function loadJS(files, done) {
   // Get the head tag
   const head = document. getElementsByTagName('head')[0];
   Promise.all(files.map(file => {
     return new Promise(resolve => {
       // create script tag and add to head
       const s = document.createElement('script');
       s.type = "text/javascript";
       s.async = true;
       s.src = file;
       // Listen to the load event, resolve if the loading is complete
       s. addEventListener('load', (e) => resolve(), false);
       head.appendChild(s);
     });
   })).then(done); // everything is done, execute the user's callback event
}
loadJS(["test1.js", "test2.js"], () => {
   // user's callback logic
});There are two core points in the code above. One is to use Promise to process asynchronous logic, but to use script tags to load and execute js.

// This is a dynamic template that contains js code
var template =
'My avorite sports:' +
'<%if(this.showSports) {%>' +
     '<% for(var index in this.sports) { %>' +
     '<a><%this.sports[index]%></a>' +
     '<%}%>' +
'<%} else {%>' +
     '<p>none</p>' +
'<%}%>';
// This is the function string we're going to concatenate
const code = `with(obj) {
   var r=[];
   r.push("My avorite sports:");
   if(this. showSports) {
     for(var index in this. sports) {
       r. push("<a>");
       r.push(this.sports[index]);
       r. push("</a>");
     }
   } else {
     r.push("<span>none</span>");
   }
   return r.join("");
}`
// dynamically rendered data
const options = {
   sports: ["swimming", "basketball", "football"],
   showSports: true
}
// Build a feasible function and pass in parameters to change the direction of this when the function is executed
result = new Function("obj", code).apply(options, [options]);
console. log(result);

const arr = [
    { classId: "1", name: "Jack", age: 16 },
    { classId: "1", name: "Jon", age: 15 },
    { classId: "2", name: "Jenny", age: 16 },
    { classId: "3", name: "Jim", age: 15 },
    { classId: "2", name: "Zoe", age: 16 }
];
groupArrayByKey(arr, "classId");
function groupArrayByKey(arr = [], key) {
    return arr.reduce((t, v) => (!t[v[key]] && (t[v[key]] = []), t[v[key]].push(v), t), {})
}

function double() {
     return value *2
}
// If not passed, give a default value of 0
function double(value = 0) {
     return value * 2
}
// The user must pass a parameter, and an error will be thrown if no parameter is passed
const required = () => {
     throw new Error("This function requires one parameter.")
}
function double(value = required()) {
     return value * 2
}
double(3) // 6
double() // throw Error

export function once (fn) {
   // Use the closure to determine whether the function has been executed
   let called = false
   return function () {
     if (! called) {
       called = true
       fn. apply(this, arguments)
     }
   }
}

function curry(fn) {
  return function curried(...args) {
    if (args.length >= fn.length) {
      return fn.apply(this, args);
    } else {
      return function(...args2) {
        return curried.apply(this, args.concat(args2));
      };
    }
  };
}
function add(x, y) {
  return x + y;
}
const curriedAdd = curry(add);
console.log(curriedAdd(1)(2)); // output 3
console.log(curriedAdd(1, 2)); // output 3

let cache;
class A {
  // ...
}
function getInstance() {
  if (cache) return cache;
  return cache = new A();
}
const x = getInstance();
const y = getInstance();
console.log(x === y); // true

// id: full file name
const path = require('path');
const fs = require('fs');
function Module(id){
     // Used to uniquely identify the module
     this.id = id;
     // Properties and methods used to export modules
     this.exports = {};
}
function myRequire(filePath) {
     // Directly call the static method of Module to load the file
     return Module._load(filePath);
}
Module._cache = {};
Module._load = function(filePath) {
     // First address the absolute path of the file through the filePath passed in by the user
     // Because in CommnJS, the unique identifier of the module is the absolute path of the file
     const realPath = Module._resoleveFilename(filePath);
     // Cache priority, if it exists in the cache, it will directly return the exports property of the module
     let cacheModule = Module._cache[realPath];
     if(cacheModule) return cacheModule. exports;
     // If it is loaded for the first time, a new module is required, and the parameter is the absolute path of the file
     let module = new Module(realPath);
     // Call the load method of the module to compile the module
     module.load(realPath);
     return module. exports;
}
// The node file is not discussed yet
Module._extensions = {
    // Process the js file
   ".js": handleJS,
   // process the json file
   ".json": handleJSON
}
function handleJSON(module) {
  // If it is a json file, read it directly with fs.readFileSync,
  // Then use JSON.parse to convert and return directly
   const json = fs.readFileSync(module.id, 'utf-8')
   module.exports = JSON.parse(json)
}
function handleJS(module) {
   const js = fs. readFileSync(module. id, 'utf-8')
   let fn = new Function('exports', 'myRequire', 'module', '__filename', '__dirname', js)
   let exports = module. exports;
   // The assembled function can be executed directly
   fn.call(exports, exports, myRequire, module, module.id, path.dirname(module.id))
}
Module._resolveFilename = function (filePath) {
   // Splice the absolute path, and then search it, if it exists, it will return
   let absPath = path. resolve(__dirname, filePath);
   let exists = fs.existsSync(absPath);
   if (exists) return absPath;
   // If it does not exist, try splicing .js, .json, .node in sequence
   let keys = Object.keys(Module._extensions);
   for (let i = 0; i < keys. length; i++) {
     let currentPath = absPath + keys[i];
     if (fs.existsSync(currentPath)) return currentPath;
   }
};
Module.prototype.load = function(realPath) {
   // Get the file extension and hand it over to the corresponding method for processing
   let extname = path.extname(realPath)
   Module._extensions[extname](this)
}

const user = {
   info: {
     name: "Jacky",
     address: { home: "MLB", company: "AI" },
   },
};
// obj is the object to get the property, path is the path, and fallback is the default value
function get(obj, path, fallback) {
   const parts = path. split(".");
   const key = parts. shift();
   if (typeof obj[key] !== "undefined") {
     return parts. length > 0 ?
       get(obj[key], parts. join("."), fallback) :
       obj[key];
   }
   // return fallback if key not found
   return fallback;
}
console.log(get(user, "info.name")); // Jacky
console.log(get(user, "info.address.home")); // MLB
console.log(get(user, "info.address.company")); // AI
console.log(get(user, "info.address.abc", "fallback")); // fallback