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我尝试了很多不同的方法来避免从全局变量中获取 undefined ，但它会弄乱其余的代码。除了少数之外，我可以在所有函数中调用变量 this.userHunger。我知道我一定在某个地方出错了：

<html>
<head>
<script>
function user() 
{
    this.userHunger = 100;
    this.userHealth = 100;
    this.userFoodNumber = 10;

    this.checkForHungerHealthData = function() 
    {
        document.getElementById('userHealthDisplay').innerHTML = "Your Health:" + this.userHealth;
        document.getElementById('HungerBarVisual').innerHTML = "Your Hunger" + this.userHunger;
        document.getElementById('amountOfFood').innerHTML = "Amount of food" + this.userFoodNumber;
    }

    this.signInButton = function()
    {
        document.getElementById('myButtonSignIn').style.visibility="hidden";
        this.checkForHungerHealthData(); // displays current hunger and health
    }

    this.eatFood = function() 
    {
        if(this.userFoodNumber  > 0) 
        {
            this.userHealth = this.userHealth + 10;
            this.userHunger = this.userHunger + 50;
            this.userFoodNumber--;
            document.getElementById('amountOfFood').innerHTML = "Amount of food:" + this.userFoodNumber;
            document.getElementById('userHealthDisplay').innerHTML = "Your Health:" + this.userHealth;
            document.getElementById('HungerBarVisual').innerHTML = "Your Hunger is currently " + this.userHunger + "%";
        }
    }

    this.start=function()
    {
        this.hungerClockStart();
    }

    this.hungerClockStart=function()
    {
        this.setUserHungerClock(setInterval(this.updateHungerScreen,1000));
    }

    this.updateHungerScreen=function()
    {
        alert(this.userHunger); //Returns undefined
        if(this.userHunger >= 0) 
        {
            var subtractAmount=1;
            document.getElementById('HungerBarVisual').innerHTML = "Hunger is Currently at: " + this.userHunger + "%";
            this.userHunger--;
        }
    }
}

var user1 = new user();

</script>
</head>
<h1 id = "userHealthDisplay">Your Health:<br></h1>
<h1 id = "HungerBarVisual">Your Hunger:<br></h1>

<input id="myButtonSignIn" style="visibility:visible" type="button" value="Sign in" onclick="user1.signInButton(); user1.hungerClockStart()">

<body>
<h1 id = 'Notifyer1'><br></h1>
<h1 id = 'Notifyer2'><br></h1>
<h1 id = 'amountOfFood'>Amount of Food:</h1>
<input id="eatFoodButton" style="visibility:visible" type="button" value="Eat some food!" onclick="user1.eatFood()">

</body>
</html>

我在 this.hungerScreenUpdate 中不断收到错误，不明白为什么。

4

1 回答 1

2 
this.setUserHungerClock(setInterval(this.updateHungerScreen,1000));

需要是

this.setUserHungerClock(setInterval(this.updateHungerScreen.bind(this),1000));

在 JavaScript 中，this它是动态的，而不是词法的，因此this可以根据调用方式而改变。当您这样做时setInterval(this.updateHungerScreen,1000)，它不会updateHungerScreen以正确的this值调用。您可以使用bind将函数绑定到特定上下文。

正如评论者所建议的那样，如果您需要它在 IE8 及更低版本上工作，您需要填充bind或使用 Phrogz 的存储this为变量的技术。

更新

this有关JavaScript 工作原理的概述，请参阅https://stackoverflow.com/a/13373383/1662998

就个人而言，我建议在bindIE8 及以下版本中进行填充。这样您就可以继续使用新功能，同时保持与旧浏览器的向后兼容性。只需将其添加到页面顶部的脚本中，就可以了：

if (!Function.prototype.bind)
    Function.prototype.bind = function(context/*, ...preArgs */) {

        var f = this,
            preArgs = Array.prototype.slice.call(arguments, 1);

        if (typeof f != 'function')
            throw new TypeError('Function expected.');

        if (context != null)
            context = Object(context);

        return function bound(/* ...args */) {
            var args = Array.prototype.slice.call(arguments);
            return f.apply(context, preArgs.concat(args));
        };

    };
于 2012-11-24T17:56:02.817 回答