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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](https://github.com/eslint/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.
Prerequisites: [Node.js](https://nodejs.org/) (`^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.)
After running `eslint --init`, you'll have a `.eslintrc` file in your directory. In it, you'll see some rules configured like this:
```json
{
"rules": {
"semi": ["error", "always"],
"quotes": ["error", "double"]
}
}
```
The names `"semi"` and `"quotes"` are the names of [rules](https://eslint.org/docs/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](https://eslint.org/docs/user-guide/configuring)).
We have prepared a [migration guide](https://eslint.org/docs/user-guide/migrating-from-jscs) 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.
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.
Yes, ESLint natively supports parsing JSX syntax (this must be enabled in [configuration](https://eslint.org/docs/user-guide/configuring)). 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](https://www.npmjs.com/package/eslint-plugin-react) if you are using React and want React semantics.
ESLint has full support for ECMAScript 3, 5 (default), 2015, 2016, 2017, 2018, and 2019. You can set your desired ECMAScript syntax (and other settings, like global variables or your target environments) through [configuration](https://eslint.org/docs/user-guide/configuring).
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](https://github.com/babel/babel-eslint) parser and [eslint-plugin-babel](https://github.com/babel/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](https://tc39.github.io/process-document/)), we will accept issues and pull requests related to the new feature, subject to our [contributing guidelines](https://eslint.org/docs/developer-guide/contributing). Until then, please use the appropriate parser and plugin(s) for your experimental feature.
### Where to ask for help?
Join our [Mailing List](https://groups.google.com/group/eslint) or [Chatroom](https://gitter.im/eslint/eslint).
We have scheduled releases every two weeks on Friday or Saturday. You can follow a [release issue](https://github.com/eslint/eslint/issues?q=is%3Aopen+is%3Aissue+label%3Arelease) for updates about the scheduling of any particular release.
ESLint follows [semantic versioning](https://semver.org). 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 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 errors.
* A new rule is created.
* A new option to an existing rule that does not result in ESLint reporting more 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 option to an existing rule that results in ESLint reporting more errors by default.
* An existing formatter is removed.
* Part of the public API is removed or changed in an incompatible way.
According to our policy, any minor update may report more 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.
The following companies, organizations, and individuals support ESLint's ongoing maintenance and development. [Become a Sponsor](https://opencollective.com/eslint) to get your logo on our README and website.
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