Javascript prototype function failure

Question

I'm playing with prototype functions and can't understand why the simple example below isn't returning the negative of a number.

Number.prototype.neg = function(x) { return -x };

var num = -1;
var num2 = 1234;

console.log(num.neg());   // returns NaN (and not 1)
console.log(num2.neg());  // returns NaN (and not -1234);

Any idea where I've made the mistake? I know that I could use property getters instead, but I'm working my way through the basics first (and hopefully learning from the mistakes).

Seriously? Someone voted that down?

Jon M
– Jon M

2016-02-06 14:13:42 +00:00
Commented Feb 6, 2016 at 14:13 — Jon M
– Jon M, Commented Feb 6, 2016 at 14:13

Omri Aharon · Accepted Answer · 2016-02-06 14:08:43Z

3

You are not passing any parameter to the function, so according to how you declared the function, you need to do:

num.neg(-1);

Returning -x when x is undefined gives you NaN. But that kind of defeats the purpose. You need to declare the function a bit differently:

Number.prototype.neg = function() { return -this };

answered Feb 6, 2016 at 14:08

Omri Aharon

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2 Comments

Omri Aharon Over a year ago

To whoever's hiding behind the downvote, please explain your action.

Jon M Over a year ago

Thanks Omri. Really obvious once it's pointed it. I'm now referring to -this with no arguments and it works fine.

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