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eslint

An AST-based pattern checker for JavaScript.

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.

You can install ESLint using npm:

npm install -g eslint

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.

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).

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

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

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

The most significant difference is that ESlint has pluggable linting rules. That means you can use the rules it comes with, or you can extend it with rules created by others or by yourself!

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.

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.

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.

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

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.

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

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.

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