webpack 懒加载原理

分析

__webpack_require__.f = {}
__webpack_require__.e = (chunkId) => {
  return Promise.all(Object.keys(__webpack_require__.f).reduce((promises, key) => {
    // promises = [], key = "j"
    // chunkId = src_title_js, promises = []
    __webpack_require__.f[key](chunkId, promises);
    return promises;
  }, []));
};

var installedChunks = {
  "main": 0
};
__webpack_require__.f.j = (chunkId, promises) => {
  var installedChunkData = installedChunks[chunkId]
  if(installedChunkData) {
    promises.push(installedChunkData[2]);
  } else {
    var promise = new Promise((resolve, reject) => (installedChunkData = installedChunks[chunkId] = [resolve, reject]));
    promises.push(installedChunkData[2] = promise);
    __webpack_require__.l(url);
  }
}

__webpack_require__.l = (url) => {
  var script;
  script = document.createElement('script');
  script.src = url;
  document.head.appendChild(script);
}

当懒加载的时候会调用push方法

// install a JSONP callback for chunk loading
var webpackJsonpCallback = (parentChunkLoadingFunction, data) => {
  var [chunkIds, moreModules, runtime] = data;
  var moduleId, chunkId, i = 0;
  for(moduleId in moreModules) {
    if(__webpack_require__.o(moreModules, moduleId)) {
      // 把返回的模块定义合并到当前的模块定义里面
      __webpack_require__.m[moduleId] = moreModules[moduleId];
    }
  }
  if(runtime) var result = runtime(__webpack_require__);
  if(parentChunkLoadingFunction) parentChunkLoadingFunction(data);
  for(;i < chunkIds.length; i++) {
    chunkId = chunkIds[i];
    if(__webpack_require__.o(installedChunks, chunkId) && installedChunks[chunkId]) {
      // installedChunks[chunkId][0] 是 resolve 方法，调用之后就加载成功
      installedChunks[chunkId][0]();
    }
    installedChunks[chunkIds[i]] = 0;
  }

}

var chunkLoadingGlobal = self["webpackChunkwebpack5"] = self["webpackChunkwebpack5"] || [];
chunkLoadingGlobal.forEach(webpackJsonpCallback.bind(null, 0));
chunkLoadingGlobal.push = webpackJsonpCallback.bind(null, chunkLoadingGlobal.push.bind(chunkLoadingGlobal));

源码

src_titile_js.bundle.js

(self["webpackChunkwebpack5"] = self["webpackChunkwebpack5"] || []).push([["src_title_js"],{

/***/ "./src/title.js":
/*!**********************!*\
  !*** ./src/title.js ***!
  \**********************/
/***/ ((__unused_webpack_module, __webpack_exports__, __webpack_require__) => {

"use strict";
__webpack_require__.r(__webpack_exports__);
/* harmony export */ __webpack_require__.d(__webpack_exports__, {
/* harmony export */   "default": () => (__WEBPACK_DEFAULT_EXPORT__)
/* harmony export */ });
/* harmony default export */ const __WEBPACK_DEFAULT_EXPORT__ = ('hello world');

/***/ })

}]);

bundle.js

(() => { // webpackBootstrap
  var __webpack_modules__ = ({});
/************************************************************************/
  // The module cache
  var __webpack_module_cache__ = {};
  
  // The require function
  function __webpack_require__(moduleId) {
    // Check if module is in cache
    var cachedModule = __webpack_module_cache__[moduleId];
    if (cachedModule !== undefined) {
      return cachedModule.exports;
    }
    // Create a new module (and put it into the cache)
    var module = __webpack_module_cache__[moduleId] = {
      // no module.id needed
      // no module.loaded needed
      exports: {}
    };
  
    // Execute the module function
    __webpack_modules__[moduleId](module, module.exports, __webpack_require__);
  
    // Return the exports of the module
    return module.exports;
  }
  
  // expose the modules object (__webpack_modules__)
  __webpack_require__.m = __webpack_modules__;
  
/************************************************************************/
  /* webpack/runtime/define property getters */
  (() => {
    // define getter functions for harmony exports
    __webpack_require__.d = (exports, definition) => {
      for(var key in definition) {
        if(__webpack_require__.o(definition, key) && !__webpack_require__.o(exports, key)) {
          Object.defineProperty(exports, key, { enumerable: true, get: definition[key] });
        }
      }
    };
  })();
  
  /* webpack/runtime/ensure chunk */
  (() => {
    __webpack_require__.f = {};
    // This file contains only the entry chunk.
    // The chunk loading function for additional chunks
    __webpack_require__.e = (chunkId) => {
      return Promise.all(Object.keys(__webpack_require__.f).reduce((promises, key) => {
        __webpack_require__.f[key](chunkId, promises);
        return promises;
      }, []));
    };
  })();
  
  /* webpack/runtime/get javascript chunk filename */
  (() => {
    // This function allow to reference async chunks
    __webpack_require__.u = (chunkId) => {
      // return url for filenames based on template
      return "" + chunkId + ".bundle.js";
    };
  })();
  
  /* webpack/runtime/global */
  (() => {
    __webpack_require__.g = (function() {
      if (typeof globalThis === 'object') return globalThis;
      try {
        return this || new Function('return this')();
      } catch (e) {
        if (typeof window === 'object') return window;
      }
    })();
  })();
  
  /* webpack/runtime/hasOwnProperty shorthand */
  (() => {
    __webpack_require__.o = (obj, prop) => (Object.prototype.hasOwnProperty.call(obj, prop))
  })();
  
  /* webpack/runtime/load script */
  (() => {
    var inProgress = {};
    var dataWebpackPrefix = "webpack5:";
    // loadScript function to load a script via script tag
    __webpack_require__.l = (url, done, key, chunkId) => {
      if(inProgress[url]) { inProgress[url].push(done); return; }
      var script, needAttach;
      if(key !== undefined) {
        var scripts = document.getElementsByTagName("script");
        for(var i = 0; i < scripts.length; i++) {
          var s = scripts[i];
          if(s.getAttribute("src") == url || s.getAttribute("data-webpack") == dataWebpackPrefix + key) { script = s; break; }
        }
      }
      if(!script) {
        needAttach = true;
        script = document.createElement('script');
    
        script.charset = 'utf-8';
        script.timeout = 120;
        if (__webpack_require__.nc) {
          script.setAttribute("nonce", __webpack_require__.nc);
        }
        script.setAttribute("data-webpack", dataWebpackPrefix + key);
        script.src = url;
      }
      inProgress[url] = [done];
      var onScriptComplete = (prev, event) => {
        // avoid mem leaks in IE.
        script.onerror = script.onload = null;
        clearTimeout(timeout);
        var doneFns = inProgress[url];
        delete inProgress[url];
        script.parentNode && script.parentNode.removeChild(script);
        doneFns && doneFns.forEach((fn) => (fn(event)));
        if(prev) return prev(event);
      }
      ;
      var timeout = setTimeout(onScriptComplete.bind(null, undefined, { type: 'timeout', target: script }), 120000);
      script.onerror = onScriptComplete.bind(null, script.onerror);
      script.onload = onScriptComplete.bind(null, script.onload);
      needAttach && document.head.appendChild(script);
    };
  })();
  
  /* webpack/runtime/make namespace object */
  (() => {
    // define __esModule on exports
    __webpack_require__.r = (exports) => {
      if(typeof Symbol !== 'undefined' && Symbol.toStringTag) {
        Object.defineProperty(exports, Symbol.toStringTag, { value: 'Module' });
      }
      Object.defineProperty(exports, '__esModule', { value: true });
    };
  })();
  
  /* webpack/runtime/publicPath */
  (() => {
    var scriptUrl;
    if (__webpack_require__.g.importScripts) scriptUrl = __webpack_require__.g.location + "";
    var document = __webpack_require__.g.document;
    if (!scriptUrl && document) {
      if (document.currentScript)
        scriptUrl = document.currentScript.src
      if (!scriptUrl) {
        var scripts = document.getElementsByTagName("script");
        if(scripts.length) scriptUrl = scripts[scripts.length - 1].src
      }
    }
    // When supporting browsers where an automatic publicPath is not supported you must specify an output.publicPath manually via configuration
    // or pass an empty string ("") and set the __webpack_public_path__ variable from your code to use your own logic.
    if (!scriptUrl) throw new Error("Automatic publicPath is not supported in this browser");
    scriptUrl = scriptUrl.replace(/#.*$/, "").replace(/\?.*$/, "").replace(/\/[^\/]+$/, "/");
    __webpack_require__.p = scriptUrl;
  })();
  
  /* webpack/runtime/jsonp chunk loading */
  (() => {
    // no baseURI
    
    // object to store loaded and loading chunks
    // undefined = chunk not loaded, null = chunk preloaded/prefetched
    // [resolve, reject, Promise] = chunk loading, 0 = chunk loaded
    var installedChunks = {
      "main": 0
    };
    
    __webpack_require__.f.j = (chunkId, promises) => {
        // JSONP chunk loading for javascript
        var installedChunkData = __webpack_require__.o(installedChunks, chunkId) ? installedChunks[chunkId] : undefined;
        if(installedChunkData !== 0) { // 0 means "already installed".
    
          // a Promise means "currently loading".
          if(installedChunkData) {
            promises.push(installedChunkData[2]);
          } else {
            if(true) { // all chunks have JS
              // setup Promise in chunk cache
              var promise = new Promise((resolve, reject) => (installedChunkData = installedChunks[chunkId] = [resolve, reject]));
              promises.push(installedChunkData[2] = promise);
    
              // start chunk loading
              var url = __webpack_require__.p + __webpack_require__.u(chunkId);
              // create error before stack unwound to get useful stacktrace later
              var error = new Error();
              var loadingEnded = (event) => {
                if(__webpack_require__.o(installedChunks, chunkId)) {
                  installedChunkData = installedChunks[chunkId];
                  if(installedChunkData !== 0) installedChunks[chunkId] = undefined;
                  if(installedChunkData) {
                    var errorType = event && (event.type === 'load' ? 'missing' : event.type);
                    var realSrc = event && event.target && event.target.src;
                    error.message = 'Loading chunk ' + chunkId + ' failed.\n(' + errorType + ': ' + realSrc + ')';
                    error.name = 'ChunkLoadError';
                    error.type = errorType;
                    error.request = realSrc;
                    installedChunkData[1](error);
                  }
                }
              };
              __webpack_require__.l(url, loadingEnded, "chunk-" + chunkId, chunkId);
            } else installedChunks[chunkId] = 0;
          }
        }
    };
    
    // install a JSONP callback for chunk loading
    var webpackJsonpCallback = (parentChunkLoadingFunction, data) => {
      var [chunkIds, moreModules, runtime] = data;
      // add "moreModules" to the modules object,
      // then flag all "chunkIds" as loaded and fire callback
      var moduleId, chunkId, i = 0;
      for(moduleId in moreModules) {
        if(__webpack_require__.o(moreModules, moduleId)) {
          __webpack_require__.m[moduleId] = moreModules[moduleId];
        }
      }
      if(runtime) var result = runtime(__webpack_require__);
      if(parentChunkLoadingFunction) parentChunkLoadingFunction(data);
      for(;i < chunkIds.length; i++) {
        chunkId = chunkIds[i];
        if(__webpack_require__.o(installedChunks, chunkId) && installedChunks[chunkId]) {
          installedChunks[chunkId][0]();
        }
        installedChunks[chunkIds[i]] = 0;
      }
    
    }
    
    var chunkLoadingGlobal = self["webpackChunkwebpack5"] = self["webpackChunkwebpack5"] || [];
    chunkLoadingGlobal.forEach(webpackJsonpCallback.bind(null, 0));
    chunkLoadingGlobal.push = webpackJsonpCallback.bind(null, chunkLoadingGlobal.push.bind(chunkLoadingGlobal));
  })();
  
/************************************************************************/
var __webpack_exports__ = {};
/*!**********************!*\
  !*** ./src/index.js ***!
  \**********************/
let root = document.getElementById('root')
root.onclick = function () {
  let res = __webpack_require__.e(/*! import() */ "src_title_js").then(__webpack_require__.bind(__webpack_require__, /*! ./title */ "./src/title.js"))
  console.log(res)
}

})()
;