Using ESLint and Prettier

A guide on configuring ESLint and Prettier to format Expo apps.

ESLint is a JavaScript linter that helps you find and fix errors in your code. It's a great tool to help you write better code and catch mistakes before they make it to production. In conjunction, you can use Prettier, a code formatter that ensures all the code files follow a consistent styling.

This guide provides steps to set up and configure ESLint and Prettier.

ESLint

Setup

To set up ESLint in your Expo project, you can use the Expo CLI to install the necessary dependencies. Running this command also creates a eslint.config.js file at the root of your project which extends configuration from eslint-config-expo.

From SDK 53 onwards, the created ESLint config file will use the Flat config format. However, legacy config will also be supported.

module.exports = {
  extends: 'expo',
};

{
  "scripts": {
    "lint": "expo lint"
  }
}

Usage

Recommended: If you're using VS Code, install the ESLint extension to lint your code as you type.

You can lint your code manually from the command line with the npx expo lint script:

Environment configuration

ESLint is generally configured for a single environment. However, the source code is written in JavaScript in an Expo app that runs in multiple different environments. For example, the app.config.js, metro.config.js, babel.config.js, and app/+html.tsx files are run in a Node.js environment. It means they have access to the global __dirname variable and can use Node.js modules such as path. Standard Expo project files like app/index.js can be run in Hermes, Node.js, or the web browser.

You can add the eslint-env comment directive to the top of a file to tell ESLint which environment the file is running in. For example, to tell ESLint that a file is run in Node.js, add the following comment to the top of the file:

/* eslint-env node */
const { getDefaultConfig } = require('expo/metro-config');

/** @type {import('expo/metro-config').MetroConfig} */
const config = getDefaultConfig(
  __dirname
);

module.exports = config;

Prettier

Installation

To install Prettier in your project:

Setup

const { defineConfig } = require('eslint/config');
const expoConfig = require('eslint-config-expo/flat');
const eslintPluginPrettierRecommended = require('eslint-plugin-prettier/recommended');

module.exports = defineConfig([
  expoConfig,
  eslintPluginPrettierRecommended,
  {
    ignores: ['dist/*'],
  },
]);

module.exports = {
  extends: ['expo', 'prettier'],
  plugins: ['prettier'],
  rules: {
    'prettier/prettier': 'error',
  },
};

Now, when you run npx expo lint, anything that is not aligned with Prettier formatting will be caught as an error.

To customize Prettier settings, create a .prettierrc file at the root of your project and add your configuration.

Troubleshooting

ESLint is not updating in VS Code

If you're using VS Code, install the ESLint extension to lint your code as you type. You can try restarting the ESLint server by running the command ESLint: Restart ESLint Server from the command palette.

ESLint is slow

ESLint can be slow to run on large projects. The easiest way to speed up the process is to lint fewer files. Add a .eslintignore file to your project root to ignore certain files and directories such as:

/.expo
node_modules

Migration to Flat config

Note: Flat config is supported in Expo SDK 53 and above.

Upgrade ESLint and eslint-config-expo:

If you haven't customized your ESLint config at all, delete your .eslintrc.js and generate the new config with:

Alternatively, migrate your config based on the ESLint's migration guide. npx expo lint supports both legacy and flat config, so the new config will automatically be picked up by the CLI.