<form name="form">
<input name="s1" onchange="zb()" />
<input name="s2" onchange="zb()" />
<input name="s3" onchange="zb()" />
<input name="s4" onchange="zb()" />
<input name="result"/>
</form>
function zb()
{
var s1=parseInt(document.form.s1.value);
var s2=parseInt(document.form.s2.value);
var s3=parseInt(document.form.s3.value);
var s4=parseInt(document.form.s4.value);
document.form.result.value=s1+s2+s3+s4;}
<input name="s1" onchange="zb()" />
<input name="s2" onchange="zb()" />
<input name="s3" onchange="zb()" />
<input name="s4" onchange="zb()" />
<input name="result"/>
</form>
function zb()
{
var s1=parseInt(document.form.s1.value);
var s2=parseInt(document.form.s2.value);
var s3=parseInt(document.form.s3.value);
var s4=parseInt(document.form.s4.value);
document.form.result.value=s1+s2+s3+s4;}
<input name="s1"/>
<input name="s2"/>
<input name="s3"/>
<input name="s4"/> <br>
<input name="result"/> <input type="button" value="求和" onclick="Sum()" /><script>var Sum = function(){
var intReturn = 0;
for(var i=1;i<5;i++){
if(document.all("s"+ i).value != ""){
intReturn += parseInt(document.all("s"+ i).value);
}
}
document.all("result").value = intReturn;
}
</script>