Applying constructor attribute have no result

Question

I'm explicitly define constructor property of just created object as the following:

var fooProto={foo:"prototype"};
function f(){ };
f.prototype=fooProto;
var object= new f();
object.constructor.prototype.bar="bar";
console.log(object.foo, Object.prototype.bar);

In the console we have prototype,bar, but i'm explicit undefined,undifined. Please explain why it's occuring?

I don't understand the question. What does *"but i'm explicit undefined,undifined."* mean? — Felix Kling, Dec 16 '13 at 01:32

score 1 · Accepted Answer · edited May 23 '17 at 11:49

You're adding bar to the Object.prototype:

console.log(fooProto.constructor===Object);//true
console.log(object.constructor.prototype===fooProto);//true

After setting the prototype of f() you should repair the constructor:

f.prototype=...;
f.prototype.constructor=f;

If you were expecting undefined, undefined then I've just answered why the second one doesn't print undefined.

Why the first one isn't undefined is answered here in the introduction to constructor functions. Short answer; when you request members on an object and that object doesn't have that member/property then JS will look for it on the prototype of the constructor that created the object.

For example:

var test = {};
console.log(test.hasOwnProperty);//=hasOwnProperty(), not undefined

Test doesn't have a hasOwnProperty so where did it come from?

console.log(test.constructor);//=Object()
console.log(test.constructor.prototype.hasOwnProperty
  ===test.hasOwnProperty);//=true

So test got hasOwnProperty from Object.prototype and Object is the constructor of test

Applying constructor attribute have no result

1 Answers1