nodejs/tools/eslint

ESLint

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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.

Installation

You can install ESLint using npm:

npm install -g eslint

Usage

If it's your first time using ESLint, you should set up a config file using --init:

eslint --init

After that, you can run ESLint on any JavaScript file:

eslint test.js test2.js

Note: eslint --init is intended for setting up and configuring ESLint on a per-project basis and will perform a local installation of ESLint and its plugins in the directory in which it is run. If you prefer using a global installation of ESLint, any plugins used in your configuration must also be installed globally.

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 number is the error level of the rule and can be one of the three 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).

Sponsors

• Development is sponsored by Box
• Site search (eslint.org) is sponsored by Algolia

Team

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

• Nicholas C. Zakas (@nzakas) - project lead
• Ilya Volodin (@ilyavolodin) - reviewer
• Brandon Mills (@btmills) - reviewer
• Gyandeep Singh (@gyandeeps) - reviewer
• Toru Nagashima (@mysticatea) - reviewer
• Alberto Rodríguez (@alberto) - reviewer
• Mathias Schreck (@lo1tuma) - committer
• Jamund Ferguson (@xjamundx) - committer
• Ian VanSchooten (@ianvs) - committer
• Burak Yiğit Kaya (@byk) - committer
• Kai Cataldo (@kaicataldo) - committer
• Michael Ficarra (@michaelficarra) - committer
• Mark Pedrotti (@pedrottimark) - committer
• Oleg Gaidarenko (@markelog) - committer
• Mike Sherov @mikesherov) - committer
• Henry Zhu (@hzoo) - committer
• Marat Dulin (@mdevils) - committer
• Alexej Yaroshevich (@zxqfox) - committer

Releases

We have scheduled releases every two weeks on Friday or Saturday.

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

Why don't you like JSHint???

I do like JSHint. And I like Anton and Rick. Neither of those were deciding factors in creating this tool. The fact is that I've had a dire need for a JavaScript tool with pluggable linting rules. I had hoped JSHint would be able to do this, however after chatting with Anton, I found that the planned plugin infrastructure wasn't going to suit my purpose.

I'm not giving up JSHint for this!

That's not really a question, but I got it. I'm not trying to convince you that ESLint is better than JSHint. The only thing I know is that ESLint is better than JSHint for what I'm doing. In the off chance you're doing something similar, it might be better for you. Otherwise, keep using JSHint, I'm certainly not going to tell you to stop using it.

How does ESLint performance compare to JSHint?

ESLint is slower than JSHint, usually 2-3x slower on a single file. This is because ESLint uses Espree to construct an AST before it can evaluate your code whereas JSHint evaluates your code as it's being parsed. The speed is also based on the number of rules you enable; the more rules you enable, the slower the process.

Despite being slower, we believe that ESLint is fast enough to replace JSHint without causing significant pain.

I heard ESLint is going to replace JSCS?

Yes. Since we are solving the same problems, ESLint and JSCS teams have decided to join forces and work together in the development of ESLint instead of competing with each other. You can read more about this in both ESLint and JSCS announcements.

So, should I stop using JSCS and start using ESLint?

Not yet. We are still working to smooth the transition. You can see our progress here. We’ll announce when all of the changes necessary to support JSCS users in ESLint are complete and will start encouraging JSCS users to switch to ESLint at that time. Meanwhile, we recommend you to upgrade to JSCS 3.0 and provide feedback to the team.

Is ESLint just linting or does it also check style?

ESLint does both traditional linting (looking for problematic patterns) and style checking (enforcement of conventions). You can use it for both.

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 about ECMAScript 6 support?

ESLint has full support for ECMAScript 6. By default, this support is off. You can enable ECMAScript 6 support through configuration.

What about experimental features?

ESLint doesn't natively support experimental ECMAScript language features. You can use babel-eslint to use any option available in Babel.

Once a language feature has been adopted into the ECMAScript standard, 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