Struggling with Prototypes/Inheritance in JavaScript

Question

I have a little practice project, where I am developing a Blackjack Game. I am using JS prototypes for OOP purposes, which works great so far. I have a Game, Card and Hand (all the cards a player has) prototype object. Now I have a problem with my Hand prototype. It returns my card values "undefined". I don't know what is wrong since my Hand constructor returns the right values.

I have to admit my understanding of JS prototypes is maybe not the best yet, so this might be my problem which I am struggling with?!

This is the relevant snippet from my code:

var suits = ["club", "diamond", "heart", "spade"];
var cardValue = ["ace", 2, 3, 4, 5, 6, 7, 8, 9, "jack", "queen", "king"];

function Game() {
}

Game.prototype = {

    deal: function() {
        var cardSuit = Math.floor(Math.random() * suits.length);
        var cardNumber = Math.floor(Math.random() * cardValue.length);

        return cardSuit, cardNumber;
    },


    createHand: function() {
        var newHand = new Hand();
        var userCards = newHand.getHand();

        return newHand;
    }

};

var card1, card2, handCards;

function Hand() {
    var card1 = Game.prototype.deal(); //works! return two values
    var card2 = Game.prototype.deal(); //works! return two values
    var handCards = [card1, card2]; //works! creates array from card 1 and 2
}

Hand.prototype = {

    getFirstCard: function() {
        return card1; // IS "UNDEFINED"!
    },

    getSecondCard: function() {
        return card2; // IS "UNDEFINED"!
    },

    getHand: function() {
        return handCards; // IS "UNDEFINED"!
    }
};

Can you please help me? Thank you very much in advance!

What's this supposed to mean? return cardSuit, cardNumber;. That will simply return cardNumber. — elclanrs
– elclanrs, Commented Dec 24, 2013 at 1:09

nnnnnn · Accepted Answer · 2013-12-24 01:16:35Z

3

You are declaring some global variables without assigning values:

var card1, card2, handCards;

Then in hour Hand() function you declare variables of the same name that are local to that function:

function Hand() {
    var card1 = Game.prototype.deal();
    // etc.

Then your other functions try to access the globals which have never been assigned a value:

getFirstCard: function() {
    return card1; // IS "UNDEFINED"!
},

If you want to access these values from the methods of your "class" these variables should be instance variables:

function Hand() {
    this.card1 = Game.prototype.deal(); //works! return two values
    this.card2 = Game.prototype.deal(); //works! return two values
    this.handCards = [this.card1, this.card2]; //works! creates array from card 1 and 2
}
...
getFirstCard: function() {
    return this.card1;
},

...and so forth in the other methods that use those variables. And you can delete the var card1, card2, handCards; line.

Or it might be tidier to do this:

function Hand() {
    this.handCards = [Game.prototype.deal(), Game.prototype.deal()];
}
...
getFirstCard: function() {
    return this.handCards[0];
},
getSecondCard: function() {
    return this.handCards[1];
},

(There are other problems in your code, e.g.:

your cardValue array is missing 10
return cardSuit, cardNumber; will just return cardNumber
your createHand() function declares and sets a userCards variable that is never used
I don't see the point in having a Game() constructor if you never instantiate any Game objects and just call the methods of the prototype like Game.prototype.deal()
you don't keep track of what cards are left in the deck, so (however unlikely) the random card selection could deal the same card twice

...and so forth.)

edited Dec 24, 2013 at 1:16

answered Dec 24, 2013 at 1:09

nnnnnn

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5 Comments

PSL Over a year ago

Shouldn't it be this.handCards = [this.card1, this.card2];

christophe Over a year ago

the this.handCards array must be [this.card1, this.card2], right?

nnnnnn Over a year ago

@PSL and christophe - Correct. Fixed.

christophe Over a year ago

Works great, I got it! I wasn't aware of the variables mess I am causing! ;) Thanks a lot!

christophe Over a year ago

thanks also for your other hints. I just began with the code and my first step was just to display any of the random dealt cards ;) You are absolutely right with the Game constructor! Anyway, very good hints and corrections, thank you!

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