eslint loader for webpack

$ npm install eslint-loader

In your webpack configuration

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

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: {
    loaders: [
      {test: /\.js$/, loader: "babel-loader", exclude: /node_modules/},
      {test: /\.js$/, loader: "eslint-loader", exclude: /node_modules/}
    ]
  }
  // ...
}

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

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

You can pass eslint options directly by

• Adding a query string to the loader for this loader usabe only

{
  module: {
    preLoaders: [
      {
        test: /\.js$/,
        loader: "eslint-loader?{rules:[{semi:0}]}",
        exclude: /node_modules/,
      },
    ],
  },
}

• Adding an eslint entry in your webpack config for global options:

module.exports = {
  eslint: {
    configFile: 'path/.eslintrc'
  }
}

Note that you can use both method in order to benefit from global & specific options

This option will enable ESLint autofix feature.

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

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: {
    // ...
  }
  eslint: {
    // 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"
    }
  }
}

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:

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

module.exports = {
  entry: "...",
  module: {
    // ...
  },
  eslint: {
    emitError: true
  }
}

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

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

module.exports = {
  entry: "...",
  module: {
    // ...
  },
  eslint: {
    quiet: true
  }
}

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

module.exports = {
  entry: "...",
  module: {
    // ...
  },
  eslint: {
    failOnWarning: true
  }
}

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

module.exports = {
  entry: "...",
  module: {
    // ...
  },
  eslint: {
    failOnError: true
  }
}

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.