<input name="f1" type="text" id="f1" value="5" size="15" onblur="func()">
<br>
<br> <input name="f2" type="text" id="f2" value="10" size="15" onblur="func()">
<br>
<br> <input name="f3" type="text" id="f3" value="0" size="15" onblur="func()">
<br>
<br>
<input name="f4" type="text" id="f4" value="0" size="15" onblur="func()">
<br>
<br> <input name="f5" type="text" id="f5" value="0" size="15" onblur="func()">
<font color="#FF0000"><strong><div id="div1">15</div></strong></font>
function func(){
var f1=document.getElementById("f1").value
var f2=document.getElementById("f2").value
...
var f5=document.getElementById("f5").value
var total=0;
if(f1 !='' || f2!='' || f3!='' || f4!='' || f5!=''){
total=f1+f2+f3+f4+f5
document.getElementById("div1").innerText=total
}}
<br>
<br> <input name="f2" type="text" id="f2" value="10" size="15" onblur="func()">
<br>
<br> <input name="f3" type="text" id="f3" value="0" size="15" onblur="func()">
<br>
<br>
<input name="f4" type="text" id="f4" value="0" size="15" onblur="func()">
<br>
<br> <input name="f5" type="text" id="f5" value="0" size="15" onblur="func()">
<font color="#FF0000"><strong><div id="div1">15</div></strong></font>
function func(){
var f1=document.getElementById("f1").value
var f2=document.getElementById("f2").value
...
var f5=document.getElementById("f5").value
var total=0;
if(f1 !='' || f2!='' || f3!='' || f4!='' || f5!=''){
total=f1+f2+f3+f4+f5
document.getElementById("div1").innerText=total
}}
function sum(){
var v1=isNaN(parseInt(f1.value))?0:parseInt(f1.value,10);
var v2=isNaN(parseInt(f1.value))?0:parseInt(f2.value,10);
var v3=isNaN(parseInt(f1.value))?0:parseInt(f3.value,10);
var v4=isNaN(parseInt(f1.value))?0:parseInt(f4.value,10);
var v5=isNaN(parseInt(f1.value))?0:parseInt(f5.value,10);
f6.value=v1+v2+v3+v4+v5;
alert(isNaN(this.value) + this.value);
}
</script>
<input name="f1" type="text" value="0" size="15" onblur="sum();">
<br><br>
<input name="f2" type="text" value="0" size="15" onblur="sum();">
<br><br>
<input name="f3" type="text" value="0" size="15" onblur="sum();">
<br><br>
<input name="f4" type="text" value="0" size="15" onblur="sum();">
<br><br>
<input name="f5" type="text" value="0" size="15" onblur="sum();">
<br><br>
<input name="f6" type="text" value="0" size="15">
这是调试用的