javascript background-color onmousemove attachevent

this.style.backgroundColor don't work in IE7/8

my code is:

<!DOCTYPE html>
<html>
<head>
    <title>Demo</title>
    <meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
    <style type="text/css">
        p{
            border:1px solid #CCC;
            margin:5px;
            padding:5px;
        }
    </style>
    <script type="text/javascript">
        window.onload = changeColor;
        function changeColor() {
            for(var i =0; i < document.getElementById("main").getElementsByTagName("p").length; i++) {
                var obj = document.getElementById("main").getElementsByTagName("p")[i];
                if (window.addEventListener) {
                    obj.addEventListener('mousemove', function () {
                        this.style.backgroundColor ="#EEE";
                    }, false);
                    obj.addEventListener('mouseout', function () {
                        this.style.backgroundColor ="#FFF";
                    }, false);
                } else if (window.attachEvent) {
                    //for ie
                    obj.attachEvent('onmousemove', function () {
                        this.style.backgroundColor ="#EEE";
                    });
                    obj.attachEvent('onmouseout', function () {
                        this.style.backgroundColor ="#FFF";
                    });
                }
            }
        }
    </script>
</head>
<body>
    <div>
        <p>1</p>
        <div id="main">
            <p>2.1</p>
            <p>2.2</p>
            <p>2.3</p>
        </div>
    </div>
</body>
</html>

it work well in Chrome、FireFox and ie9,but can't work in IE7/8

the error message is:Unable to set the property value of the "backgroundColor": the object is null or undefined

what's rong with me?

Solution

When using attachEvent in IE, this is set to the window object, not to the object that the event happened on.

In IE, the global variable window.event.srcElement will contain the target object for the event.

You could code a work-around like this to make all the event handlers work the same:

function hookEvent(event, obj, fn) {
    if (obj.addEventListener) {
        obj.addEventListener(event, fn, false);
    } else {
        obj.attachEvent("on" + event, function() {return(fn.call(obj, window.event));});
    }
}

This will make it so that this is set to the source object of the event and that the argument to the event handler is the event object.