JavaScript 添加侦听器时丢失引用

Question

下面是我的代码，当我将对象方法 keydown 添加到事件监听器时，我丢失了对对象的引用有人知道为什么会这样吗？正确的方法是什么

<html>
<head>
<title>Testpage</title>

<script type="text/javascript"> 
Person = function(fname, lname)
    {
    this.lastname=fname;
    this.firstname=lname;
    }

Person.prototype.toString = function()
    {
    return this.firstname+" "+this.lastname;
    }

Person.prototype.keydown = function (event) 
    {
    alert(this.firstname);
    //return this.firstname+" "+this.lastname;
    }

function init() 
    {
    var k=new Person("Paul", "Nistor");
    document.getElementById("nowpressed").value=k.toString();

    window.addEventListener("keydown", k.keydown, true);
    }
</script>
</head>
<body onLoad="init();">
<form>
    Now pressed: <input id="nowpressed" name="nowpressed" type="text" size="20" value="">
</form>
</body>
</html>

Answer 1

一种方法是提供匿名函数：

window.addEventListener("keydown", function() {
    k.keydown();
}, true);

另一种方法是将 k.keydown 方法显式绑定到适当的上下文：

window.addEventListener("keydown", k.keydown.bind(k), true);

JavaScript 添加侦听器时丢失引用

JavaScript losing reference when adding an listener

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