关于 JavaScript 中 Object.create() 和“new”方法的输出的问题

Question

最近学习了原型、＿proto＿和继承的概念。 我试图找出下面的输出，但失败了......

function test(){
  var a = 1;
  this.b =2;
}
var o1 = new test();
var o2 = Object.create(test);
console.log(test.a);  //undefined
console.log(test.b);  //undefined
console.log(o1.a);    //undefined
console.log(o1.b);    //2
console.log(o2.a);    //undefined
console.log(o2.b);    //undefined

作为一个javascript菜鸟，谁能告诉我

为什么只有“o1.b”能够访问值 2？

Answer 1

首先，您在这里做错了一些事情。让我分解一下。查看每行后面的 cmets。

function test(){
  var a = 1; //this variable a is local to this function test only.so it is not part of the o1,o2 objects.
  this.b =2; //this keyword refers to the object on which this function is called.
}
var o1 = new test(); 
var o2 = Object.create(test);//this is not a way to create object using Object.create().. Object.create() takes another object as an argument to create a new object and here you are using test which is a function.
console.log(test.a);  //test is a function not an object
console.log(test.b);  //test is a function not an object
console.log(o1.a);    //since a is local to the function that's why you are getting undefined here
console.log(o1.b);    //here output is 2 because b is an object property and you have assigned its value to 2 in the test function.
console.log(o2.a);    //undefined because o2 is not an object it is a function.
console.log(o2.b);    //undefined because o2 is not an object it is a function.

你可以尝试这样实现上面的代码：

var Test=function test(){
  this.a = 1;
  this.b =2;
}
var o1 = new Test();
var o2 = Object.create(o1);

console.log(o1.a);    
console.log(o1.b);    
console.log(o2.a);    
console.log(o2.b);

Answer 2

在 javascript 中，函数也是对象，所以 test、o1 和 o2 是对象，它们具有属性，这是在控制台上可视化时的结果：

console.log(test.a);  //undefined ---> test has no a nor b property
console.log(o1.b);    //2         ---> o1 has b property
console.log(o2.a);    //undefined ---> o2 has no a nor b property