Getting Started with ESLint

ESLint is a tool for identifying and reporting on patterns found in ECMAScript/JavaScript code, with the goal of making code more consistent and avoiding bugs.

ESLint is completely pluggable. Every single rule is a plugin and you can add more at runtime. You can also add community plugins, configurations, and parsers to extend the functionality of ESLint.

Prerequisites

To use ESLint, you must have Node.js (^18.18.0, ^20.9.0, or >=21.1.0) installed and built with SSL support. (If you are using an official Node.js distribution, SSL is always built in.)

Quick start

You can install and configure ESLint using this command:

npm init @eslint/config
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If you want to use a specific shareable config that is hosted on npm, you can use the --config option and specify the package name:

# use `eslint-config-standard` shared config

# npm 7+
npm init @eslint/config -- --config eslint-config-standard

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Note: npm init @eslint/config assumes you have a package.json file already. If you don’t, make sure to run npm init or yarn init beforehand.

After that, you can run ESLint on any file or directory like this:

npx eslint yourfile.js

# or

yarn run eslint yourfile.js
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Configuration

Note: If you are coming from a version before 9.0.0 please see the migration guide.

After running npm init @eslint/config, you’ll have an eslint.config.js file in your directory. In it, you’ll see some rules configured like this:

// eslint.config.js
export default [
    {
        rules: {
            "no-unused-vars": "error",
            "no-undef": "error"
        }
    }
];
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The names "no-unused-vars" and "no-undef" are the names of rules in ESLint. The first value is the error level of the rule and can be one of these values:

"off" or 0 - turn the rule off
"warn" or 1 - turn the rule on as a warning (doesn’t affect exit code)
"error" or 2 - turn the rule on as an error (exit code will be 1)

The three error levels allow you fine-grained control over how ESLint applies rules (for more configuration options and details, see the configuration docs).

Your eslint.config.js configuration file will also include a recommended configuration, like this:

// eslint.config.js
import js from "@eslint/js";

export default [
    js.configs.recommended,

    {
        rules: {
            "no-unused-vars": "warn",
            "no-undef": "warn"
        }
    }
];
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The js.configs.recommended object contains configuration to ensure that all of the rules marked as recommended on the rules page will be turned on. Alternatively, you can use configurations that others have created by searching for “eslint-config” on npmjs.com. ESLint will not lint your code unless you extend from a shared configuration or explicitly turn rules on in your configuration.

Global Install

It is also possible to install ESLint globally, rather than locally, using npm install eslint --global. However, this is not recommended, and any plugins or shareable configs that you use must still be installed locally if you install ESLint globally.

Manual Set Up

You can also manually set up ESLint in your project.

Before you begin, you must already have a package.json file. If you don’t, make sure to run npm init or yarn init to create the file beforehand.

Install the ESLint packages in your project:

npm install --save-dev eslint @eslint/js
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Add an eslint.config.js file:

# Create JavaScript configuration file
touch eslint.config.js
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Add configuration to the eslint.config.js file. Refer to the Configure ESLint documentation to learn how to add rules, custom configurations, plugins, and more.

import js from "@eslint/js";

export default [
    js.configs.recommended,

   {
       rules: {
           "no-unused-vars": "warn",
           "no-undef": "warn"
       }
   }
];
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Lint code using the ESLint CLI:
```
npx eslint project-dir/ file1.js
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```
For more information on the available CLI options, refer to Command Line Interface.

Next Steps

Learn about advanced configuration of ESLint.
Get familiar with the command line options.
Explore ESLint integrations into other tools like editors, build systems, and more.
Can’t find just the right rule? Make your own custom rule.
Make ESLint even better by contributing.

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