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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.
**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.
The names `"semi"` and `"quotes"` are the names of [rules](http://eslint.org/docs/rules) in ESLint. The first value is the error level of the rule and can be one of these values:
The three error levels allow you fine-grained control over how ESLint applies rules (for more configuration options and details, see the [configuration docs](http://eslint.org/docs/user-guide/configuring)).
ESLint follows [semantic versioning](http://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.
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 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](http://eslint.org/blog/2016/04/welcoming-jscs-to-eslint) and [JSCS](https://medium.com/@markelog/jscs-end-of-the-line-bc9bf0b3fdb2#.u76sx334n) announcements.
### So, should I stop using JSCS and start using ESLint?
Maybe, depending on how much you need it. [JSCS has reached end of life](http://eslint.org/blog/2016/07/jscs-end-of-life), but if it is working for you then there is no reason to move yet. We are still working to smooth the transition. You can see our progress [here](https://github.com/eslint/eslint/milestones/JSCS%20Compatibility). 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.
If you are having issues with JSCS, you can try to move to ESLint. We are focusing our time and energy on JSCS compatibility issues.
Yes, ESLint natively supports parsing JSX syntax (this must be enabled in [configuration](http://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 6. By default, this support is off. You can enable ECMAScript 6 support through [configuration](http://eslint.org/docs/user-guide/configuring).
ESLint doesn't natively support experimental ECMAScript language features. You can use [babel-eslint](https://github.com/babel/babel-eslint) to use any option available in Babel.
