js(javascript)判断图片是否存在，如果不存在，显示默认图片

<script  src="http://apps.bdimg.com/libs/jquery/2.1.4/jquery.min.js " ></script> 
<img class="top-img" src="" alt="" />

<script type="text/javascript">
	var img1 = $(".top-img").attr("src");
	if(img1 == ""){
	
		$("img").attr("src","img2.jpg");
	}
</script>