用on事件，就是onmousemove(每一个像素移动)
原型：Element.onmousemove = function (){...
}
实例：window.onmousemove = function (){
alert('鼠标移动了！')
}
另外，拖动（理解不了题目具体）用on事件，就是ondragstart(选择元素并拖动了)
原型：Element.ondragstart = function (){...
}
实例：window.ondragstart = function (){
alert('鼠标拖动了元素！')
}
getElementsByClassName ，IE下不支持
<!DOCTYPE html><html>
<head>
<meta charset=utf-8 />
<title>test</title>
<style>
div{width: 500px height: 300px border: 1px solid Yellow}
.tt{display: block}
</style>
<script>
window.onload = initClass
function initClass() {
var classTest = document.getElementsByTagName("div")
for (var i = 0 i < classTest.length i++) {
if (classTest[i].className == "t") {
var tts = classTest[i].querySelectorAll(".tt") || classTest[i].getElementsByTagName("div")
classTest[i].onmouseover = function() {
overTest(tts[0])
}
classTest[i].onmouseout = function() {
outTest(tts[0])
}
}
}
}
function overTest(tt) {
tt.style.display = "block" /*鼠标移上*/
}
function outTest(tt) {
tt.style.display = "none" /*鼠标移走*/
}
</script>
<body>
<div class="t">ddddddddddddd
<div class="tt">fffffffffffffffff</div>
</div>
</body>
</html>
