SOURCE

function add(a,b){
   return(a+b)
}
function minus(a,b){
    return(a-b)
}
function mul(a,b){
   return(a*b)
}
function div(a,b){
    return(a/b)
}
function caculter(a,b,f){
return f(parseInt(a),parseInt(b))

}
function jisuan(){
var num1=document.getElementById("num1").value
var op=document.getElementById("op").value
var num2=document.getElementById("num2").value
switch(op){
case "add":

document.getElementById("res").value=caculter(num1,num2,add)
break
case "minus":
document.getElementById("res").value=caculter(num1,num2,minus)
break
case "mul":
document.getElementById("res").value=caculter(num1,num2,mul)
break
case "div":
document.getElementById("res").value=caculter(num1,num2,div)
break
}
}

<form>
<input id="num1"/>
<select id="op">
<option value="add">+</option>
<option value="minus">-</option>
<option value="mul">*</option>
<option value="div">/</option>
</select>
<input id="num2"/>
<label>=</label>
<input id="res"/>
<input type="button" value="计算" onclick="jisuan()"/>
</form>