js代码

<script>
$(function(){
	var a = $('#nav'),
		b =a.offset();
	$(document).on('scroll',function(){
		var	c = $(document).scrollTop();
		if(b.top<=c){
			a.css({'position':'fixed','top':'0px'})
			}else{
				a.css({'position':'absolute','top':'300px'})
				}
		})
	})
</script>