JavaScript Objects get overwritten when using prototyping

Question

I have a Pers(on) and an Employee, who is derived from Pers.

Pers = function(options){
  this.Name;
  this.ID;
  this.init = function(options){
    this.Name=options.Name;
    this.ID=options.ID;
  }
}

Employee = function(options){
  this.Sal;
  this.init = function(options){
    this.Sal=options.Sal;
    this.__proto__.init(options);
  }
  this.init(options);
}

Employee.prototype=new Pers();

Now when i create new objects...

var o=new Employee({Name:"Nik",ID:"1",Sal:100});
var p=new Employee({Name:"Tanja",ID:"2",Sal:200});

and alert their Name, i will get two times "Tanja".

Has anyone an idea?

Pretty sure you shouldn't be using __proto__ anymore.

gen_Eric
– gen_Eric

2012-07-17 16:11:04 +00:00
Commented Jul 17, 2012 at 16:11 — gen_Eric
– gen_Eric, Commented Jul 17, 2012 at 16:11

johusman · Accepted Answer · 2012-07-17 16:29:08Z

3

this.__proto__.init(options);

will call the init method on the prototype with the prototype itself as this, causing you to modify the prototype. Try

this.__proto__.init.apply(this, [options]);

Edit

To avoid __proto__ you can save a reference to the prototype init function before you shadow it:

Employee = function(options){
  this.Sal;
  var protoInit = this.init;
  this.init = function(options){
    this.Sal=options.Sal;
    protoInit.apply(this, [options]);
  }
  this.init(options);
}

edited Jul 17, 2012 at 16:29

answered Jul 17, 2012 at 16:23

johusman

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1 Comment

OrangeDog Over a year ago

call(this, options) is probably better in this case

OrangeDog · Accepted Answer · 2012-07-18 13:09:29Z

2

You're calling init in the wrong scope. Try something like this.

function Person(opt) {
    this.name = opt.name;
    this.id = opt.id;
}

function Employee(opt) {
    Person.call(this, opt);
    this.sal = opt.sal;
}

Employee.prototype = Object.create(Person.prototype, {});

You can now set properties of Person.prototype and Employee.prototype and they should behave as expected.

This avoids using hacky deprecated properties (__proto__) and should be a lot clearer. Object.create is used to make an instance using the super-constructor's prototype without actually calling the super-constructor (removing the need for init calls). You could include semi-standard property definitions such as super and constructor while you're doing it, as many libraries' inherits implementations do.

edited Jul 18, 2012 at 13:09

answered Jul 17, 2012 at 16:28

OrangeDog

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2 Comments

Niklas Hoesl Over a year ago

this is not working, when i'm creating new objects: var o=new Employee({name:"Nik",id:"1",sal:100}); var p=new Employee({name:"Tanja",id:"2",sal:200}); o.name will not be found. what i'm doing wrong?

OrangeDog Over a year ago

In my haste to re-write without init, I made the same mistake you did. Answer edited.

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