<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>无标题</title>
<script type="text/javascript">
function sum(obj) {
var z = document.getElementById("z");
var a = document.getElementById("a");
var b = document.getElementById("b");
var c = document.getElementById("c");
all.value=parseInt(z.value);
if(a.value!='')
{
y.value=parseInt(a.value);
h.value=parseInt(z.value)-parseInt(a.value);
}
if(a.value!=''&&b.value!='')
{
y.value=parseInt(b.value)+parseInt(a.value);
h.value=parseInt(z.value)-parseInt(a.value)-parseInt(b.value);
}
if(a.value!=''&&b.value!=''&&c.value!='')
{
y.value=parseInt(b.value)+parseInt(a.value)+parseInt(c.value);
h.value=parseInt(z.value)-parseInt(a.value)-parseInt(b.value)-parseInt(c.value);
}
}
</script>
</head>
<body>
总分：<input type="text" id="z" onkeyup="sum(this);" />
<input type="text" id="a" onkeyup="sum(this);" />
<input type="text" id="b" onkeyup="sum(this);" />
<input type="text" id="c" onkeyup="sum(this);" />
总分：<input type='text' id='all' style="border:0px solid white; width:25px" />
已选：<input type='text' id='y' style="border:0px solid white; width:25px" />
还剩：<input type='text' id='h' style="border:0px solid white; width:25px" />
</body>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>无标题</title>
<script type="text/javascript">
function sum(obj) {
var z = document.getElementById("z");
var a = document.getElementById("a");
var b = document.getElementById("b");
var c = document.getElementById("c");
all.value=parseInt(z.value);
if(a.value!='')
{
y.value=parseInt(a.value);
h.value=parseInt(z.value)-parseInt(a.value);
}
if(a.value!=''&&b.value!='')
{
y.value=parseInt(b.value)+parseInt(a.value);
h.value=parseInt(z.value)-parseInt(a.value)-parseInt(b.value);
}
if(a.value!=''&&b.value!=''&&c.value!='')
{
y.value=parseInt(b.value)+parseInt(a.value)+parseInt(c.value);
h.value=parseInt(z.value)-parseInt(a.value)-parseInt(b.value)-parseInt(c.value);
}
}
</script>
</head>
<body>
总分：<input type="text" id="z" onkeyup="sum(this);" />
<input type="text" id="a" onkeyup="sum(this);" />
<input type="text" id="b" onkeyup="sum(this);" />
<input type="text" id="c" onkeyup="sum(this);" />
总分：<input type='text' id='all' style="border:0px solid white; width:25px" />
已选：<input type='text' id='y' style="border:0px solid white; width:25px" />
还剩：<input type='text' id='h' style="border:0px solid white; width:25px" />
</body>
</html>
效果图：
实时显示的！！！
普及小知识：
onkeyup：键盘按下的时候会触发onkeydown，松开键盘时会出发onkeyup。
parseInt：parseInt() 函数可解析一个字符串，并返回一个整数。
parseInt("10"); //返回 10
parseInt("19",10); //返回 19 (10+9)