simple demo

// add.js
const add = (a, b) => {
  return a + b;
};

export default add;

// index.js
import add from "./add";

console.log("sum", add(1, 3));

output

(() => {
 // 运用的模块列表，当成一个map存起来，一是缓存，而是把执行结果给export
  var __webpack_modules__ = {
    "./src/add.js": (
      __unused_webpack_module, //模块信心，id，isload，export
      __webpack_exports__, //把执行结果复制给exports这个变量
      __webpack_require__ // 模块里面可以继续引用其他模块
    ) => {
        // 1. 把这个标示为es模块 {esmodule: true}
      __webpack_require__.r(__webpack_exports__);
       // 2: define 给这个export负值 export = {default: add, esmodule: true}
      __webpack_require__.d(__webpack_exports__, {
        default: () => __WEBPACK_DEFAULT_EXPORT__,
      });
      const add = (a, b) => {
        console.log("kkkk");
        return a + b;
      };

      const __WEBPACK_DEFAULT_EXPORT__ = add;
    },
  };
  
// 一个模块只需要执行一次就可以得到export，所以做个缓存
  var __webpack_module_cache__ = {};

  // 核心功能
  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: {},
    });

   
    // 执行模块，得到exports
    __webpack_modules__[moduleId](module, module.exports, __webpack_require__);

    // Return the exports of the module
    return module.exports;
  }

  /* 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/hasOwnProperty shorthand */
  (() => {
    __webpack_require__.o = (obj, prop) =>
      Object.prototype.hasOwnProperty.call(obj, prop);
  })();

  /* 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 });
    };
  })();

  var __webpack_exports__ = {};
  // This entry need to be wrapped in an IIFE because it need to be isolated against other modules in the chunk.
  (() => {
    __webpack_require__.r(__webpack_exports__);
    var _add__WEBPACK_IMPORTED_MODULE_0__ = __webpack_require__("./src/add.js");

    console.log("sum", (0, _add__WEBPACK_IMPORTED_MODULE_0__.default)(1, 3));
  })();
})();