表单控件中元素input的属性checked的操作实例是什么呢？

<input type="radio" name="radio" value="1" checked>
<input type="radio" name="radio" value="2">
<input type="checkbox" name="checkbox" value="1">
<input type="checkbox" name="checkbox" value="2">
<script>
var oInput = document.getElementsByTagName('input');
for(var i = 0,len = oInput.length; i < len; i++){
    oInput[i].onmouseover = function(){
        this.checked = 'checked';
    }
}    
</script>