XO Linter

2.0.2 · active · verified Tue Apr 21

XO is an opinionated JavaScript and TypeScript linter that wraps ESLint, providing a zero-configuration setup with strict, readable code defaults. It aims to streamline code style enforcement and eliminate style-related discussions in pull requests. The current stable version is 2.0.2, with a relatively active release cadence to incorporate updates and maintain compatibility with its underlying dependencies. Key differentiators include its batteries-included approach, which bundles many useful ESLint plugins (e.g., `unicorn`, `import-x`, `n`), automatic caching for performance, first-class TypeScript support, and flexible integration options, including optional Prettier usage. It mandates an ESM-only project structure for optimal functionality.

Common errors

```
Error [ERR_REQUIRE_ESM]: require() of ES Module <path_to_xo>/index.js from <your_file> not supported.
```
cause Attempting to import XO programmatically using CommonJS `require()` syntax in an ES module context, or running XO in a project that is not configured as an ES module.

fix Change your import statement to `import xo from 'xo';` if using XO programmatically. Ensure your `package.json` has `"type": "module"` if your `.js` files are intended to be ESM.
```
Your current Node.js version is <version>. XO requires Node.js >= 20.19.
```
cause The installed Node.js version is older than the minimum required by XO v2.0.0+.

fix Update your Node.js installation to version 20.19 or newer. Consider using a version manager like `nvm` or `volta`.
```
ESLint couldn't find the plugin "eslint-plugin-promise".
```
cause This specific plugin's rules were temporarily removed from XO v2.0.0 due to compatibility issues with ESLint 10.

fix This is expected behavior in XO v2.0.0. You cannot re-enable these rules until XO re-integrates them in a future release. Review your linting expectations accordingly.
```
ESLint couldn't find the config "<config_name>" to extend from.
```
cause A configuration file (e.g., `xo.config.js`) might be trying to extend a CJS-specific configuration or an older config that's no longer compatible with ESLint 10.

fix Verify your `xo.config.js` or similar file is using correct ESM syntax and extending valid, compatible configurations. Ensure there are no CJS-style `module.exports` or `require()` calls within the config file itself.

Warnings

breaking XO v2.0.0 and later require Node.js version 20.19 or higher. Older Node.js versions are not supported.
Fix: Upgrade your Node.js environment to version 20.19 or newer.
breaking XO v2.0.0 and later require ESLint version 10. Ensure your project's ESLint dependency (if any) is compatible or allow XO to manage its own ESLint peer dependency.
Fix: Update your local ESLint installation to `eslint@^10.0.0` or higher, or remove it if you intend for XO to manage it exclusively.
breaking CommonJS (CJS) config files are no longer supported. Configuration must be provided in ES module format (e.g., `.js`, `.mjs`, `.ts`, `.mts`) as XO is an ESM-only package.
Fix: Migrate your `xo.config.cjs` or similar CJS configuration files to `.js`, `.mjs`, or TypeScript equivalents, ensuring they use `export default` syntax.
breaking The `eslint-plugin-promise` rules were temporarily removed in v2.0.0 due to compatibility issues with ESLint 10. This may affect existing projects relying on these specific rules.
Fix: Be aware that these rules are currently disabled. Monitor the XO changelog for re-introduction once ESLint 10 compatibility is resolved for the plugin.
gotcha XO is strictly an ESM-only package. Attempting to use `require()` or run XO in a non-ESM Node.js environment will result in errors.
Fix: Ensure your project's `package.json` contains `"type": "module"` if you are using `.js` files, or use `.mjs` or TypeScript files. For CLI usage, `npx xo` should handle the environment correctly.

Install

npm install xo npm
yarn add xo yarn
pnpm add xo pnpm

Imports

xo
wrong:
```
const xo = require('xo');
```
correct:
```
import xo from 'xo';
```
XO is an ESM-only package since v2.0.0. Direct programmatic usage is typically for advanced scenarios, as it's primarily a CLI tool.
create-xo
```
npm init xo
```
This is a command-line utility for quickly adding XO to a new or existing project, not a direct JavaScript import.
eslint-config-xo-react
```
/* configured via 'react' option or implicitly */
```
XO integrates React linting via the `--react` CLI flag or `react: true` in its configuration, which internally uses `eslint-config-xo-react`. It's not typically imported directly by the user.

Quickstart

Demonstrates how to install XO as a dev dependency, configure it in `package.json`, and run it from the command line, including using the `--fix` flag to automatically resolve issues.

{
  "name": "my-project",
  "version": "1.0.0",
  "type": "module",
  "devDependencies": {
    "xo": "^2.0.0"
  },
  "scripts": {
    "lint": "xo",
    "lint:fix": "xo --fix"
  }
}

// src/index.js
const myVar =  'hello' ;  // Linting issue: extra space, single quotes

function MyComponent(props) { // Linting issue: function-style component for React
  return (<div>{props.message}</div>);
}

console.log(myVar, MyComponent);

// To run:
// npm install
// npm run lint
// npm run lint:fix

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