react-dev-utils

What is react-dev-utils?

The react-dev-utils package provides a set of utilities designed to enhance the development experience of building React applications, especially when used in conjunction with Create React App. These utilities include webpack configuration helpers, development server helpers, and an assortment of other small utilities designed to make common tasks more convenient.

What are react-dev-utils's main functionalities?

Interacting with Webpack

This feature allows developers to interact with Webpack more easily, for example, by choosing a port for the development server. The code sample demonstrates how to use the `choosePort` function to select a port for running the development server.

const { choosePort } = require('react-dev-utils/WebpackDevServerUtils');
choosePort('localhost', 3000).then(port => console.log(`Port selected: ${port}`));

Formatting Webpack Messages

This utility helps in formatting the output of Webpack messages to be more readable. The code sample shows how to format raw Webpack messages into a more digestible format, separating out errors and warnings.

const { formatWebpackMessages } = require('react-dev-utils/formatWebpackMessages');
const rawMessages = webpackCompiler.run();
const messages = formatWebpackMessages(rawMessages);
console.log(messages.errors); console.log(messages.warnings);

Opening Browser

This feature provides a simple way to programmatically open the developer's browser to a specified URL, which is particularly useful when starting a development server. The code sample demonstrates opening the default browser to 'http://localhost:3000'.

const { openBrowser } = require('react-dev-utils/openBrowser');
openBrowser('http://localhost:3000');

Other packages similar to react-dev-utils

webpack-dev-server

Similar to some functionalities provided by react-dev-utils, webpack-dev-server offers a development server that provides live reloading. It is more focused on being a standalone server for webpack projects, whereas react-dev-utils offers utilities that are more specifically tailored to React applications and can be used in conjunction with webpack-dev-server.

env-cmd

This package allows you to specify and manage environment variables for your development environment, similar to how react-dev-utils allows for environment variable manipulation and other configuration tweaks. However, env-cmd is more focused on environment variables and does not offer the wide range of utilities related to webpack and development server management.

cross-env

Cross-env provides a cross-platform way to set and use environment variables in npm scripts, which is a narrower scope compared to react-dev-utils. While react-dev-utils includes utilities for working with environment variables among its features, cross-env is specifically designed for this purpose and does not include the broader set of development utilities.

README

react-dev-utils

This package includes some utilities used by Create React App.
Please refer to its documentation:

• Getting Started – How to create a new app.
• User Guide – How to develop apps bootstrapped with Create React App.

Usage in Create React App Projects

These utilities come by default with Create React App. You don’t need to install it separately in Create React App projects.

Usage Outside of Create React App

If you don’t use Create React App, or if you ejected, you may keep using these utilities. Their development will be aligned with Create React App, so major versions of these utilities may come out relatively often. Feel free to fork or copy and paste them into your projects if you’d like to have more control over them, or feel free to use the old versions. Not all of them are React-specific, but we might make some of them more React-specific in the future.

Entry Points

There is no single entry point. You can only import individual top-level modules.

new InterpolateHtmlPlugin(htmlWebpackPlugin: HtmlWebpackPlugin, replacements: {[key:string]: string})

This webpack plugin lets us interpolate custom variables into index.html.
It works in tandem with HtmlWebpackPlugin 2.x via its events.

var path = require('path');
var HtmlWebpackPlugin = require('html-webpack-plugin');
var InterpolateHtmlPlugin = require('react-dev-utils/InterpolateHtmlPlugin');

// webpack config
var publicUrl = '/my-custom-url';

module.exports = {
  output: {
    // ...
    publicPath: publicUrl + '/',
  },
  // ...
  plugins: [
    // Generates an `index.html` file with the <script> injected.
    new HtmlWebpackPlugin({
      inject: true,
      template: path.resolve('public/index.html'),
    }),
    // Makes the public URL available as %PUBLIC_URL% in index.html, e.g.:
    // <link rel="icon" href="%PUBLIC_URL%/favicon.ico">
    new InterpolateHtmlPlugin(HtmlWebpackPlugin, {
      PUBLIC_URL: publicUrl,
      // You can pass any key-value pairs, this was just an example.
      // WHATEVER: 42 will replace %WHATEVER% with 42 in index.html.
    }),
    // ...
  ],
  // ...
};

new InlineChunkHtmlPlugin(htmlWebpackPlugin: HtmlWebpackPlugin, tests: Regex[])

This webpack plugin inlines script chunks into index.html.
It works in tandem with HtmlWebpackPlugin 4.x.

var path = require('path');
var HtmlWebpackPlugin = require('html-webpack-plugin');
var InlineChunkHtmlPlugin = require('react-dev-utils/InlineChunkHtmlPlugin');

// webpack config
var publicUrl = '/my-custom-url';

module.exports = {
  output: {
    // ...
    publicPath: publicUrl + '/',
  },
  // ...
  plugins: [
    // Generates an `index.html` file with the <script> injected.
    new HtmlWebpackPlugin({
      inject: true,
      template: path.resolve('public/index.html'),
    }),
    // Inlines chunks with `runtime` in the name
    new InlineChunkHtmlPlugin(HtmlWebpackPlugin, [/runtime/]),
    // ...
  ],
  // ...
};

new ModuleScopePlugin(appSrc: string | string[], allowedFiles?: string[])

This webpack plugin ensures that relative imports from app's source directories don't reach outside of it.

var path = require('path');
var ModuleScopePlugin = require('react-dev-utils/ModuleScopePlugin');

module.exports = {
  // ...
  resolve: {
    // ...
    plugins: [
      new ModuleScopePlugin(paths.appSrc, [paths.appPackageJson]),
      // ...
    ],
    // ...
  },
  // ...
};

checkRequiredFiles(files: Array<string>): boolean

Makes sure that all passed files exist.
Filenames are expected to be absolute.
If a file is not found, prints a warning message and returns false.

var path = require('path');
var checkRequiredFiles = require('react-dev-utils/checkRequiredFiles');

if (
  !checkRequiredFiles([
    path.resolve('public/index.html'),
    path.resolve('src/index.js'),
  ])
) {
  process.exit(1);
}

clearConsole(): void

Clears the console, hopefully in a cross-platform way.

var clearConsole = require('react-dev-utils/clearConsole');

clearConsole();
console.log('Just cleared the screen!');

eslintFormatter(results: Object): string

This is our custom ESLint formatter that integrates well with Create React App console output.
You can use the default one instead if you prefer so.

const eslintFormatter = require('react-dev-utils/eslintFormatter');

// In your webpack config:
// ...
module: {
  rules: [
    {
      test: /\.(js|jsx)$/,
      include: paths.appSrc,
      enforce: 'pre',
      use: [
        {
          loader: 'eslint-loader',
          options: {
            // Pass the formatter:
            formatter: eslintFormatter,
          },
        },
      ],
    },
  ];
}

FileSizeReporter

measureFileSizesBeforeBuild(buildFolder: string): Promise<OpaqueFileSizes>

Captures JS and CSS asset sizes inside the passed buildFolder. Save the result value to compare it after the build.

printFileSizesAfterBuild(webpackStats: WebpackStats, previousFileSizes: OpaqueFileSizes, buildFolder: string, maxBundleGzipSize?: number, maxChunkGzipSize?: number)

Prints the JS and CSS asset sizes after the build, and includes a size comparison with previousFileSizes that were captured earlier using measureFileSizesBeforeBuild(). maxBundleGzipSize and maxChunkGzipSizemay may optionally be specified to display a warning when the main bundle or a chunk exceeds the specified size (in bytes).

var {
  measureFileSizesBeforeBuild,
  printFileSizesAfterBuild,
} = require('react-dev-utils/FileSizeReporter');

measureFileSizesBeforeBuild(buildFolder).then(previousFileSizes => {
  return cleanAndRebuild().then(webpackStats => {
    printFileSizesAfterBuild(webpackStats, previousFileSizes, buildFolder);
  });
});

formatWebpackMessages({errors: Array<string>, warnings: Array<string>}): {errors: Array<string>, warnings: Array<string>}

Extracts and prettifies warning and error messages from webpack stats object.

var webpack = require('webpack');
var config = require('../config/webpack.config.dev');
var formatWebpackMessages = require('react-dev-utils/formatWebpackMessages');

var compiler = webpack(config);

compiler.hooks.invalid.tap('invalid', function () {
  console.log('Compiling...');
});

compiler.hooks.done.tap('done', function (stats) {
  var rawMessages = stats.toJson({}, true);
  var messages = formatWebpackMessages(rawMessages);
  if (!messages.errors.length && !messages.warnings.length) {
    console.log('Compiled successfully!');
  }
  if (messages.errors.length) {
    console.log('Failed to compile.');
    messages.errors.forEach(e => console.log(e));
    return;
  }
  if (messages.warnings.length) {
    console.log('Compiled with warnings.');
    messages.warnings.forEach(w => console.log(w));
  }
});

printBuildError(error: Object): void

Prettify some known build errors. Pass an Error object to log a prettified error message in the console.

  const printBuildError = require('react-dev-utils/printBuildError')
  try {
    build()
  } catch(e) {
    printBuildError(e) // logs prettified message
  }

getProcessForPort(port: number): string

Finds the currently running process on port. Returns a string containing the name and directory, e.g.,

create-react-app
in /Users/developer/create-react-app

var getProcessForPort = require('react-dev-utils/getProcessForPort');

getProcessForPort(3000);

launchEditor(fileName: string, lineNumber: number): void

On macOS, tries to find a known running editor process and opens the file in it. It can also be explicitly configured by REACT_EDITOR, VISUAL, or EDITOR environment variables. For example, you can put REACT_EDITOR=atom in your .env.local file, and Create React App will respect that.

noopServiceWorkerMiddleware(servedPath: string): ExpressMiddleware

Returns Express middleware that serves a ${servedPath}/service-worker.js that resets any previously set service worker configuration. Useful for development.

redirectServedPathMiddleware(servedPath: string): ExpressMiddleware

Returns Express middleware that redirects to ${servedPath}/${req.path}, if req.url does not start with servedPath. Useful for development.

openBrowser(url: string): boolean

Attempts to open the browser with a given URL.
On Mac OS X, attempts to reuse an existing Chrome tab via AppleScript.
Otherwise, falls back to opn behavior.

var path = require('path');
var openBrowser = require('react-dev-utils/openBrowser');

if (openBrowser('http://localhost:3000')) {
  console.log('The browser tab has been opened!');
}

printHostingInstructions(appPackage: Object, publicUrl: string, publicPath: string, buildFolder: string, useYarn: boolean): void

Prints hosting instructions after the project is built.

Pass your parsed package.json object as appPackage, your URL where you plan to host the app as publicUrl, output.publicPath from your webpack configuration as publicPath, the buildFolder name, and whether to useYarn in instructions.

const appPackage = require(paths.appPackageJson);
const publicUrl = paths.publicUrlOrPath;
const publicPath = config.output.publicPath;
printHostingInstructions(appPackage, publicUrl, publicPath, 'build', true);

WebpackDevServerUtils

choosePort(host: string, defaultPort: number): Promise<number | null>

Returns a Promise resolving to either defaultPort or next available port if the user confirms it is okay to do. If the port is taken and the user has refused to use another port, or if the terminal is not interactive and can’t present user with the choice, resolves to null.

createCompiler(args: Object): WebpackCompiler

Creates a webpack compiler instance for WebpackDevServer with built-in helpful messages.

The args object accepts a number of properties:

• appName string: The name that will be printed to the terminal.
• config Object: The webpack configuration options to be provided to the webpack constructor.
• urls Object: To provide the urls argument, use prepareUrls() described below.
• useYarn boolean: If true, yarn instructions will be emitted in the terminal instead of npm.
• useTypeScript boolean: If true, TypeScript type checking will be enabled. Be sure to provide the devSocket argument above if this is set to true.
• webpack function: A reference to the webpack constructor.

prepareProxy(proxySetting: string, appPublicFolder: string, servedPathname: string): Object

Creates a WebpackDevServer proxy configuration object from the proxy setting in package.json.

prepareUrls(protocol: string, host: string, port: number, pathname: string = '/'): Object

Returns an object with local and remote URLs for the development server. Pass this object to createCompiler() described above.

webpackHotDevClient

This is an alternative client for WebpackDevServer that shows a syntax error overlay.

It currently supports only webpack 3.x.

// webpack development config
module.exports = {
  // ...
  entry: [
    // You can replace the line below with these two lines if you prefer the
    // stock client:
    // require.resolve('webpack-dev-server/client') + '?/',
    // require.resolve('webpack/hot/dev-server'),
    'react-dev-utils/webpackHotDevClient',
    'src/index',
  ],
  // ...
};

getCSSModuleLocalIdent(context: Object, localIdentName: String, localName: String, options: Object): string

Creates a class name for CSS Modules that uses either the filename or folder name if named index.module.css.

For MyFolder/MyComponent.module.css and class MyClass the output will be MyComponent.module_MyClass__[hash] For MyFolder/index.module.css and class MyClass the output will be MyFolder_MyClass__[hash]

const getCSSModuleLocalIdent = require('react-dev-utils/getCSSModuleLocalIdent');

// In your webpack config:
// ...
module: {
  rules: [
    {
      test: /\.module\.css$/,
      use: [
        require.resolve('style-loader'),
        {
          loader: require.resolve('css-loader'),
          options: {
            importLoaders: 1,
            modules: {
              getLocalIdent: getCSSModuleLocalIdent,
            },
          },
        },
        {
          loader: require.resolve('postcss-loader'),
          options: postCSSLoaderOptions,
        },
      ],
    },
  ];
}

getCacheIdentifier(environment: string, packages: string[]): string

Returns a cache identifier (string) consisting of the specified environment and related package versions, e.g.,

var getCacheIdentifier = require('react-dev-utils/getCacheIdentifier');

getCacheIdentifier('prod', ['react-dev-utils', 'chalk']); // # => 'prod:react-dev-utils@5.0.0:chalk@3.0.0'