Modifying object's prototype not working

Question

I've been trying to figure out why this won't work. Would appreciate if some could help me out!

function Person(name, age) {
    this.name = name;
    this.age = age;
    var ageInTenYears = age + 10; 
    this.sayNameAndAge = function() {
      console.log(name + age);
    }
}

Person.prototype.sayAge = function() {
   console.log(this.age); 
}

Person.prototype = { 
    sayName : function(){
       console.log(this.name); 
    },
    sayNameAfterTimeOut : function(time) {
       setTimeout(this.sayName, time);
    },
    sayAgeInTenYears : function() { 
       console.log(ageInTenYears);
    } 
}

var bob = new Person('bob', 30); 
bob.sayName();

I get this error:

  Uncaught TypeError: Object #<Object> has no method 'sayAge'

Ingo Bürk · Accepted Answer · 2013-11-03 15:13:22Z

6

You are overwriting the entire prototype by doing

Person.prototype = { /* ... */ };

which means that the sayAge method you added before is lost again. Either reverse the order of those assignments or move the sayAge into the other assignment as well.

answered Nov 3, 2013 at 15:13

Ingo Bürk

20.2k7 gold badges72 silver badges103 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

HMR Over a year ago

There are several other obvious things wrong with the code: ageInTenYears is not defined (not the same as having a value of undefined) in sayAgeInTenYears. The value for this is window in sayName when called from sayNameAfterTimeOut causing this.name to have a value of undefined

Šime Vidas · Accepted Answer · 2013-11-03 15:13:33Z

3

With Person.prototype = { … };, you're rewriting the prototype object, i.e. replacing the old one with a completely new object. Cou can do that, but then make sure that you're not defining any methods beforehand (like you do with .sayAge above).

answered Nov 3, 2013 at 15:13

Šime Vidas

187k66 gold badges290 silver badges392 bronze badges

Comments

Community · Accepted Answer · 2017-05-23 12:29:13Z

There are several things wrong with the code, I made some comments where I corrected it. If you have any questions you can comment on this answer:

function Person(name, age) {
    this.name = name;
    this.age = age;
    //var ageInTenYears = age + 10; //<--Why var, you can't
    // use this anywhere but in the Person constuctor body
    this.ageInTenYears=age+10;
}

Person.prototype = { 
    sayName : function(){
       console.log(this.name); 
    },
    sayNameAfterTimeOut : function(time) {
       // check out this link about what this can be
       // https://stackoverflow.com/a/19068438/1641941
       var me=this;
       setTimeout(function(){
         me.sayName();
       }, time);
    },
    sayAgeInTenYears : function() { 
      // you defined it as var so cannot use
      // ageInTenYears outside the constructor body
      //console.log(ageInTenYears);
      console.log(this.ageInTenYears);
    } 
};
Person.prototype.sayAge = function() {
   console.log(this.age); 
};
Person.prototype.sayNameAndAge = function() {
  console.log(this.name + this.age);
};
//just for good measure, someone may do
// Person.prototype.haveBaby=function(){
//   return new this.constructor();
Person.prototype.constructor=Person;

var bob = new Person('bob', 30); 
bob.sayName();

More on prototype, inheritance/mixin, overriding and calling super: https://stackoverflow.com/a/16063711/1641941

Collectives™ on Stack Overflow

Modifying object's prototype not working

3 Answers 3

1 Comment

Comments

Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

1 Comment

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related