事件

function count(){

var a=document.getElementById("txt1").value;

var b=document.getElementById("txt2").value;

var f=document.getElementById("select").value;

switch(f)

{

case '+':document.getElementById("fruit").value=a+b;

break;

case '-':document.getElementById("fruit").value=a-b;

break;

case '*':document.getElementById("fruit").value=a*b;

break;

case '/':document.getElementById("fruit").value=a/b;

break;

}

}

+

-

*

/