代码：
<!DOCTYPE html>
<html lang="en">
<head><meta charset="UTF-8"><title>Title</title>
</head>
<body><input type="text"id="user"><script>document.getElementById("user").onblur=function (anyparameter) {if(this.value==""){alert("输入为空!");}}
</script>
</body>
</html>运行结果：
![[SDIO].SDIO总线详解](https://img2018.cnblogs.com/blog/917884/201906/917884-20190616214049134-1561040047.jpg)
