JS手写bind之处理new的情况详解

Function.prototype.myBind = function(thisArg, ...prefixArgs) {
  const fn = this;
  return function(...args) {
    return fn.call(thisArg, ...prefixArgs, ...args);
  }
}

function sum(a, b) {
  return a + b;
}

function Person(name, age) {
 this.name = name;
 this.age = age;
}

const person = new Person('前端西瓜哥', 100)
// Person {name: '前端西瓜哥', age: 100}

function Person() {
  if (this instanceof Person) {
    console.log('通过 new 构建实例');
  } else {
    console.log('普通调用')
  }
}

Person()     // 输出：普通调用
new Person() // 输出：通过 new 构建实例

function Vue(options) {
  if (__DEV__ && !(this instanceof Vue)) {
    warn('Vue is a constructor and should be called with the `new` keyWord')
  }
  this._init(options)
}

function Person(name, age) {
 this.name = name;
 this.age = age;
}

const BoundPerson = Person.bind(null, '前端西瓜哥');
const boundPerson = new BoundPerson(100);
// Person {name: '前端西瓜哥', age: 100}

boundPerson.__proto__ === Person.prototype
// true

Function.prototype.myBind = function(thisArg, ...prefixArgs) {
  const fn = this;
  const boundFn = function(...args) {
    // 通过 new 使用当前函数
    if (this instanceof boundFn) {
      return new fn(...prefixArgs, ...args);
    }
    // 普通的方法调用当前函数
    return fn.call(thisArg, ...prefixArgs, ...args);
  }
  boundFn.prototype = fn.prototype;
  return boundFn;
}

// `Function.prototype.bind` method implementation
// https://tc39.es/ecma262/#sec-function.prototype.bind
module.exports = Function.bind || function bind(that ) {
  var F = aCallable(this);
  var Prototype = F.prototype;
  var partArgs = arraySlice(arguments, 1);
  var boundFunction = function bound() {
    var args = concat(partArgs, arraySlice(arguments));
    return this instanceof boundFunction ? construct(F, args.length, args) : F.apply(that, args);
  };
  if (isObject(Prototype)) boundFunction.prototype = Prototype;
  return boundFunction;
};