eslint-loader

What is eslint-loader?

eslint-loader is a webpack loader that integrates ESLint into the webpack build process. It allows you to lint your JavaScript code as part of the build process, ensuring that your code adheres to specified coding standards and conventions.

What are eslint-loader's main functionalities?

Linting JavaScript files

This feature allows you to lint JavaScript files using ESLint as part of the webpack build process. The configuration specifies that all `.js` files, excluding those in `node_modules`, should be processed by `eslint-loader`.

module.exports = {
  module: {
    rules: [
      {
        test: /\.js$/,
        exclude: /node_modules/,
        use: [
          {
            loader: 'eslint-loader',
            options: {
              // eslint options (if necessary)
            }
          }
        ]
      }
    ]
  }
};

Custom ESLint options

This feature allows you to specify custom ESLint options, such as the path to the ESLint configuration file (`configFile`) and whether to emit warnings instead of errors (`emitWarning`).

module.exports = {
  module: {
    rules: [
      {
        test: /\.js$/,
        exclude: /node_modules/,
        use: [
          {
            loader: 'eslint-loader',
            options: {
              configFile: 'path/to/.eslintrc',
              emitWarning: true
            }
          }
        ]
      }
    ]
  }
};

Other packages similar to eslint-loader

eslint-webpack-plugin

eslint-webpack-plugin is a modern alternative to eslint-loader. It provides similar functionality by integrating ESLint into the webpack build process but is designed to work with webpack 5 and above. It offers better performance and more features compared to eslint-loader.

stylelint-webpack-plugin

stylelint-webpack-plugin is a webpack plugin that integrates Stylelint into the webpack build process. While it focuses on linting CSS and other style files, it provides similar functionality to eslint-loader for stylesheets.

tslint-loader

tslint-loader is a webpack loader that integrates TSLint into the webpack build process. It is similar to eslint-loader but is specifically designed for linting TypeScript files.

README

eslint-loader

eslint loader for webpack

Install

$ npm install eslint-loader --save-dev

Usage

In your webpack configuration

module.exports = {
  // ...
  module: {
    rules: [
      {
        test: /\.js$/,
        exclude: /node_modules/,
        loader: "eslint-loader",
        options: {
          // eslint options (if necessary)
        }
      },
    ],
  },
  // ...
}

When using with transpiling loaders (like babel-loader), make sure they are in correct order (bottom to top). Otherwise files will be check after being processed by babel-loader

module.exports = {
  // ...
  module: {
    rules: [
      {
        test: /\.js$/,
        exclude: /node_modules/,
        use: [
          "babel-loader",
          "eslint-loader",
        ],
      },
    ],
  },
  // ...
}

To be safe, you can use enforce: "pre" section to check source files, not modified by other loaders (like babel-loader)

module.exports = {
  // ...
  module: {
    rules: [
      {
        enforce: "pre",
        test: /\.js$/,
        exclude: /node_modules/,
        loader: "eslint-loader",
      },
      {
        test: /\.js$/,
        exclude: /node_modules/,
        loader: "babel-loader",
      },
    ],
  },
  // ...
}

Options

You can pass eslint options using standard webpack loader options.

Note that the config option you provide will be passed to the CLIEngine. This is a different set of options than what you'd specify in package.json or .eslintrc. See the eslint docs for more detail.

fix (default: false)

This option will enable ESLint autofix feature.

Be careful: this option might cause webpack to enter an infinite build loop if some issues cannot be fixed properly.

cache (default: false)

This option will enable caching of the linting results into a file. This is particularly useful in reducing linting time when doing a full build.

The cache file is written to the ./node_modules/.cache directory.

formatter (default: eslint stylish formatter)

Loader accepts a function that will have one argument: an array of eslint messages (object). The function must return the output as a string. You can use official eslint formatters.

module.exports = {
  entry: "...",
  module: {
    rules: [
      {
        test: /\.js$/,
        exclude: /node_modules/,
        loader: "eslint-loader",
        options: {
          // several examples !

          // default value
          formatter: require("eslint/lib/formatters/stylish"),

          // community formatter
          formatter: require("eslint-friendly-formatter"),

          // custom formatter
          formatter: function(results) {
            // `results` format is available here
            // http://eslint.org/docs/developer-guide/nodejs-api.html#executeonfiles()

            // you should return a string
            // DO NOT USE console.*() directly !
            return "OUTPUT"
          }
        }
      },
    ],
  },
}

eslintPath (default: "eslint")

Path to eslint instance that will be used for linting.

module.exports = {
  entry: "...",
  module: {
    rules: [
      {
        test: /\.js$/,
        exclude: /node_modules/,
        loader: "eslint-loader",
        options: {
          eslintPath: path.join(__dirname, "reusable-eslint-rules.js"),
        }
      },
    ],
  },
  }

Errors and Warning

By default the loader will auto adjust error reporting depending on eslint errors/warnings counts. You can still force this behavior by using emitError or emitWarning options:

emitError (default: false)

Loader will always return errors if this option is set to true.

module.exports = {
  entry: "...",
  module: {
    rules: [
      {
        test: /\.js$/,
        exclude: /node_modules/,
        loader: "eslint-loader",
        options: {
          emitError: true,
        }
      },
    ],
  },
}

emitWarning (default: false)

Loader will always return warnings if option is set to true.

quiet (default: false)

Loader will process and report errors only and ignore warnings if this option is set to true

module.exports = {
  entry: "...",
  module: {
    rules: [
      {
        test: /\.js$/,
        exclude: /node_modules/,
        loader: "eslint-loader",
        options: {
          quiet: true,
        }
      },
    ],
  },
}

failOnWarning (default: false)

Loader will cause the module build to fail if there are any eslint warnings.

module.exports = {
  entry: "...",
  module: {
    rules: [
      {
        test: /\.js$/,
        exclude: /node_modules/,
        loader: "eslint-loader",
        options: {
          failOnWarning: true,
        }
      },
    ],
  },
}

failOnError (default: false)

Loader will cause the module build to fail if there are any eslint errors.

module.exports = {
  entry: "...",
  module: {
    rules: [
      {
        test: /\.js$/,
        exclude: /node_modules/,
        loader: "eslint-loader",
        options: {
          failOnError: true,
        }
      },
    ],
  },
}

outputReport (default: false)

Write the output of the errors to a file, for example a checkstyle xml file for use for reporting on Jenkins CI

The filePath is relative to the webpack config: output.path You can pass in a different formatter for the output file, if none is passed in the default/configured formatter will be used

module.exports = {
  entry: "...",
  module: {
    rules: [
      {
        test: /\.js$/,
        exclude: /node_modules/,
        loader: "eslint-loader",
        options: {
          outputReport: {
            filePath: 'checkstyle.xml',
            formatter: require('eslint/lib/formatters/checkstyle')
          }
        }
      },
    ],
  },
}

Gotchas

NoErrorsPlugin

NoErrorsPlugin prevents webpack from outputting anything into a bundle. So even ESLint warnings will fail the build. No matter what error settings are used for eslint-loader.

So if you want to see ESLint warnings in console during development using WebpackDevServer remove NoErrorsPlugin from webpack config.

Defining configFile or using eslint -c path/.eslintrc

Bear in mind that when you define configFile, eslint doesn't automatically look for .eslintrc files in the directory of the file to be linted. More information is available in official eslint documentation in section Using Configuration Files. See #129.

---

Changelog

License

Changelog

1.9.0 - 2017-07-06

• Added: eslintPath option so you can provide your own eslint path. (#183 - @trungdq88)