Prototypal in heritance in Javascript

Question

Below is the Javascript object that I have:

var calc = {
    title : 'Calculator',
    plus : function( a, b ) {
        return (
           console.log(this),    // calc
           console.log(arguments), // [ 3, 5 ]
           console.log(a+b), // 8
           console.log(this.title) // Calculator
        )
    }
};

And I am trying to access the prototype of this object calc by the following:

calc.prototype.getNum = function( numStr ) {
  return isNaN(parseInt(numStr)) ? 'Not a valid number!' : parseInt(numStr);
};

But it keeps giving the following error, whenever I execute it!

TypeError: Cannot set property 'getNum' of undefined

Can anyone please tell me what I'm doing wrong here?

Why do you want to add something to the prototype, if you have only one "instance" of your object literal? — Bergi, Mar 15 '14 at 12:33
@Bergi because that's what you do, when you are learning something... — Kamran Ahmed, Mar 15 '14 at 13:40
If there is ever going to be only one calc then you can leave it as object literal, put all functions in the body of that and forget about prototype. If you're planning to create several calc instances then you may use prototype. For an explanation about prototype and constructor functions you can read this answer: http://stackoverflow.com/questions/16063394/prototypical-inheritance-writing-up/16063711#16063711 — HMR, Mar 15 '14 at 13:53
possible duplicate of [How does JavaScript .prototype work?](http://stackoverflow.com/questions/572897/how-does-javascript-prototype-work) — Bergi, Mar 15 '14 at 13:54

thefourtheye · Accepted Answer · 2014-03-15T11:59:50.010

0

prototype is a property of functions. It will not be available on object literals. So, you might want to convert the calc to a function, like this

function calc() {
    this.title = 'Calculator';
}

calc.prototype.plus = function( a, b ) {
  console.log(this);
  console.log(arguments);
  console.log(a+b);
  console.log(this.title);
}

Now, you should be able to add the getNum to the prototype of calc.

edited Mar 15 '14 at 11:59

answered Mar 15 '14 at 11:28

thefourtheye

233,700
52
457
497

I will be better to put plus method to prototype ( right now new instance of method will be created for each calc object ) – Vasiliy vvscode Vanchuk Mar 15 '14 at 11:57

score -1 · Answer 2 · answered Mar 15 '14 at 11:27

-1

Use calc.constructor.prototype

answered Mar 15 '14 at 11:27

Paul Kotets

199
5

1

As it is the answer might not be very useful to the readers. Could you please explain the answer a bit more? – thefourtheye Mar 15 '14 at 11:32
No, because `calc.constructor === Object`. – Bergi Mar 15 '14 at 12:33
Nooo you'll be adding to Object.prototype if you do that – HMR Mar 15 '14 at 13:48

Prototypal in heritance in Javascript

2 Answers2