEquals": "error",
        "noThenProperty": "error",
        "useIsArray": "error"
      }
    }
  },
  "javascript": { "globals": ["Global1"] },
  "overrides": [
    {
      "include": ["tests/**"],
      "linter": { "rules": { "suspicious": { "noDoubleEquals": "off" } } }
    }
  ]
}

For now, biome migrate eslint doesn’t support configuration written in YAML.

By default, Biome doesn’t migrate inspired rules. You can use the CLI flag --include-inspired to migrate them.

npm

npx @biomejs/biome migrate eslint --write --include-inspired

pnpm

pnpx @biomejs/biome migrate eslint --write --include-inspired

bun

bunx --bun @biomejs/biome migrate eslint --write --include-inspired

deno

deno run -A npm:@biomejs/biome migrate eslint --write --include-inspired

yarn

yarn exec biome -- migrate eslint --write --include-inspired

Note that you are unlikely to get exactly the same behavior as ESLint because Biome has chosen not to implement some rule options or to deviate slightly from the original implementation.

Since ESLint takes VCS ignore files into account, we recommend that you enable Biome’s VCS integration.

Some plugins or shared configurations may export an object with a cyclic reference. Biome may fail to load such a configuration.

Workaround: If that happens, temporarily comment out the plugins or shared configurations may cause the issue, run the migration, and then enable them again one by one to identify the problematic configuration.

For example, if your eslint.config.js looks like this, you can comment out the problematic entries:

eslint.config.js

import { defineConfig } from 'eslint';
import jsxA11y from 'eslint-plugin-jsx-a11y';
import react from '@eslint-react/eslint-plugin';
import stylistic from '@stylistic/eslint-plugin';
import tsEslint from 'typescript-eslint';

export default defineConfig([
  react.configs.recommended,   // may cause cyclic reference
  jsxA11y.flatConfigs.recommended, // may cause cyclic reference
  // react.configs.recommended,
  // jsxA11y.flatConfigs.recommended,
  {
    plugins: {
      '@stylistic': stylistic,           // may cause cyclic reference
      '@typescript-eslint': tsEslint.plugin, // may cause cyclic reference
      // '@stylistic': stylistic,
      // '@typescript-eslint': tsEslint.plugin,
    },
  }
]);

Once the migration is complete, you can restore and manually configure any rules from those plugins in your new biome.json.

Migrate from Prettier

Biome tries to match the Prettier formatter as closely as possible. However, Biome uses different defaults for its formatter. For example, it uses tabs for indentation instead of spaces. You can easily migrate to Biome by running biome migrate prettier --write.

Given the following Prettier configuration:

.prettierrc.json

{
  "useTabs": false,
  "singleQuote": true,
  "overrides": [
    {
          "files": ["*.json"],
          "options": { "tabWidth": 2 }
      }
  ]
}

Run the following command to migrate your Prettier configuration to Biome.

npm

npx @biomejs/biome migrate prettier --write

pnpm

pnpx @biomejs/biome migrate prettier --write

bun

bunx --bun @biomejs/biome migrate prettier --write

deno

deno run -A npm:@biomejs/biome migrate prettier --write

yarn

yarn exec biome -- migrate prettier --write

This results in the following Biome configuration:

biome.json

{
  "formatter": {
    "enabled": true,
    "formatWithErrors": false,
    "indentStyle": "space",
    "indentWidth": 2,
    "lineEnding": "lf",
    "lineWidth": 80,
    "attributePosition": "auto"
  },
  "organizeImports": { "enabled": true },
  "linter": { "enabled": true, "rules": { "recommended": true } },
  "javascript": {
    "formatter": {
      "jsxQuoteStyle": "double",
      "quoteProperties": "asNeeded",
      "trailingCommas": "all",
      "semicolons": "asNeeded",
      "arrowParentheses": "always",
      "bracketSpacing": true,
      "bracketSameLine": false,
      "quoteStyle": "single",
      "attributePosition": "auto"
    }
  },
  "overrides": [
    {
      "include": ["*.json"],
      "formatter": {
        "indentWidth": 2
      }
    }
  ]
}

The subcommand needs Node.js to load JavaScript configurations such as .prettierrc.js. biome migrate prettier doesn’t support configuration written in JSON5, TOML, or YAML.

Since Prettier takes VCS ignore files into account, we recommend that you enable Biome’s VCS integration.