Javascript constructor function

Question

When trying executing the two following code blocks separately: The first one:

function Hallo() {
}
var some_obj = {
 name: "Fred",
 age: 23,
}
Hallo.prototype = some_obj;
var obj = new Hallo();
obj.constructor;

And the second one:

 function Hallo() {
 }
 Hallo.prototype.name = 'Khanh';
 Hallo.prototype.age = 23;
 var obj = new Hallo();
 obj.constructor;

I got the result in firebug's console is "Object{}" for the first and "Hallo()" for the second. While the second one is pretty simple to understand but the first one is really odd. Because as I know the constructor of the obj Object in the first one is still the same (that is Hallo() function). However I got Object() function in result. I really cann't understand why. Could you help me with it? Thank you!

• Blender: You're overwriting the entire prototype in the first example, but in the second you're only adding two new properties.
• Lac Viet: Yes, I know it, but the constructor is still the same, isn't it? I concern about constructor.
• Blender: constructor is overwritten by the constructor of your object when you overwrite the entire prototype.

Answer 1

The reason is:

When you do var obj = new Hallo();, then

console.log(obj.constructor === Hallo.prototype.constructor); // true

In your first example, you assigned Hallo.prototype with a new object, whose constructor is function Object (function Object(){...}).

In your second example, the Hallo.prototype.constructor is still function Hallo() {...}

• Lac Viet: Thank xdazz, I did test you answer using firebug. and yes, it return exactly as you said. So if I do: var obj = new Hallo(), the constructor of obj is NOT really Hallo() function but always the Hallo.prototype constructor function. However, I read a book call Object Oriented Javascript, and it said: If I do: var obj = new Hallo(), then the obj Object is created by Hallo() constructor function. So it differ from you answer. Could you explain me about this confusion?

Answer 2

prototype will get a reference that point to the constructor by default, int the first function you overwrite the prototype to some_obj, the constructor reference changes at the same time to some_obj's constructor reference --Object's constructor Object()