I can't change a property in a constructor function

Question

I'm a newbie with JavaScript and I'm facing a problem with constructor functions, my problem is that I can't overwrite an old function's property with a new one!

Below is my code:

function myFun() {
  this.anotherFun = function() {
    return true;
  }
}

var myVar = new myFun();

console.log(myVar.anotherFun()); // returns 'true' as expected; 

myFun.prototype.anotherFun = function() {
  return false;
}

console.log(myVar.anotherFun()); // is returns 'true' why not 'false'?

You have two functions, an own property and a prototype property. Own properties as dandavis points out are looked up first. — elclanrs, Jul 01 '16 at 17:57

score 4 · Accepted Answer · answered Jul 01 '16 at 18:11

4

Because when the same property appears multiple times in the prototypical chain, it makes sense to use the closest one.

Your instance has an own property, so you won't be able to override it by adding an inherited one.

You might have not wanted to add myFun as an own property

function myFun(){}
myFun.prototype.anotherFun = function(){return true};
var myVar = new myFun();
console.log(myVar.anotherFun()); // true
myFun.prototype.anotherFun = function(){return false};
console.log(myVar.anotherFun()); // false

answered Jul 01 '16 at 18:11

Oriol

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Thanks, that works for me!, but why don't we write function myFun(){this.prototype.anotherFun = function(){return true};} instead of function myFun(){} myFun.prototype.anotherFun = function(){return true}; – M.Taki_Eddine Jul 02 '16 at 13:38
1

@M.Taki_Eddine Because there is no reason to run that code each time you create an instance. So it should be outside the constructor. – Oriol Jul 02 '16 at 15:39

score 1 · Answer 2 · edited May 23 '17 at 12:22

What you are trying to do will not work with your code as own properties will always be looked up before prototype properties, so the change your are making to the prototype property will have no visible effect. In order for this to work, you can change your code to always use prototype properties:

function myFun(){} 
myFun.prototype.anotherFun = function(){return true;}
var myVar=new myFun();
console.log(myVar.anotherFun()); // returns 'true' as expected; 
myFun.prototype.anotherFun=function(){return false;}
console.log(myVar.anotherFun()); // now returns 'false' as expected

If you want to know more about this subject, check out this Question.

Thanks, that works for me!, but why don't we write function myFun(){this.prototype.anotherFun = function(){return true};} instead of function myFun(){} myFun.prototype.anotherFun = function(){return true}; – — M.Taki_Eddine, Jul 02 '16 at 13:39

I can't change a property in a constructor function

2 Answers2