javascript - Player 不会从 JavaScript 中的 CardHolder 继承

Question

我是第一次在 JavaScript 中练习 OOP，不明白为什么继承不起作用。

代码:

function Card(s, v) {
    if (arguments.length === 0) {
        this.suit = SUITS[Math.floor(Math.random()*SUITS_LENGTH)];
        this.val = VALS[Math.floor(Math.random()*VALS_LENGTH)];
    }
    else {
        this.suit = s;
        this.val = v;
    }
}
Card.prototype = {
    constructor: Card,
    toString: function() {
        return this.val + " of " + this.suit;
    },
    lowVal: function() {
        if (this.val === "A") { return 1; }
        else if (this.val === "J" || this.val === "Q" || this.val === "K") { return 10; }
        else { return parseInt(this.val); }
    },
    highVal: function() {
        if (this.val === "A") { return 11; }
        else if (this.val === "J" || this.val === "Q" || this.val === "K") { return 10; }
        else { return parseInt(this.val)}
    }
};

function CardHolder() {
    this.status = "in";
    this.cards = [];
}
CardHolder.prototype = {
    constructor: CardHolder,
    deal: function() {
        this.cards.push(new Card());
    },
    lowVal: function() {
        var lowVal = 0;
        for (var i = 0, len = this.cards.length; i < len; i++) {
            lowVal += this.cards[i].lowVal();
        }
        return lowVal;
    },
    highVal: function() {
        var highVal = 0;
        for (var i = 0, len = this.cards.length; i < len; i++) {
            highVal += this.cards[i].highVal();
        }
        return highVal;
    },
    score: function() {
        if (this.highVal() > 21) { return this.lowVal(); }
        else { return this.highVal(); }
    }
};
function Player(id) {
    CardHolder.call(this);
    if (typeof(id)) {
        this.id = id;
    }
}
Player.prototype = Object.create(CardHolder.prototype);
Player.prototype = {
    constructor: Player,
    toString: function() {
        var returnString = "Player " + this.id + ":\n";
        for (var i = 0, len = this.cards.length; i < len; i++) {
            returnString += this.cards[i].toString() + "\n"
        }
        return returnString;
    }
}

输出

var p = new Player();
p.deal();
console.log(p.toString());

输出 Uncaught TypeError: undefined is not a function。我认为这意味着 p 没有从 CardHolder 继承 deal 函数。

为什么它不起作用？

Answer 1

问题是

Player.prototype = {
    constructor: Player,
    toString: function() {
        var returnString = "Player " + this.id + ":\n";
        for (var i = 0, len = this.cards.length; i < len; i++) {
            returnString += this.cards[i].toString() + "\n"
        }
        return returnString;
    }
}

正在覆盖在

中分配给 Player.prototype 的值

Player.prototype = Object.create(CardHolder.prototype);

为避免这种情况，您可以这样做:

Player.prototype = Object.create(CardHolder.prototype);

Player.prototype.constructor = Player;

Player.prototype.toString = function() {
        var returnString = "Player " + this.id + ":\n";
        for (var i = 0, len = this.cards.length; i < len; i++) {
            returnString += this.cards[i].toString() + "\n"
        }
        return returnString;
};