babel-eslint

What is babel-eslint?

The babel-eslint npm package is a parser that allows you to lint all valid Babel code with the ESLint linter. It is designed to be used with ESLint and Babel, providing compatibility between the two and allowing developers to use ESLint on code that contains Babel-specific syntax.

What are babel-eslint's main functionalities?

Parsing Babel Code for ESLint

This feature allows developers to configure ESLint to use babel-eslint as the parser, enabling ESLint to understand and lint code that includes Babel-specific syntax that is not yet supported by ESLint's default parser.

module.exports = { parser: 'babel-eslint', rules: { /* ESLint rules */ } };

Experimental Syntax Support

babel-eslint can parse experimental syntax such as class properties and async functions, which might not be supported by ESLint's default parser. This allows developers to use ESLint on projects that make use of the latest JavaScript features.

class MyClass { async myMethod() { /* ... */ } }

Custom Babel Configuration

babel-eslint allows developers to specify a custom Babel configuration file, ensuring that the parser understands the code in the same way Babel does when it transpiles the code.

{ "parserOptions": { "babelOptions": { "configFile": "path/to/your/babel.config.js" } } }

Other packages similar to babel-eslint

@typescript-eslint/parser

This package is a parser that allows ESLint to lint TypeScript code. It is similar to babel-eslint in that it extends ESLint's capabilities to understand syntax not natively supported by ESLint. However, it is specifically designed for TypeScript, whereas babel-eslint is focused on Babel-specific JavaScript syntax.

eslint-plugin-flowtype

This package is an ESLint plugin that provides linting rules for Flow, a static type checker for JavaScript. Like babel-eslint, it extends ESLint's functionality to additional syntax features, but it is tailored for Flow type annotations rather than Babel's JavaScript transformations.

prettier

Prettier is an opinionated code formatter that supports many languages, including JavaScript. While it is not a linter like ESLint or a parser like babel-eslint, it is often used in conjunction with ESLint to enforce consistent code formatting. Prettier can parse and format code with Babel-specific syntax when used with the appropriate plugins.

README

babel-eslint

babel-eslint allows you to lint ALL valid Babel code with the fantastic ESLint.

Note: You don't need to use babel-eslint if you are only using ES6/ES2015. ESLint actually supports ES6/ES7, JSX, and object rest/spread by default now.

At the moment, you'll need it if you use stuff like class properties, decorators, async/await, types.

If there is an issue, first check if it can be reproduced with the regular parser or with the latest versions of eslint and babel-eslint!

For questions and support please visit the #linting babel slack channel (sign up here)!

Note that the ecmaFeatures config property may still be required for ESLint to work properly with features not in ECMAScript 5 by default. Examples are globalReturn and modules)

Known Issues

Flow:

Check out eslint-plugin-flow-vars: An eslint plugin that makes flow type annotations global variables and marks declarations as used. Solves the problem of false positives with no-undef and no-unused-vars.

• no-undef for global flow types: ReactElement, ReactClass #130
  • Workaround: define types as globals in .eslintrc or define types and import them import type ReactElement from './types'
• no-unused-vars/no-undef with Flow declarations (declare module A {}) #132

Modules/strict mode

• no-unused-vars: [2, {vars: local}] #136

Please check out eslint-plugin-react for React/JSX issues

• no-unused-vars with jsx

Please check out eslint-plugin-babel for other issues such as (and more):

• generator-star with async/await functions #78
• object-shorthand with spread operator #131

How does it work?

ESLint allows custom parsers. This is great but some of the syntax nodes that Babel supports aren't supported by ESLint. When using this plugin, ESLint is monkeypatched and your code is transformed into code that ESLint can understand. All location info such as line numbers, columns is also retained so you can track down errors with ease.

Basically babel-eslint exports an index.js that a linter can use. It just needs to export a parse method that takes in a string of code and outputs an AST.

Usage

Install

$ npm install eslint@1.x babel-eslint@5 --save-dev

$ npm install eslint@2.x babel-eslint@6 --save-dev

Setup

.eslintrc

{
  "parser": "babel-eslint",
  "rules": {
    "strict": 0
  }
}

Check out the ESLint docs for all possible rules.

Configuration

sourceType can be set to 'module'(default) or 'script' if your code isn't using ECMAScript modules. allowImportExportEverywhere can be set to true to allow import and export declarations to appear anywhere a statement is allowed if your build environment supports that. By default, import and export declarations can only appear at a program's top level.

.eslintrc

{
  "parser": "babel-eslint",
  "parserOptions": {
    "sourceType": "module",
    "allowImportExportEverywhere": false
  }
}

Run

$ eslint your-files-here