【attribute】
<!DOCTYPE html>
<html>
<head>
<script
src="http://ajax.googleapis.com/ajax/libs/jquery/1/jquery.min.js"></script>
</head>
<body>
<img class="a" />
<script>
$(".a").attr('src',"http://www.yjpub.cn/style/download.png");
console.log($(".a").length);
</script>
</body>
</html>
