this.id返回什么？

Question

function test(){
alert(this);//alerts oblect
alert(this.id);//alerts undefined

}

<input type="text" onKeyPress="test();" id="text1">

why does alert(this.id) alert undefined ? Is this because this returns the document object?

Thank You

Answer 1

Your code should be.

function test(objRef){
  alert(objRef);//alerts oblect
  alert(objRef.id);//alerts undefined
}

<input type="text" onKeyPress="test(this);" id="txtNum" />

EDIT: You may use following code too.

<script type="text/javascript">
        window.onload = function() {
            var txtnum = document.getElementById("txtNum");
            txtnum.onkeypress = function() {
                alert(this);
                alert(this.id);
            };
        };
 </script>

 <input type="text" id="txtNum" />

Answer 2

this is the window instance. Try it in your browser:

javascript:alert(this)

I get an [object Window]

Answer 3

在全局范围内，上下文是窗口，因为窗口没有未定义的称为“ id”的属性。