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.

Breaking change in v11.x.x

As of the v11.x.x release, babel-eslint now requires Babel as a peer dependency and expects a valid Babel configuration file to exist. This ensures that the same Babel configuration is used during both linting and compilation.

When should I use babel-eslint?

ESLint's default parser and core rules only suppport the latest final ECMAScript standard and do not support experimental (such as new features) and non-standard (such as Flow or TypeScript types) syntax provided by Babel. babel-eslint is a parser that allows ESLint to run on source code that is transformed by Babel.

Note: You only need to use babel-eslint if you are using Babel to transform your code. If this is not the case, please use the relevant parser for your chosen flavor of ECMAScript (note that the default parser supports all non-experimental syntax as well as JSX).

How does it work?

ESLint allows for the use of custom parsers. When using this plugin, your code is parsed by Babel's parser (using the configuration specified in your Babel configuration file) and the resulting AST is transformed into an ESTree-compliant structure that ESLint can understand. All location info such as line numbers, columns is also retained so you can track down errors with ease.

Note: ESLint's core rules do not support experimental syntax and may therefore not work as expected when using babel-eslint. Please use the companion eslint-plugin-babel plugin for core rules that you have issues with.

Usage

Installation

$ npm install eslint babel-eslint --save-dev
# or
$ yarn add eslint babel-eslint -D

Note: babel-eslint requires babel/core@>=7.2.0 and a valid Babel configuration file to run. If you do not have this already set up, please see the Babel Usage Guide.

Setup

To use babel-eslint, "babel-eslint" must be specified as the parser in your ESLint configuration file (see here for more detailed information).

.eslintrc.js

module.exports = {
  parser: "babel-eslint",
};

With the parser set, your configuration can be configured as described in the Configuring ESLint documentation.

Note: The parserOptions described in the official documentation are for the default parser and are not necessarily supported by babel-eslint. Please see the section directly below for supported parserOptions.

Additional parser configuration

Additional configuration options can be set in your ESLint configuration under the parserOptions key. Please note that the ecmaFeatures config property may still be required for ESLint to work properly with features not in ECMAScript 5 by default.

• requireConfigFile (default true) can be set to false to allow babel-eslint to run on files that do not have a Babel configuration associated with them. This can be useful for linting files that are not transformed by Babel (such as tooling configuration files), though we recommend using the default parser via glob-based configuration. Note: babel-eslint will not parse any experimental syntax when no configuration file is found.
• sourceType can be set to "module"(default) or "script" if your code isn't using ECMAScript modules.
• allowImportExportEverywhere (default false) can be set to true to allow import and export declarations to appear anywhere a statement is allowed if your build environment supports that. Otherwise import and export declarations can only appear at a program's top level.
• ecmaFeatures.globalReturn (default false) allow return statements in the global scope when used with sourceType: "script".
• babelOptions passes through Babel's configuration loading and merging options (for instance, in case of a monorepo). When not defined, babel-eslint will use Babel's default configuration file resolution logic.

.eslintrc.js

module.exports = {
  parser: "babel-eslint",
  parserOptions: {
    sourceType: "module",
    allowImportExportEverywhere: false,
    ecmaFeatures: {
      globalReturn: false,
    },
    babelOptions: {
      configFile: "path/to/config.js",
    },
  },
};

Run

$ ./node_modules/.bin/eslint yourfile.js

Known issues

Flow:

Check out eslint-plugin-flowtype: 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: ["error", { 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.

Questions and support

If you have an issue, please first check if it can be reproduced with the default parser and with the latest versions of eslint and babel-eslint. If it is not reproducible with the default parser, it is most likely an issue with babel-eslint.

For questions and support please visit the #discussion Babel Slack channel (sign up here) or the ESLint Gitter.