init 阶段：options

// lib/config/normalization.js
const getNormalizedWebpackOptions = configs => {
  // ......
  return normalizedConfigs;
};

// Sets a constant default value when undefined
const D = (obj, prop, value) => {
 if (obj[prop] === undefined) {
  obj[prop] = value;
 }
};
// Sets a dynamic default value when undefined, by calling the factory function
const F = (obj, prop, factory) => {
 if (obj[prop] === undefined) {
  obj[prop] = factory();
 }
};
// 
const A = (obj, prop, factory) => {
 const value = obj[prop];
 if (value === undefined) {
  obj[prop] = factory();
 } else if (Array.isArray(value)) {
  let newArray = undefined;
  for (let i = 0; i < value.length; i++) {
   const item = value[i];
   if (item === "...") {
    if (newArray === undefined) {
     newArray = value.slice(0, i);
     //  注意：newArray 是应用，赋值之后 newArray 仍然在变化
     obj[prop] = newArray;
    }
    const items = factory();
    if (items !== undefined) {
     for (const item of items) {
      newArray.push(item);
     }
    }
   } else if (newArray !== undefined) {
    newArray.push(item);
   }
  }
 }
};

class NodeEnvironmentPlugin {
 apply(compiler) {
  const { infrastructureLogging } = this.options;
  // 初始化创建 infrastructureLogger
  compiler.infrastructureLogger = createConsoleLogger(/** ...... */);
  // 初始化文件系统相关，inputFileSystem 和 watchFileSystem
  compiler.inputFileSystem = new CachedInputFileSystem(fs, 60000);
  const inputFileSystem = compiler.inputFileSystem;
  compiler.outputFileSystem = fs;
  compiler.intermediateFileSystem = fs;
  // this.watcher = new Watchpack(this.watcherOptions);
  compiler.watchFileSystem = new NodeWatchFileSystem(
   compiler.inputFileSystem
  );
  // 注册 Callback：Hook.beforeRun
  compiler.hooks.beforeRun.tap("NodeEnvironmentPlugin", (compiler) => {
   if (compiler.inputFileSystem === inputFileSystem) {
    compiler.fsStartTime = Date.now();
    inputFileSystem.purge();
   }
  });
 }
}