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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 (^10.12.0, or >=12.0.0) 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

© JS Foundation and other contributors
Licensed under the MIT License.
https://eslint.org/docs/user-guide/getting-started