eslint

What is eslint?

ESLint is a static code analysis tool for identifying problematic patterns found in JavaScript code. It is primarily used for finding and fixing problems in JavaScript code, enforcing coding standards, and improving code quality.

What are eslint's main functionalities?

Linting JavaScript Files

Run ESLint on all JavaScript files in the 'src' directory and its subdirectories.

eslint 'src/**/*.js'

Fixing Problems Automatically

Automatically fix problems in JavaScript files that ESLint is capable of fixing.

eslint --fix 'src/**/*.js'

Customizable Configuration

Customize ESLint rules and extend from recommended presets in the ESLint configuration file.

{ 'extends': 'eslint:recommended', 'rules': { 'eqeqeq': 'warn', 'no-unused-vars': 'error' } }

Integrating with Build Tools

Integrate ESLint with build tools like Grunt by using the corresponding plugin.

grunt.loadNpmTasks('grunt-eslint');

Using Plugins

Extend ESLint's capabilities with plugins for specific libraries or frameworks, such as React.

{ 'plugins': ['react'], 'rules': { 'react/jsx-uses-vars': 'error' } }

Other packages similar to eslint

jshint

JSHint is a community-driven tool that detects errors and potential problems in JavaScript code. It is less configurable than ESLint and does not support custom rule creation.

jscs

JSCS is a code style linter for programmatically enforcing your style guide. It has been deprecated and merged with ESLint, which now covers its functionality.

prettier

Prettier is an opinionated code formatter that supports many languages and integrates with most editors. Unlike ESLint, it does not check for code errors but focuses on maintaining a consistent code style.

tslint

TSLint was a linter for TypeScript, providing similar functionality to ESLint but specifically for TypeScript code. It has been deprecated in favor of typescript-eslint, which allows ESLint to be used with TypeScript.

standard

Standard is a JavaScript style guide, linter, and formatter with a set of predefined rules. It enforces a strict coding standard but is less configurable than ESLint.

README

ESLint

Website | Configuring | Rules | Contributing | Reporting Bugs | Code of Conduct | Twitter | Mailing List | Chat Room

ESLint is a tool for identifying and reporting on patterns found in ECMAScript/JavaScript code. 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.

Table of Contents

• Installation and Usage
• Configuration
• Code of Conduct
• Filing Issues
• Frequently Asked Questions
• Releases
• Security Policy
• Semantic Versioning Policy
• License
• Team
• Sponsors
• Technology Sponsors

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:

$ npm install eslint --save-dev

You should then set up a configuration file:

$ ./node_modules/.bin/eslint --init

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

$ ./node_modules/.bin/eslint yourfile.js

Configuration

After running eslint --init, you'll have a .eslintrc 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).

Code of Conduct

ESLint adheres to the JS Foundation Code of Conduct.

Filing Issues

Before filing an issue, please be sure to read the guidelines for what you're reporting:

• Bug Report
• Propose a New Rule
• Proposing a Rule Change
• Request a Change

Frequently Asked Questions

I'm using JSCS, should I migrate to ESLint?

Yes. JSCS has reached end of life and is no longer supported.

We have prepared a migration guide to help you convert your JSCS settings to an ESLint configuration.

We are now at or near 100% compatibility with JSCS. If you try ESLint and believe we are not yet compatible with a JSCS rule/configuration, please create an issue (mentioning that it is a JSCS compatibility issue) and we will evaluate it as per our normal process.

Does Prettier replace ESLint?

No, ESLint does both traditional linting (looking for problematic patterns) and style checking (enforcement of conventions). You can use ESLint for everything, or you can combine both using Prettier to format your code and ESLint to catch possible errors.

Why can't ESLint find my plugins?

• Make sure your plugins (and ESLint) are both in your project's package.json as devDependencies (or dependencies, if your project uses ESLint at runtime).
• Make sure you have run npm install and all your dependencies are installed.
• Make sure your plugins' peerDependencies have been installed as well. You can use npm view eslint-plugin-myplugin peerDependencies to see what peer dependencies eslint-plugin-myplugin has.

Does ESLint support JSX?

Yes, ESLint natively supports parsing JSX syntax (this must be enabled in configuration). Please note that supporting JSX syntax is not the same as supporting React. React applies specific semantics to JSX syntax that ESLint doesn't recognize. We recommend using eslint-plugin-react if you are using React and want React semantics.

What ECMAScript versions does ESLint support?

ESLint has full support for ECMAScript 3, 5 (default), 2015, 2016, 2017, 2018, 2019, and 2020. You can set your desired ECMAScript syntax (and other settings, like global variables or your target environments) through configuration.

What about experimental features?

ESLint's parser only officially supports the latest final ECMAScript standard. We will make changes to core rules in order to avoid crashes on stage 3 ECMAScript syntax proposals (as long as they are implemented using the correct experimental ESTree syntax). We may make changes to core rules to better work with language extensions (such as JSX, Flow, and TypeScript) on a case-by-case basis.

In other cases (including if rules need to warn on more or fewer cases due to new syntax, rather than just not crashing), we recommend you use other parsers and/or rule plugins. If you are using Babel, you can use the babel-eslint parser and eslint-plugin-babel to use any option available in Babel.

Once a language feature has been adopted into the ECMAScript standard (stage 4 according to the TC39 process), we will accept issues and pull requests related to the new feature, subject to our contributing guidelines. Until then, please use the appropriate parser and plugin(s) for your experimental feature.

Where to ask for help?

Join our Mailing List or Chatroom.

Releases

We have scheduled releases every two weeks on Friday or Saturday. You can follow a release issue for updates about the scheduling of any particular release.

Security Policy

ESLint takes security seriously. We work hard to ensure that ESLint is safe for everyone and that security issues are addressed quickly and responsibly. Read the full security policy.

Semantic Versioning Policy

ESLint follows semantic versioning. However, due to the nature of ESLint as a code quality tool, it's not always clear when a minor or major version bump occurs. To help clarify this for everyone, we've defined the following semantic versioning policy for ESLint:

• Patch release (intended to not break your lint build)
  • A bug fix in a rule that results in ESLint reporting fewer linting errors.
  • A bug fix to the CLI or core (including formatters).
  • Improvements to documentation.
  • Non-user-facing changes such as refactoring code, adding, deleting, or modifying tests, and increasing test coverage.
  • Re-releasing after a failed release (i.e., publishing a release that doesn't work for anyone).
• Minor release (might break your lint build)
  • A bug fix in a rule that results in ESLint reporting more linting errors.
  • A new rule is created.
  • A new option to an existing rule that does not result in ESLint reporting more linting errors by default.
  • A new addition to an existing rule to support a newly-added language feature (within the last 12 months) that will result in ESLint reporting more linting errors by default.
  • An existing rule is deprecated.
  • A new CLI capability is created.
  • New capabilities to the public API are added (new classes, new methods, new arguments to existing methods, etc.).
  • A new formatter is created.
  • eslint:recommended is updated and will result in strictly fewer linting errors (e.g., rule removals).
• Major release (likely to break your lint build)
  • eslint:recommended is updated and may result in new linting errors (e.g., rule additions, most rule option updates).
  • A new option to an existing rule that results in ESLint reporting more linting errors by default.
  • An existing formatter is removed.
  • Part of the public API is removed or changed in an incompatible way. The public API includes:
    • Rule schemas
    • Configuration schema
    • Command-line options
    • Node.js API
    • Rule, formatter, parser, plugin APIs

According to our policy, any minor update may report more linting errors than the previous release (ex: from a bug fix). As such, we recommend using the tilde (~) in package.json e.g. "eslint": "~3.1.0" to guarantee the results of your builds.

License

Team

These folks keep the project moving and are resources for help.

Technical Steering Committee (TSC)

The people who manage releases, review feature requests, and meet regularly to ensure ESLint is properly maintained.

Nicholas C. Zakas

Brandon Mills

Toru Nagashima

Milos Djermanovic

Reviewers

The people who review and implement new features.

薛定谔的猫

Committers

The people who review and fix bugs and help triage issues.

Pig Fang

Anix

YeonJuan

Sponsors

The following companies, organizations, and individuals support ESLint's ongoing maintenance and development. Become a Sponsor to get your logo on our README and website.

Platinum Sponsors

Gold Sponsors

Silver Sponsors

Bronze Sponsors

Technology Sponsors

• Site search (eslint.org) is sponsored by Algolia
• Hosting for (eslint.org) is sponsored by Netlify
• Password management is sponsored by 1Password