📦

React がビルドされるまでの流れを理解したい

ツチノコ

2023/12/17に公開

株式会社アイデミーの土屋です。
Lab Bank という化学業界の研究室向け SaaS 開発と、
Modeloy という DX プロジェクトの伴走支援をしています。

https://labbank.jp/
https://www.modeloy.ai/

はじめに

React(TypeScript)で開発を行う際に、基本的には何かしらのフレームワークを利用することになると思います。
その際、ビルドに必要な設定はフレームワークが最初から設定してくれていることも多いため、
あまり意識することはないかもしれません。

これまでなんとなく
「TypeScript が JavaScript に変換される」
「ESModules と CommonJS で書き方が違う」
「 Webpack などのモジュールバンドラーが一つのファイルにまとめてくれる」
ということは知ってはいたのですが、
具体的にどのような流れで実行されているのかは理解していませんでした。

このままフレームワークの温室育ちのままではいけないと感じたため、
React がビルドされるまでの流れを整理してみようと思います。

主な登場人物

まずは、React がビルドされるまでに関わる主な登場人物を整理してみます。
今回は実行までの流れにフォーカスするため、
細かい設定値などについては割愛します。

Webpack

Webpack はモジュールバンドラと呼ばれるツールであり、
複数のモジュールを一つにまとめる(=バンドル)して出力ことができます。

これにより開発時のモジュールごとの分離と、
本番環境でのネットワーク負荷軽減を両立することができます。

Loaders と呼ばれるプラグインを利用することで、
TypeScript や JSX を JavaScript に変換することもできますが、
メインの役割としてはモジュールをバンドルすることになります。

https://webpack.js.org/

Babel

Babel は JavaScript のコンパイラ(トランスパイラ)であり、
ES2015(ES6)以上のコードを ES5 に変換することができます。

例えば、アロー関数は ES6 から導入された構文ですが、
Babel を利用することで ES5 の形式に変換することができます。

// Babel Input: ES2015 arrow function
[1, 2, 3].map((n) => n + 1);

// Babel Output: ES5 equivalent
[1, 2, 3].map(function (n) {
  return n + 1;
});

他にも、 Webpack と同様に
JSX や TypeScript を JavaScript に変換することもできます。

https://babeljs.io/docs

tsc

tsc は TypeScript のコンパイラで、
tsconfig.json に記載された設定に従って、
TypeScript を JavaScript に変換します。
また、TypeScript の型チェックなども行います。

ざっくりと流れを理解する

上記の登場人物の主な役割とできることを整理すると、
下記のようになります。

ツール	役割	できること
Webpack	複数のファイルを 1 つのファイルにまとめる	- JSX や TypeScript を JavaScript に変換 - ES2015 以上のコードを ES5 に変換 etc
Babel	ES2015 以上のコードを ES5 に変換	- JSX や TypeScript を JavaScript に変換 etc
tsc	TypeScript を JavaScript に変換	- JSX や TypeScript を JavaScript に変換 - 型チェック

それぞれのツールのメインの役割は異なっていますが、
できることについては若干被りがあります。

JSX や TypeScript をどのツールが変換するかという違いはありますが、
Webpack のLoadersの説明には

Loaders are transformations that are applied to the source code of a module. They allow you to pre-process files as you import or “load” them. Thus, loaders are kind of like “tasks” in other build tools and provide a powerful way to handle front-end build steps. Loaders can transform files from a different language (like TypeScript) to JavaScript or load inline images as data URLs. Loaders even allow you to do things like import CSS files directly from your JavaScript modules!

ローダーは、モジュールのソースコードに適用される変換である。ローダーによって、ファイルをインポートしたり「ロード」したりするときに、前処理を行うことができる。このように、ローダーは他のビルドツールにおける「タスク」のようなもので、フロントエンドのビルドステップを処理する強力な方法を提供します。ローダーは、異なる言語（TypeScript など）のファイルを JavaScript に変換したり、インライン画像をデータ URL として読み込んだりできる。ローダーは JavaScript モジュールから直接 CSS ファイルをインポートするようなこともできる！(DeepL 翻訳)

とあります。
要約すると、「前処理として、異なる言語（TypeScript など）のファイルを JavaScript に変換したり、インライン画像をデータ URL として読み込んだりできる。」
ということになります。

つまり、流れとしては

JSX や TypeScript を何かしらのツールで JavaScript に変換する
1. その際に、ES6 以上のコードを ES5 に変換する
変換された JavaScript を Webpack などのモジュールバンドラで一つのファイルにまとめる
ビルドされた JavaScript を実行する

という流れになります。

React をビルドした時の流れ

では、実際に React をビルドした時の流れを見ていきたいと思います。

環境構築

下記のコマンドで環境を構築します。

npx create-react-app my-app --template typescript

出来上がった環境を見ると、
webpack.config.js や babel.config.js などの設定ファイルが存在しません。
ですが、node_modules には babel や webpack に関連するパッケージが存在します。

調べてみると、
npm run eject というコマンドを実行することで、
create react appでインストールされたモジュールを、直接インストールした状態にすることができるようです。

npm run eject

npm run eject

上記のコマンドを実行すると、package.jsonのdependencies が更新され、
config ディレクトリ配下にwebpack.config.jsが作成されました。

この時点でのディレクトリ構成は

tree . -I node_modules
.
├── README.md
├── config
│   ├── env.js
│   ├── getHttpsConfig.js
│   ├── jest
│   │   ├── babelTransform.js
│   │   ├── cssTransform.js
│   │   └── fileTransform.js
│   ├── modules.js
│   ├── paths.js
│   ├── webpack
│   │   └── persistentCache
│   │       └── createEnvironmentHash.js
│   ├── webpack.config.js
│   └── webpackDevServer.config.js
├── package-lock.json
├── package.json
├── public
│   ├── favicon.ico
│   ├── index.html
│   ├── logo192.png
│   ├── logo512.png
│   ├── manifest.json
│   └── robots.txt
├── scripts
│   ├── build.js
│   ├── start.js
│   └── test.js
├── src
│   ├── App.css
│   ├── App.test.tsx
│   ├── App.tsx
│   ├── index.css
│   ├── index.tsx
│   ├── logo.svg
│   ├── react-app-env.d.ts
│   ├── reportWebVitals.ts
│   └── setupTests.ts
└── tsconfig.json

になっています。

webpack.config.js(長いです)

config/webpack.config.js

'use strict';

const fs = require('fs');
const path = require('path');
const webpack = require('webpack');
const resolve = require('resolve');
const HtmlWebpackPlugin = require('html-webpack-plugin');
const CaseSensitivePathsPlugin = require('case-sensitive-paths-webpack-plugin');
const InlineChunkHtmlPlugin = require('react-dev-utils/InlineChunkHtmlPlugin');
const TerserPlugin = require('terser-webpack-plugin');
const MiniCssExtractPlugin = require('mini-css-extract-plugin');
const CssMinimizerPlugin = require('css-minimizer-webpack-plugin');
const { WebpackManifestPlugin } = require('webpack-manifest-plugin');
const InterpolateHtmlPlugin = require('react-dev-utils/InterpolateHtmlPlugin');
const WorkboxWebpackPlugin = require('workbox-webpack-plugin');
const ModuleScopePlugin = require('react-dev-utils/ModuleScopePlugin');
const getCSSModuleLocalIdent = require('react-dev-utils/getCSSModuleLocalIdent');
const ESLintPlugin = require('eslint-webpack-plugin');
const paths = require('./paths');
const modules = require('./modules');
const getClientEnvironment = require('./env');
const ModuleNotFoundPlugin = require('react-dev-utils/ModuleNotFoundPlugin');
const ForkTsCheckerWebpackPlugin =
  process.env.TSC_COMPILE_ON_ERROR === 'true'
    ? require('react-dev-utils/ForkTsCheckerWarningWebpackPlugin')
    : require('react-dev-utils/ForkTsCheckerWebpackPlugin');
const ReactRefreshWebpackPlugin = require('@pmmmwh/react-refresh-webpack-plugin');

const createEnvironmentHash = require('./webpack/persistentCache/createEnvironmentHash');

// Source maps are resource heavy and can cause out of memory issue for large source files.
const shouldUseSourceMap = process.env.GENERATE_SOURCEMAP !== 'false';

const reactRefreshRuntimeEntry = require.resolve('react-refresh/runtime');
const reactRefreshWebpackPluginRuntimeEntry = require.resolve(
  '@pmmmwh/react-refresh-webpack-plugin'
);
const babelRuntimeEntry = require.resolve('babel-preset-react-app');
const babelRuntimeEntryHelpers = require.resolve(
  '@babel/runtime/helpers/esm/assertThisInitialized',
  { paths: [babelRuntimeEntry] }
);
const babelRuntimeRegenerator = require.resolve('@babel/runtime/regenerator', {
  paths: [babelRuntimeEntry],
});

// Some apps do not need the benefits of saving a web request, so not inlining the chunk
// makes for a smoother build process.
const shouldInlineRuntimeChunk = process.env.INLINE_RUNTIME_CHUNK !== 'false';

const emitErrorsAsWarnings = process.env.ESLINT_NO_DEV_ERRORS === 'true';
const disableESLintPlugin = process.env.DISABLE_ESLINT_PLUGIN === 'true';

const imageInlineSizeLimit = parseInt(
  process.env.IMAGE_INLINE_SIZE_LIMIT || '10000'
);

// Check if TypeScript is setup
const useTypeScript = fs.existsSync(paths.appTsConfig);

// Check if Tailwind config exists
const useTailwind = fs.existsSync(
  path.join(paths.appPath, 'tailwind.config.js')
);

// Get the path to the uncompiled service worker (if it exists).
const swSrc = paths.swSrc;

// style files regexes
const cssRegex = /\.css$/;
const cssModuleRegex = /\.module\.css$/;
const sassRegex = /\.(scss|sass)$/;
const sassModuleRegex = /\.module\.(scss|sass)$/;

const hasJsxRuntime = (() => {
  if (process.env.DISABLE_NEW_JSX_TRANSFORM === 'true') {
    return false;
  }

  try {
    require.resolve('react/jsx-runtime');
    return true;
  } catch (e) {
    return false;
  }
})();

// This is the production and development configuration.
// It is focused on developer experience, fast rebuilds, and a minimal bundle.
module.exports = function (webpackEnv) {
  const isEnvDevelopment = webpackEnv === 'development';
  const isEnvProduction = webpackEnv === 'production';

  // Variable used for enabling profiling in Production
  // passed into alias object. Uses a flag if passed into the build command
  const isEnvProductionProfile =
    isEnvProduction && process.argv.includes('--profile');

  // We will provide `paths.publicUrlOrPath` to our app
  // as %PUBLIC_URL% in `index.html` and `process.env.PUBLIC_URL` in JavaScript.
  // Omit trailing slash as %PUBLIC_URL%/xyz looks better than %PUBLIC_URL%xyz.
  // Get environment variables to inject into our app.
  const env = getClientEnvironment(paths.publicUrlOrPath.slice(0, -1));

  const shouldUseReactRefresh = env.raw.FAST_REFRESH;

  // common function to get style loaders
  const getStyleLoaders = (cssOptions, preProcessor) => {
    const loaders = [
      isEnvDevelopment && require.resolve('style-loader'),
      isEnvProduction && {
        loader: MiniCssExtractPlugin.loader,
        // css is located in `static/css`, use '../../' to locate index.html folder
        // in production `paths.publicUrlOrPath` can be a relative path
        options: paths.publicUrlOrPath.startsWith('.')
          ? { publicPath: '../../' }
          : {},
      },
      {
        loader: require.resolve('css-loader'),
        options: cssOptions,
      },
      {
        // Options for PostCSS as we reference these options twice
        // Adds vendor prefixing based on your specified browser support in
        // package.json
        loader: require.resolve('postcss-loader'),
        options: {
          postcssOptions: {
            // Necessary for external CSS imports to work
            // https://github.com/facebook/create-react-app/issues/2677
            ident: 'postcss',
            config: false,
            plugins: !useTailwind
              ? [
                  'postcss-flexbugs-fixes',
                  [
                    'postcss-preset-env',
                    {
                      autoprefixer: {
                        flexbox: 'no-2009',
                      },
                      stage: 3,
                    },
                  ],
                  // Adds PostCSS Normalize as the reset css with default options,
                  // so that it honors browserslist config in package.json
                  // which in turn let's users customize the target behavior as per their needs.
                  'postcss-normalize',
                ]
              : [
                  'tailwindcss',
                  'postcss-flexbugs-fixes',
                  [
                    'postcss-preset-env',
                    {
                      autoprefixer: {
                        flexbox: 'no-2009',
                      },
                      stage: 3,
                    },
                  ],
                ],
          },
          sourceMap: isEnvProduction ? shouldUseSourceMap : isEnvDevelopment,
        },
      },
    ].filter(Boolean);
    if (preProcessor) {
      loaders.push(
        {
          loader: require.resolve('resolve-url-loader'),
          options: {
            sourceMap: isEnvProduction ? shouldUseSourceMap : isEnvDevelopment,
            root: paths.appSrc,
          },
        },
        {
          loader: require.resolve(preProcessor),
          options: {
            sourceMap: true,
          },
        }
      );
    }
    return loaders;
  };

  return {
    target: ['browserslist'],
    // Webpack noise constrained to errors and warnings
    stats: 'errors-warnings',
    mode: isEnvProduction ? 'production' : isEnvDevelopment && 'development',
    // Stop compilation early in production
    bail: isEnvProduction,
    devtool: isEnvProduction
      ? shouldUseSourceMap
        ? 'source-map'
        : false
      : isEnvDevelopment && 'cheap-module-source-map',
    // These are the "entry points" to our application.
    // This means they will be the "root" imports that are included in JS bundle.
    entry: paths.appIndexJs,
    output: {
      // The build folder.
      path: paths.appBuild,
      // Add /* filename */ comments to generated require()s in the output.
      pathinfo: isEnvDevelopment,
      // There will be one main bundle, and one file per asynchronous chunk.
      // In development, it does not produce real files.
      filename: isEnvProduction
        ? 'static/js/[name].[contenthash:8].js'
        : isEnvDevelopment && 'static/js/bundle.js',
      // There are also additional JS chunk files if you use code splitting.
      chunkFilename: isEnvProduction
        ? 'static/js/[name].[contenthash:8].chunk.js'
        : isEnvDevelopment && 'static/js/[name].chunk.js',
      assetModuleFilename: 'static/media/[name].[hash][ext]',
      // webpack uses `publicPath` to determine where the app is being served from.
      // It requires a trailing slash, or the file assets will get an incorrect path.
      // We inferred the "public path" (such as / or /my-project) from homepage.
      publicPath: paths.publicUrlOrPath,
      // Point sourcemap entries to original disk location (format as URL on Windows)
      devtoolModuleFilenameTemplate: isEnvProduction
        ? info =>
            path
              .relative(paths.appSrc, info.absoluteResourcePath)
              .replace(/\\/g, '/')
        : isEnvDevelopment &&
          (info => path.resolve(info.absoluteResourcePath).replace(/\\/g, '/')),
    },
    cache: {
      type: 'filesystem',
      version: createEnvironmentHash(env.raw),
      cacheDirectory: paths.appWebpackCache,
      store: 'pack',
      buildDependencies: {
        defaultWebpack: ['webpack/lib/'],
        config: [__filename],
        tsconfig: [paths.appTsConfig, paths.appJsConfig].filter(f =>
          fs.existsSync(f)
        ),
      },
    },
    infrastructureLogging: {
      level: 'none',
    },
    optimization: {
      minimize: isEnvProduction,
      minimizer: [
        // This is only used in production mode
        new TerserPlugin({
          terserOptions: {
            parse: {
              // We want terser to parse ecma 8 code. However, we don't want it
              // to apply any minification steps that turns valid ecma 5 code
              // into invalid ecma 5 code. This is why the 'compress' and 'output'
              // sections only apply transformations that are ecma 5 safe
              // https://github.com/facebook/create-react-app/pull/4234
              ecma: 8,
            },
            compress: {
              ecma: 5,
              warnings: false,
              // Disabled because of an issue with Uglify breaking seemingly valid code:
              // https://github.com/facebook/create-react-app/issues/2376
              // Pending further investigation:
              // https://github.com/mishoo/UglifyJS2/issues/2011
              comparisons: false,
              // Disabled because of an issue with Terser breaking valid code:
              // https://github.com/facebook/create-react-app/issues/5250
              // Pending further investigation:
              // https://github.com/terser-js/terser/issues/120
              inline: 2,
            },
            mangle: {
              safari10: true,
            },
            // Added for profiling in devtools
            keep_classnames: isEnvProductionProfile,
            keep_fnames: isEnvProductionProfile,
            output: {
              ecma: 5,
              comments: false,
              // Turned on because emoji and regex is not minified properly using default
              // https://github.com/facebook/create-react-app/issues/2488
              ascii_only: true,
            },
          },
        }),
        // This is only used in production mode
        new CssMinimizerPlugin(),
      ],
    },
    resolve: {
      // This allows you to set a fallback for where webpack should look for modules.
      // We placed these paths second because we want `node_modules` to "win"
      // if there are any conflicts. This matches Node resolution mechanism.
      // https://github.com/facebook/create-react-app/issues/253
      modules: ['node_modules', paths.appNodeModules].concat(
        modules.additionalModulePaths || []
      ),
      // These are the reasonable defaults supported by the Node ecosystem.
      // We also include JSX as a common component filename extension to support
      // some tools, although we do not recommend using it, see:
      // https://github.com/facebook/create-react-app/issues/290
      // `web` extension prefixes have been added for better support
      // for React Native Web.
      extensions: paths.moduleFileExtensions
        .map(ext => `.${ext}`)
        .filter(ext => useTypeScript || !ext.includes('ts')),
      alias: {
        // Support React Native Web
        // https://www.smashingmagazine.com/2016/08/a-glimpse-into-the-future-with-react-native-for-web/
        'react-native': 'react-native-web',
        // Allows for better profiling with ReactDevTools
        ...(isEnvProductionProfile && {
          'react-dom$': 'react-dom/profiling',
          'scheduler/tracing': 'scheduler/tracing-profiling',
        }),
        ...(modules.webpackAliases || {}),
      },
      plugins: [
        // Prevents users from importing files from outside of src/ (or node_modules/).
        // This often causes confusion because we only process files within src/ with babel.
        // To fix this, we prevent you from importing files out of src/ -- if you'd like to,
        // please link the files into your node_modules/ and let module-resolution kick in.
        // Make sure your source files are compiled, as they will not be processed in any way.
        new ModuleScopePlugin(paths.appSrc, [
          paths.appPackageJson,
          reactRefreshRuntimeEntry,
          reactRefreshWebpackPluginRuntimeEntry,
          babelRuntimeEntry,
          babelRuntimeEntryHelpers,
          babelRuntimeRegenerator,
        ]),
      ],
    },
    module: {
      strictExportPresence: true,
      rules: [
        // Handle node_modules packages that contain sourcemaps
        shouldUseSourceMap && {
          enforce: 'pre',
          exclude: /@babel(?:\/|\\{1,2})runtime/,
          test: /\.(js|mjs|jsx|ts|tsx|css)$/,
          loader: require.resolve('source-map-loader'),
        },
        {
          // "oneOf" will traverse all following loaders until one will
          // match the requirements. When no loader matches it will fall
          // back to the "file" loader at the end of the loader list.
          oneOf: [
            // TODO: Merge this config once `image/avif` is in the mime-db
            // https://github.com/jshttp/mime-db
            {
              test: [/\.avif$/],
              type: 'asset',
              mimetype: 'image/avif',
              parser: {
                dataUrlCondition: {
                  maxSize: imageInlineSizeLimit,
                },
              },
            },
            // "url" loader works like "file" loader except that it embeds assets
            // smaller than specified limit in bytes as data URLs to avoid requests.
            // A missing `test` is equivalent to a match.
            {
              test: [/\.bmp$/, /\.gif$/, /\.jpe?g$/, /\.png$/],
              type: 'asset',
              parser: {
                dataUrlCondition: {
                  maxSize: imageInlineSizeLimit,
                },
              },
            },
            {
              test: /\.svg$/,
              use: [
                {
                  loader: require.resolve('@svgr/webpack'),
                  options: {
                    prettier: false,
                    svgo: false,
                    svgoConfig: {
                      plugins: [{ removeViewBox: false }],
                    },
                    titleProp: true,
                    ref: true,
                  },
                },
                {
                  loader: require.resolve('file-loader'),
                  options: {
                    name: 'static/media/[name].[hash].[ext]',
                  },
                },
              ],
              issuer: {
                and: [/\.(ts|tsx|js|jsx|md|mdx)$/],
              },
            },
            // Process application JS with Babel.
            // The preset includes JSX, Flow, TypeScript, and some ESnext features.
            {
              test: /\.(js|mjs|jsx|ts|tsx)$/,
              include: paths.appSrc,
              loader: require.resolve('babel-loader'),
              options: {
                customize: require.resolve(
                  'babel-preset-react-app/webpack-overrides'
                ),
                presets: [
                  [
                    require.resolve('babel-preset-react-app'),
                    {
                      runtime: hasJsxRuntime ? 'automatic' : 'classic',
                    },
                  ],
                ],

                plugins: [
                  isEnvDevelopment &&
                    shouldUseReactRefresh &&
                    require.resolve('react-refresh/babel'),
                ].filter(Boolean),
                // This is a feature of `babel-loader` for webpack (not Babel itself).
                // It enables caching results in ./node_modules/.cache/babel-loader/
                // directory for faster rebuilds.
                cacheDirectory: true,
                // See #6846 for context on why cacheCompression is disabled
                cacheCompression: false,
                compact: isEnvProduction,
              },
            },
            // Process any JS outside of the app with Babel.
            // Unlike the application JS, we only compile the standard ES features.
            {
              test: /\.(js|mjs)$/,
              exclude: /@babel(?:\/|\\{1,2})runtime/,
              loader: require.resolve('babel-loader'),
              options: {
                babelrc: false,
                configFile: false,
                compact: false,
                presets: [
                  [
                    require.resolve('babel-preset-react-app/dependencies'),
                    { helpers: true },
                  ],
                ],
                cacheDirectory: true,
                // See #6846 for context on why cacheCompression is disabled
                cacheCompression: false,

                // Babel sourcemaps are needed for debugging into node_modules
                // code.  Without the options below, debuggers like VSCode
                // show incorrect code and set breakpoints on the wrong lines.
                sourceMaps: shouldUseSourceMap,
                inputSourceMap: shouldUseSourceMap,
              },
            },
            // "postcss" loader applies autoprefixer to our CSS.
            // "css" loader resolves paths in CSS and adds assets as dependencies.
            // "style" loader turns CSS into JS modules that inject <style> tags.
            // In production, we use MiniCSSExtractPlugin to extract that CSS
            // to a file, but in development "style" loader enables hot editing
            // of CSS.
            // By default we support CSS Modules with the extension .module.css
            {
              test: cssRegex,
              exclude: cssModuleRegex,
              use: getStyleLoaders({
                importLoaders: 1,
                sourceMap: isEnvProduction
                  ? shouldUseSourceMap
                  : isEnvDevelopment,
                modules: {
                  mode: 'icss',
                },
              }),
              // Don't consider CSS imports dead code even if the
              // containing package claims to have no side effects.
              // Remove this when webpack adds a warning or an error for this.
              // See https://github.com/webpack/webpack/issues/6571
              sideEffects: true,
            },
            // Adds support for CSS Modules (https://github.com/css-modules/css-modules)
            // using the extension .module.css
            {
              test: cssModuleRegex,
              use: getStyleLoaders({
                importLoaders: 1,
                sourceMap: isEnvProduction
                  ? shouldUseSourceMap
                  : isEnvDevelopment,
                modules: {
                  mode: 'local',
                  getLocalIdent: getCSSModuleLocalIdent,
                },
              }),
            },
            // Opt-in support for SASS (using .scss or .sass extensions).
            // By default we support SASS Modules with the
            // extensions .module.scss or .module.sass
            {
              test: sassRegex,
              exclude: sassModuleRegex,
              use: getStyleLoaders(
                {
                  importLoaders: 3,
                  sourceMap: isEnvProduction
                    ? shouldUseSourceMap
                    : isEnvDevelopment,
                  modules: {
                    mode: 'icss',
                  },
                },
                'sass-loader'
              ),
              // Don't consider CSS imports dead code even if the
              // containing package claims to have no side effects.
              // Remove this when webpack adds a warning or an error for this.
              // See https://github.com/webpack/webpack/issues/6571
              sideEffects: true,
            },
            // Adds support for CSS Modules, but using SASS
            // using the extension .module.scss or .module.sass
            {
              test: sassModuleRegex,
              use: getStyleLoaders(
                {
                  importLoaders: 3,
                  sourceMap: isEnvProduction
                    ? shouldUseSourceMap
                    : isEnvDevelopment,
                  modules: {
                    mode: 'local',
                    getLocalIdent: getCSSModuleLocalIdent,
                  },
                },
                'sass-loader'
              ),
            },
            // "file" loader makes sure those assets get served by WebpackDevServer.
            // When you `import` an asset, you get its (virtual) filename.
            // In production, they would get copied to the `build` folder.
            // This loader doesn't use a "test" so it will catch all modules
            // that fall through the other loaders.
            {
              // Exclude `js` files to keep "css" loader working as it injects
              // its runtime that would otherwise be processed through "file" loader.
              // Also exclude `html` and `json` extensions so they get processed
              // by webpacks internal loaders.
              exclude: [/^$/, /\.(js|mjs|jsx|ts|tsx)$/, /\.html$/, /\.json$/],
              type: 'asset/resource',
            },
            // ** STOP ** Are you adding a new loader?
            // Make sure to add the new loader(s) before the "file" loader.
          ],
        },
      ].filter(Boolean),
    },
    plugins: [
      // Generates an `index.html` file with the <script> injected.
      new HtmlWebpackPlugin(
        Object.assign(
          {},
          {
            inject: true,
            template: paths.appHtml,
          },
          isEnvProduction
            ? {
                minify: {
                  removeComments: true,
                  collapseWhitespace: true,
                  removeRedundantAttributes: true,
                  useShortDoctype: true,
                  removeEmptyAttributes: true,
                  removeStyleLinkTypeAttributes: true,
                  keepClosingSlash: true,
                  minifyJS: true,
                  minifyCSS: true,
                  minifyURLs: true,
                },
              }
            : undefined
        )
      ),
      // Inlines the webpack runtime script. This script is too small to warrant
      // a network request.
      // https://github.com/facebook/create-react-app/issues/5358
      isEnvProduction &&
        shouldInlineRuntimeChunk &&
        new InlineChunkHtmlPlugin(HtmlWebpackPlugin, [/runtime-.+[.]js/]),
      // Makes some environment variables available in index.html.
      // The public URL is available as %PUBLIC_URL% in index.html, e.g.:
      // <link rel="icon" href="%PUBLIC_URL%/favicon.ico">
      // It will be an empty string unless you specify "homepage"
      // in `package.json`, in which case it will be the pathname of that URL.
      new InterpolateHtmlPlugin(HtmlWebpackPlugin, env.raw),
      // This gives some necessary context to module not found errors, such as
      // the requesting resource.
      new ModuleNotFoundPlugin(paths.appPath),
      // Makes some environment variables available to the JS code, for example:
      // if (process.env.NODE_ENV === 'production') { ... }. See `./env.js`.
      // It is absolutely essential that NODE_ENV is set to production
      // during a production build.
      // Otherwise React will be compiled in the very slow development mode.
      new webpack.DefinePlugin(env.stringified),
      // Experimental hot reloading for React .
      // https://github.com/facebook/react/tree/main/packages/react-refresh
      isEnvDevelopment &&
        shouldUseReactRefresh &&
        new ReactRefreshWebpackPlugin({
          overlay: false,
        }),
      // Watcher doesn't work well if you mistype casing in a path so we use
      // a plugin that prints an error when you attempt to do this.
      // See https://github.com/facebook/create-react-app/issues/240
      isEnvDevelopment && new CaseSensitivePathsPlugin(),
      isEnvProduction &&
        new MiniCssExtractPlugin({
          // Options similar to the same options in webpackOptions.output
          // both options are optional
          filename: 'static/css/[name].[contenthash:8].css',
          chunkFilename: 'static/css/[name].[contenthash:8].chunk.css',
        }),
      // Generate an asset manifest file with the following content:
      // - "files" key: Mapping of all asset filenames to their corresponding
      //   output file so that tools can pick it up without having to parse
      //   `index.html`
      // - "entrypoints" key: Array of files which are included in `index.html`,
      //   can be used to reconstruct the HTML if necessary
      new WebpackManifestPlugin({
        fileName: 'asset-manifest.json',
        publicPath: paths.publicUrlOrPath,
        generate: (seed, files, entrypoints) => {
          const manifestFiles = files.reduce((manifest, file) => {
            manifest[file.name] = file.path;
            return manifest;
          }, seed);
          const entrypointFiles = entrypoints.main.filter(
            fileName => !fileName.endsWith('.map')
          );

          return {
            files: manifestFiles,
            entrypoints: entrypointFiles,
          };
        },
      }),
      // Moment.js is an extremely popular library that bundles large locale files
      // by default due to how webpack interprets its code. This is a practical
      // solution that requires the user to opt into importing specific locales.
      // https://github.com/jmblog/how-to-optimize-momentjs-with-webpack
      // You can remove this if you don't use Moment.js:
      new webpack.IgnorePlugin({
        resourceRegExp: /^\.\/locale$/,
        contextRegExp: /moment$/,
      }),
      // Generate a service worker script that will precache, and keep up to date,
      // the HTML & assets that are part of the webpack build.
      isEnvProduction &&
        fs.existsSync(swSrc) &&
        new WorkboxWebpackPlugin.InjectManifest({
          swSrc,
          dontCacheBustURLsMatching: /\.[0-9a-f]{8}\./,
          exclude: [/\.map$/, /asset-manifest\.json$/, /LICENSE/],
          // Bump up the default maximum size (2mb) that's precached,
          // to make lazy-loading failure scenarios less likely.
          // See https://github.com/cra-template/pwa/issues/13#issuecomment-722667270
          maximumFileSizeToCacheInBytes: 5 * 1024 * 1024,
        }),
      // TypeScript type checking
      useTypeScript &&
        new ForkTsCheckerWebpackPlugin({
          async: isEnvDevelopment,
          typescript: {
            typescriptPath: resolve.sync('typescript', {
              basedir: paths.appNodeModules,
            }),
            configOverwrite: {
              compilerOptions: {
                sourceMap: isEnvProduction
                  ? shouldUseSourceMap
                  : isEnvDevelopment,
                skipLibCheck: true,
                inlineSourceMap: false,
                declarationMap: false,
                noEmit: true,
                incremental: true,
                tsBuildInfoFile: paths.appTsBuildInfoFile,
              },
            },
            context: paths.appPath,
            diagnosticOptions: {
              syntactic: true,
            },
            mode: 'write-references',
            // profile: true,
          },
          issue: {
            // This one is specifically to match during CI tests,
            // as micromatch doesn't match
            // '../cra-template-typescript/template/src/App.tsx'
            // otherwise.
            include: [
              { file: '../**/src/**/*.{ts,tsx}' },
              { file: '**/src/**/*.{ts,tsx}' },
            ],
            exclude: [
              { file: '**/src/**/__tests__/**' },
              { file: '**/src/**/?(*.){spec|test}.*' },
              { file: '**/src/setupProxy.*' },
              { file: '**/src/setupTests.*' },
            ],
          },
          logger: {
            infrastructure: 'silent',
          },
        }),
      !disableESLintPlugin &&
        new ESLintPlugin({
          // Plugin options
          extensions: ['js', 'mjs', 'jsx', 'ts', 'tsx'],
          formatter: require.resolve('react-dev-utils/eslintFormatter'),
          eslintPath: require.resolve('eslint'),
          failOnError: !(isEnvDevelopment && emitErrorsAsWarnings),
          context: paths.appSrc,
          cache: true,
          cacheLocation: path.resolve(
            paths.appNodeModules,
            '.cache/.eslintcache'
          ),
          // ESLint class options
          cwd: paths.appPath,
          resolvePluginsRelativeTo: __dirname,
          baseConfig: {
            extends: [require.resolve('eslint-config-react-app/base')],
            rules: {
              ...(!hasJsxRuntime && {
                'react/react-in-jsx-scope': 'error',
              }),
            },
          },
        }),
    ].filter(Boolean),
    // Turn off performance processing because we utilize
    // our own hints via the FileSizeReporter
    performance: false,
  };
};

config/paths.js

'use strict';

const path = require('path');
const fs = require('fs');
const getPublicUrlOrPath = require('react-dev-utils/getPublicUrlOrPath');

// Make sure any symlinks in the project folder are resolved:
// https://github.com/facebook/create-react-app/issues/637
const appDirectory = fs.realpathSync(process.cwd());
const resolveApp = relativePath => path.resolve(appDirectory, relativePath);

// We use `PUBLIC_URL` environment variable or "homepage" field to infer
// "public path" at which the app is served.
// webpack needs to know it to put the right <script> hrefs into HTML even in
// single-page apps that may serve index.html for nested URLs like /todos/42.
// We can't use a relative path in HTML because we don't want to load something
// like /todos/42/static/js/bundle.7289d.js. We have to know the root.
const publicUrlOrPath = getPublicUrlOrPath(
  process.env.NODE_ENV === 'development',
  require(resolveApp('package.json')).homepage,
  process.env.PUBLIC_URL
);

const buildPath = process.env.BUILD_PATH || 'build';

const moduleFileExtensions = [
  'web.mjs',
  'mjs',
  'web.js',
  'js',
  'web.ts',
  'ts',
  'web.tsx',
  'tsx',
  'json',
  'web.jsx',
  'jsx',
];

// Resolve file paths in the same order as webpack
const resolveModule = (resolveFn, filePath) => {
  const extension = moduleFileExtensions.find(extension =>
    fs.existsSync(resolveFn(`${filePath}.${extension}`))
  );

  if (extension) {
    return resolveFn(`${filePath}.${extension}`);
  }

  return resolveFn(`${filePath}.js`);
};

// config after eject: we're in ./config/
module.exports = {
  dotenv: resolveApp('.env'),
  appPath: resolveApp('.'),
  appBuild: resolveApp(buildPath),
  appPublic: resolveApp('public'),
  appHtml: resolveApp('public/index.html'),
  appIndexJs: resolveModule(resolveApp, 'src/index'),
  appPackageJson: resolveApp('package.json'),
  appSrc: resolveApp('src'),
  appTsConfig: resolveApp('tsconfig.json'),
  appJsConfig: resolveApp('jsconfig.json'),
  yarnLockFile: resolveApp('yarn.lock'),
  testsSetup: resolveModule(resolveApp, 'src/setupTests'),
  proxySetup: resolveApp('src/setupProxy.js'),
  appNodeModules: resolveApp('node_modules'),
  appWebpackCache: resolveApp('node_modules/.cache'),
  appTsBuildInfoFile: resolveApp('node_modules/.cache/tsconfig.tsbuildinfo'),
  swSrc: resolveModule(resolveApp, 'src/service-worker'),
  publicUrlOrPath,
};



module.exports.moduleFileExtensions = moduleFileExtensions;

以降では、上記のwebpack.config.jsの一部を抜粋して読み解いていきます。

型チェック

まずは型チェックです。

型チェックは

// Check if TypeScript is setup
const useTypeScript = fs.existsSync(paths.appTsConfig);

// 省略

// TypeScript type checking
useTypeScript &&
new ForkTsCheckerWebpackPlugin({
    async: isEnvDevelopment,
    typescript: {
    typescriptPath: resolve.sync('typescript', {
        basedir: paths.appNodeModules,
    }),
    configOverwrite: {
        compilerOptions: {
        sourceMap: isEnvProduction
            ? shouldUseSourceMap
            : isEnvDevelopment,
        skipLibCheck: true,
        inlineSourceMap: false,
        declarationMap: false,
        noEmit: true,
        incremental: true,
        tsBuildInfoFile: paths.appTsBuildInfoFile,
        },
    },
    context: paths.appPath,
    diagnosticOptions: {
        syntactic: true,
    },
    mode: 'write-references',
    // profile: true,
    },
// 省略
}),

の部分で行われていそうです。
まず、tsconfig.jsonが存在するかどうかを確認し、
存在する場合はtsc経由での型チェックを行わず、WebpackのプラグインであるForkTsCheckerWebpackPluginを実行しているようです。

また、

sourceMap: isEnvProduction
            ? shouldUseSourceMap
            : isEnvDevelopment,
skipLibCheck: true,
inlineSourceMap: false,
declarationMap: false,
noEmit: true,
incremental: true,
tsBuildInfoFile: paths.appTsBuildInfoFile,

の項目については、tsconfig.jsonの上書きを行っているようでした。
(こういうことができるのも知らなかった)

コンパイル(トランスパイル)

次に、コンパイル(トランスパイル)です。
コンパイルは

// Process application JS with Babel.
// The preset includes JSX, Flow, TypeScript, and some ESnext features.
{
    test: /\.(js|mjs|jsx|ts|tsx)$/,
    include: paths.appSrc,
    loader: require.resolve('babel-loader'),
    options: {
    customize: require.resolve(
        'babel-preset-react-app/webpack-overrides'
    ),
    presets: [
        [
        require.resolve('babel-preset-react-app'),
        {
            runtime: hasJsxRuntime ? 'automatic' : 'classic',
        },
        ],
    ],

    plugins: [
        isEnvDevelopment &&
        shouldUseReactRefresh &&
        require.resolve('react-refresh/babel'),
    ].filter(Boolean),
    // This is a feature of `babel-loader` for webpack (not Babel itself).
    // It enables caching results in ./node_modules/.cache/babel-loader/
    // directory for faster rebuilds.
    cacheDirectory: true,
    // See #6846 for context on why cacheCompression is disabled
    cacheCompression: false,
    compact: isEnvProduction,
    },
},

の部分で行われており、
paths.appSrc = srcディレクトリ以下のjs|mjs|jsx|ts|tsxファイルを対象に、実行されます。

コンパイルを行っているのは
babel-preset-react-appという Babel のプリセットで、
それを Webpack のbabel-loaderを使って実行しているようです。

エントリーポイント

最後にエントリーポイントについて確認していきます。
エントリーポイントはバンドルの開始点になり、
このファイルを基準に、依存関係を辿っていきます。

config/paths.js

// Make sure any symlinks in the project folder are resolved:
// https://github.com/facebook/create-react-app/issues/637
const appDirectory = fs.realpathSync(process.cwd());
const resolveApp = relativePath => path.resolve(appDirectory, relativePath);


// config after eject: we're in ./config/
module.exports = {
  dotenv: resolveApp('.env'),
  appPath: resolveApp('.'),
  appBuild: resolveApp(buildPath),
  appPublic: resolveApp('public'),
  appHtml: resolveApp('public/index.html'),
  appIndexJs: resolveModule(resolveApp, 'src/index'),
  appPackageJson: resolveApp('package.json'),
  appSrc: resolveApp('src'),
  appTsConfig: resolveApp('tsconfig.json'),
  appJsConfig: resolveApp('jsconfig.json'),
  yarnLockFile: resolveApp('yarn.lock'),
  testsSetup: resolveModule(resolveApp, 'src/setupTests'),
  proxySetup: resolveApp('src/setupProxy.js'),
  appNodeModules: resolveApp('node_modules'),
  appWebpackCache: resolveApp('node_modules/.cache'),
  appTsBuildInfoFile: resolveApp('node_modules/.cache/tsconfig.tsbuildinfo'),
  swSrc: resolveModule(resolveApp, 'src/service-worker'),
  publicUrlOrPath,
};

webpack.config.js

// These are the "entry points" to our application.
// This means they will be the "root" imports that are included in JS bundle.
entry: paths.appIndexJs,

エントリーポイントは上記のようになっており、
config/paths.jsのappIndexJsが指定されています。
つまり、src/index.jsがデフォルトのエントリーポイントになります。

しかし、先ほどのディレクトリ構成を確認すると、
src/index.jsというファイルは存在していません。
にも関わらず、

npm run build

を実行してみると、

npm run build

> sample-app@0.1.0 build
> node scripts/build.js

Creating an optimized production build...
Compiled successfully.

が表示されます。

これは先ほどの@babel/preset-reactの中で
src/index.tsxがsrc/index.jsにコンパイルされているためです。

つまり、今回の例の場合
エントリーポイントはコンパイル後の JavaScript ファイルを指定しています。

まとめ

Create React App したものがビルドされるまでの流れをまとめると、

Webpack のプラグインfork-ts-checker-webpack-pluginが型チェックを行う
Webpack のプラグインbabel-loaderが Babel の@babel/preset-reactを使ってコンパイルを行い、JSX や TypeScript を JavaScript に変換する
Webpack がbuildフォルダ配下にバンドルしたファイルを出力する

という流れになっていました。

おわりに

今までフレームワークに任せっきりになっていた
ビルドの流れを整理してみました。
なんとなく知っていた Webpack や Babel の動きが
少し理解できた気がします。
Vite や esbuild などのツールへの解像度も高まりそうで楽しみです。

Aidemy Tech BlogPublication

株式会社アイデミーの技術ブログです。

はじめに

主な登場人物

Webpack

Babel

tsc

ざっくりと流れを理解する

React をビルドした時の流れ

環境構築

npm run eject

型チェック

コンパイル(トランスパイル)

エントリーポイント

まとめ

おわりに

Discussion