Question

So lets say I want to make a simple Price class in JS. It’s basically a number so I thought I would just inherit from a Number. Here is some code:

Number.prototype.toMoney = function (precision, decimals, thousands) {
    // Formats number...
}

function Price(val) {
    Number.call(val); // Based on MozillaDN
}

Price.sufix = ' EUR'; // To print with every Price

// Price.prototype = Number.prototype;
Price.prototype = new Number(); // any difference?

Price.prototype.toString = function() {
    return this.toMoney() + Price.sufix; // Of course it does not work.
}

var price = new Price(100.50);
alert(price.toString()); // Gives: undefined EUR
alert(price); // This fail. I thought it should work as it works with Numbers.

I probably do something wrong, but I can’t find out what.

Answer 1

You have a typo in this line I think:

Price.prototype - new Number(); // any difference?

You are assigning a new Number to be the prototype of Price so you need to use an = sign not a minus sign.

It should read:

Price.prototype = new Number(); // any difference?

Having said that, it still doesn’t work. I can’t understand why you are attempting to inherit from Number.

This works:

function Price(val) {
    this.val = val;                
}
Price.suffix = ' EUR'; // To print with every Price

Price.prototype = {
    toString: function() {
        return this.toMoney() + Price.suffix;
    },
    toMoney: function(precision, decimals, thousands){
        //fancy code here to format the number
        return "formatted number " + this.val;
    }
};

var price = new Price(100.50);
alert(price.toString()); // returns: formatted number 100.50 EUR
alert(price);