JavaScript/TypeScript linter (ESLint wrapper) with great defaults

Opinionated but configurable ESLint wrapper with lots of goodies included. Enforces strict and readable code. Never discuss code style on a pull request again! No decision-making. No eslint.config.js to manage. It just works!

It uses ESLint underneath, so issues regarding built-in rules should be opened over there.

XO requires your project to be ESM.

Highlights

• Beautiful output.
• Zero-config, but configurable when needed.
• Enforces readable code, because you read more code than you write.
• No need to specify file paths to lint as it lints all JS/TS files except for commonly ignored paths.
• Flat config customization.
• TypeScript supported by default.
• Includes many useful ESLint plugins, like unicorn, import, ava, n and more.
• Caches results between runs for much better performance.
• Super simple to add XO to a project with $ npm init xo.
• Fix many issues automagically with $ xo --fix.
• Open all files with errors at the correct line in your editor with $ xo --open.
• Specify indent and semicolon preferences easily without messing with the rule config.
• Optionally use the Prettier code style or turn off all prettier rules with the 'compat' option.
• Optionally use eslint-config-xo-react for easy jsx and react linting with zero config.
• Optionally use with eslint directly
• Great editor plugins.

Install

npm install xo --save-dev

You must install XO locally. You can run it directly with $ npx xo.

You'll need eslint-config-xo-vue for specific linting in a Vue app.

Usage

$ xo --help

  Usage
    $ xo [<file|glob> ...]

  Options
    --fix             Automagically fix issues
    --reporter        Reporter to use
    --ignore          Ignore pattern globs, can be set multiple times
    --space           Use space indent instead of tabs [Default: 2]
    --no-semicolon    Prevent use of semicolons
    --prettier        Conform to Prettier code style or turn off conflicting rules
    --react           Include React plugins and xo-react linting rules [Default: false]
    --ts              Auto configure type aware linting on unincluded ts files [Default: true]
    --open            Open files with issues in your editor
    --quiet           Show only errors and no warnings
    --cwd=<dir>       Working directory for files
    --stdin           Validate/fix code from stdin
    --stdin-filename  Specify a filename for the --stdin option
    --print-config    Print the ESLint configuration for the given file

  Examples
    $ xo
    $ xo index.js
    $ xo *.js !foo.js
    $ xo --space
    $ xo --print-config=index.js
    $ echo 'const x=true' | xo --stdin --fix

  Tips
    - Add XO to your project with `npm init xo`.
    - Put options in xo.config.js instead of using flags so other tools can read it.

Default code style

Any of these can be overridden if necessary.

• Tab indentation (or space)
• Semicolons (or not)
• Single-quotes
• Trailing comma for multiline statements
• No unused variables
• Space after keyword if (condition) {}
• Always === instead of ==

Check out an example and the ESLint rules.

Workflow

The recommended workflow is to add XO locally to your project and run it with the tests.

Simply run $ npm init xo (with any options) to add XO to create an xo.config.js.

Config

You can configure XO options by creating an xo.config.js or an xo.config.ts file in the root directory of your project. XO supports all js/ts file extensions (js,cjs,mjs,ts,cts,mts) automatically. A XO config is an extension of ESLint's Flat Config. Like ESLint, an XO config exports an array of XO config objects. XO config objects extend ESLint Configuration Objects. This means all the available configuration params for ESLint also work for XO. However, XO enhances and adds extra params to the configuration objects to make them easier to work with.

Config types

XO exports the types FlatXoConfig, XoConfigItem, and other types for you to get ts validation on your config files.

examples: xo.config.js

/** @type {import('xo').FlatXoconfig} */
const xoConfig = [...]

xo.config.ts

import {type FlatXoConfig} from 'xo';

const xoConfig: FlatXoConfig = [...]

files

type: string | string[] | undefined, default: **/*.{js,cjs,mjs,jsx,ts,cts,mts,tsx};

A glob or array of glob strings which the config object will apply. By default XO will apply the configuration to all files.

ignores

Type: string[]

Some paths are ignored by default, including paths in .gitignore. Additional ignores can be added here. For global ignores, keep ignores as the only key in the config item.

space

Type: boolean | number
Default: false (tab indentation)

Set it to true to get 2-space indentation or specify the number of spaces.

This option exists for pragmatic reasons, but I would strongly recommend you read "Why tabs are superior".

semicolon

Type: boolean
Default: true (Semicolons required)

Set it to false to enforce no-semicolon style.

prettier

Type: boolean|'compat'
Default: false

Format code with Prettier.

Prettier options will be based on your Prettier config. XO will then merge your options with its own defaults:

• semi: based on semicolon option
• useTabs: based on space option
• tabWidth: based on space option
• trailingComma: all
• singleQuote: true
• bracketSpacing: false

To stick with Prettier's defaults, add this to your Prettier config:

export default {
	trailingComma: "es5",
	singleQuote: false,
	bracketSpacing: true,
};

If contradicting options are set for both Prettier and XO, an error will be thrown.

prettier compat

If the prettier option is set to "compat", instead of formatting your code automatically, xo will turn off all rules that conflict with prettier code style and allow you to pass your formatting to the prettier tool directly.

react

Type: boolean
Default: false

Adds eslint-config-plugin-react, eslint-plugin-react-hooks and eslint-config-xo-react to get all the react best practices applied automatically

TypeScript

XO will automatically lint TypeScript files (.ts, .mts, .cts, .d.ts and .tsx) with the rules defined in eslint-config-xo-typescript#use-with-xo.

XO will handle the @typescript-eslint/parser project option automatically even if you don't have a tsconfig.json in your project.

Usage as an ESLint Configuration

With the introduction of the ESLint flat config, many of the original goals of xo were brought into the ESLint core, and shareable configs with plugins because possible. Although we highly recommend the use of the xo cli, we understand that some teams need to rely on ESLint directly.

For these purposes, you can still get most of the features of xo by using our ESLint configuration helpers.

xoToEslintConfig

The xoToEslintConfig function is designed for use in an eslint.config.js file. It is NOT for use in an xo.config.js file. This function takes a FlatXoConfig and outputs and ESLint config object. This function will not be able to automatically handle TS integration for you nor automatic prettier integration. You are responsible for configuring your other tools appropriately. The xo cli, will however, handle all of these details for you.

eslint.config.js

import Xo from "xo";

export default Xo.xoToEslintConfig([{ space: true, prettier: "compat" }]);

xo base config

As another option, you can use the base xo eslint config as a jumping off point for your eslint config.

eslint.config.js

import { config } from "xo/config";

export default [...config, ...customOptions];

Tips

The --ts option

By default, XO will handle all aspects of type aware linting, even when a file is not included in a tsconfig, which would normally error when using ESLint directly. However, this incurs a small performance penalty of having to look up the tsconfig each time in order to calculate and write an appropriate default tsconfig to use for the file. In situations where you are linting often, you may want to configure your project correctly for type aware linting. This can help performance in editor plugins.

Monorepo

Put a xo.config.js with your config at the root and do not add a config to any of your bundled packages.

Including files ignored by default

To include files that XO ignores by default, add them as negative globs in the ignores option:

const xoConfig = [{ ignores: ["!vendor/**"] }];
export default xoConfig;

FAQ

What does XO mean?

It means hugs and kisses.

Why not Standard?

The Standard style is a really cool idea. I too wish we could have one style to rule them all! But the reality is that the JS community is just too diverse and opinionated to create one code style. They also made the mistake of pushing their own style instead of the most popular one. In contrast, XO is more pragmatic and has no aspiration of being the style. My goal with XO is to make it simple to enforce consistent code style with close to no config. XO comes with my code style preference by default, as I mainly made it for myself, but everything is configurable.

Why not ESLint?

XO is based on ESLint. This project started out as just a shareable ESLint config, but it quickly grew out of that. I wanted something even simpler. Just typing xo and be done. No decision-making. No config. I also have some exciting future plans for it. However, you can still get most of the XO benefits while using ESLint directly with the ESLint shareable config.

Editor plugins

• Sublime Text
• Atom
• Vim
• TextMate 2
• VSCode
• Emacs
• WebStorm

Build-system plugins

• Gulp
• Grunt
• webpack loader
• webpack plugin
• Metalsmith
• Fly

Configs

• eslint-config-xo - ESLint shareable config for XO with tab indent
• eslint-config-xo-space - ESLint shareable config for XO with 2-space indent
• eslint-config-xo-react - ESLint shareable config for React to be used with the above
• eslint-config-xo-vue - ESLint shareable config for Vue to be used with the above
• stylelint-config-xo - Stylelint shareable config for XO with tab indent
• stylelint-config-xo-space - Stylelint shareable config for XO with 2-space indent
• eslint-config-xo-typescript - ESLint shareable config for TypeScript

Support

• Twitter

Related

• eslint-plugin-unicorn - Various awesome ESLint rules (Bundled in XO)
• xo-summary - Display output from xo as a list of style errors, ordered by count

Badge

Show the world you're using XO →

[![XO code style](https://shields.io/badge/code_style-5ed9c7?logo=xo&labelColor=gray&logoSize=auto&logoWidth=20)](https://github.com/xojs/xo)

Or customize the badge.

You can also find some nice dynamic XO badges on badgen.net.

Team

• Sindre Sorhus

Former

• James Talmage
• Michael Mayer
• Mario Nebl
• Pierre Vanduynslager