<form action="" method="post" onSubmit="return inputNull(this)">
  		<table>
	  		<p>
				<span>姓名：</span>
				<input type="text" placeholder="姓名">
	  		</p>
	  		<p>
				<span>年齡：</span>
				<input type="text" placeholder="年齡">
	  		</p>
			<input type="submit" value="登入">
	  		
  	</form>

		<script type="text/javascript">
  		function inputNull(form){
			for(i=0;i<form.length;i++){
                    //form屬性的elements的首字母e要小寫
				if(form.elements[i].value == ""){ 
					alert("親：" + form.elements[i].placeholder + "不能為空");
					form.elements[i].focus();	
					return false;
				}
			}
  	  	}
  	</script>
 

效果：