Javascript Object.create不是从父级继承

Question

I want the man object inherits from the person object. I could have done it using the new operator, but it should work with Object.create . But why it is not working? The console.log says undefined instead of the expected hello .

 function person() { this.say="hello"; } function man() { this.name="John Miler"; } man.prototype = Object.create(person); var Johnny = new man(); console.log(Johnny.say); 

Answer 1

Your problem is two-fold.

Problem 1:

Object.create should be passed the prototype , not the constructor. In this case, you should use Object.create(person.prototype); , not Object.create(person);

---

Problem 2:

The say property is added when the constructor is called, and you never call the parent constructor from the child constructor.

There are a couple ways to solve this, depending on your desired behavior.

Option 1, call parent constructor.

person.call(this);

Sample:

 function person() { this.say="hello"; } function man() { person.call(this); this.name="John Miler"; } man.prototype = Object.create(person.prototype); var Johnny = new man(); console.log(Johnny.say); 

Option 2, make it a static property.

person.prototype.say = "hello";

Sample:

 function person() { } person.prototype.say = "hello"; function man() { this.name="John Miler"; } man.prototype = Object.create(person.prototype); var Johnny = new man(); console.log(Johnny.say); 

Answer 2

If what you try to achieve is for a man object to inherit a person object, try this:

// superclass
function Person() {
  this.say = "hello";
}

// superclass method
Person.prototype.doStuff = function() {
  console.info('Stuff done.');
};

// subclass
function Man() {
  Person.call(this); // call super constructor.
  this.name="John Miler";
}

// subclass extends superclass
Man.prototype = Object.create(Person.prototype);
Man.prototype.constructor = Man;


var Johnny = new Man();

console.log(Johnny.say); // hello

Answer 3

Object.create should be passed the prototype not the constructor.

function person() {
 this.say="hello";
}

function man() {
 this.name="John Miler";
}

man.prototype = Object.create(new person());

var Johnny = new man();

console.log(Johnny.say);