function A(name) {
      this.name = name
    }

    A.prototype.getName = function () {
      console.log(this.name)
    }

    function B(model) {
      this.model = model
    }

    B.prototype.getModel = function () {
      console.log(this.model)
    }


    function create(type, param) {
      // instanceof 会判断后面的构造函数的原型是否存在当前这个对象的原型链里
      if (this instanceof create) {
        return new this[type](param)
      } else {
        return new create(type, param)
      }

    }

    create.prototype = {
      A,
      B
    }

    var c = new create('A', '123')
    console.log(c); // A {name: '123'}
    var d = create("B", 666)
    console.log(d); // B { model: 666 }