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. In many ways, it is similar to JSLint and JSHint with a few exceptions:

• ESLint uses Espree for JavaScript parsing.
• ESLint uses an AST to evaluate patterns in code.
• ESLint is completely pluggable, every single rule is a plugin and you can add more at runtime.

Installation and Usage

Prerequisites: Node.js (^12.22.0, ^14.17.0, or >=16.0.0) built with SSL support. (If you are using an official Node.js distribution, SSL is always built in.)

You can install ESLint using npm or yarn:

npm install eslint --save-dev

# or

yarn add eslint --dev

You should then set up a configuration file, and the easiest way to do that is to use the --init flag:

$ npx eslint --init

# or

$ yarn run eslint --init

Note: --init 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

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 be installed locally in either case.

Configuration

Note: If you are coming from a version before 1.0.0 please see the migration guide.

After running eslint --init, you'll have a .eslintrc.{js,yml,json} file in your directory. In it, you'll see some rules configured like this:

{
    "rules": {
        "semi": ["error", "always"],
        "quotes": ["error", "double"]
    }
}

The names "semi" and "quotes" 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 .eslintrc.{js,yml,json} configuration file will also include the line:

{
    "extends": "eslint:recommended"
}

Because of this line, all of the rules marked "✓" 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.

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.