在forEach循环中使用Object.create

Question

I am writing a card game using javascript/html5

I get the gamestate as an ajax request. this is JSON data that lists the players and what cards they have in their hand

I am trying to loop over each player and set the hand data as follows

gameState.Players.forEach(function (player, i) {    
    var inx = i + 1;
    var canvas = document.getElementById("player" + inx);
    var ctx = canvas.getContext("2d");

    var hand = Object.create(Hand);
    hand.initialiseHand(player.Hand);
    hand.setPosition(10, 10);
    hand.draw(ctx);
});

There are six canvases on the page. One for each player

I am using Object.create to create a new "instance" of a hand. And then calling the draw method, which lays out the images on the canvas

However, what actually happens is that each players data just gets added to the same instance

ie each time I go round the forEach loop, the hand object just gets assigned more and more cards

I would like to have a seperate instance for each player

So how do I achieve this?

I want to loop over the data and create a new hand for each iteration of the loop

I am guessing that the hand variable has been hoisted out of the loop and so I get the same one each time?

and this is what Hand looks like

var Hand = {

    faceDownCards: [],
    faceUpCards: [],
    inHandCards: [],

    initialiseHand: function (handData) {
        handData.FaceDownCards.forEach(function (c, i) {
            this.faceDownCards.push(Object.create(Card, pd({ rank: c.Rank, suit: c.Suit })));
        }, this);

        handData.FaceUpCards.forEach(function (c, i) {
            this.faceUpCards.push(Object.create(Card, pd({ rank: c.Rank, suit: c.Suit })));
        }, this);

        handData.InHandCards.forEach(function (c, i) {
            this.inHandCards.push(Object.create(Card, pd({ rank: c.Rank, suit: c.Suit })));
        }, this);

    },

    draw: function (context) {
        this.faceDownCards.forEach(function (c, i) {
            c.draw(context);
        });

        this.faceUpCards.forEach(function (c, i) {
            c.draw(context);
        });

        this.inHandCards.forEach(function (c, i) {
            c.draw(context);
        });
    },

    setPosition: function (x, y) {
        this.x = x;
        this.y = y;
        this.faceDownCards.forEach(function (c, i) {
            c.setPosition(x + i * 70, y);
        });
        this.faceUpCards.forEach(function (c, i) {
            c.setPosition(x + i * 70, y + 60);
        });
        this.inHandCards.forEach(function (c, i) {
            c.setPosition(x + i * 20, y + 80);
            //c.rotation = 3;
        });
    }
};

Answer 1

Your hand variable is not being hoisted out of the loop. Javascript variables have function scope and you are already conveniently using a function inside your forEach loop.

---

What is Hand ? The usual convention is having capitalized names represent constructor functions, but the Players and the Object.create make it look like Hand is just an object?

If Hand is already an object (and not a Constructor you are misusing) my best bet would be that initializeHand/setPosition are seting closed-over variables instead of accessing them via this . (Of course, this is just a wild guess without looking at the Hand code)

---

After looking at your Hand code, I now think the problem is that the hands are sharing the faceDownCards , etc, arrays. The base prototype should only be used for shared traits and the arrays and other instance-olny state should be set on initialization :

For a concrete example, change

handData.FaceDownCards.forEach(function (c, i) {
    this.faceDownCards.push(Object.create(Card, pd({ rank: c.Rank, suit: c.Suit })));
}, this);

to

 this.faceDownCards = handData.FaceDownCards.map(function (c, i) {
     return Object.create(Card, pd({ rank: c.Rank, suit: c.Suit }));
 });

---

ps.: All the Object.create going around is not exactly idiomatic Javascript. But then, I guess you should have an Idea of what you are doing, right?