<!DOCTYPE html>
<html>
<head>
<title> 事件</title>
<script type="text/javascript">
function count(){
var a = document.getElementById('text1').value;
var b = document.getElementById('select').value;
var c = document.getElementById('txt2').value;
var result ;
switch(b){
case "+":
result = parseFloat(a) + parseFloat(c);
break;
case "-":
result = parseFloat(a) - parseFloat(c);
break;
case "*":
result = parseFloat(a) * parseFloat(c);
break;
default:
result = parseFloat(a) / parseFloat(c);
}
document.getElementById("fruit").value = result;
}
</script>
</head>
<body>
<input type='text' id='txt1' />
<select id='select'>
<option value='+'>+</option>
<option value="-">-</option>
<option value="*">*</option>
<option value="/">/</option>
</select><br>
<input type='text' id='txt2' />
<input type='button' value=' = ' onclick="count()" /> <br><!--通过 = 按钮来调用创建的函数，得到结果-->
<input type='text' id='fruit' disabled/>
</body>
</html>
var a = document.getElementById('text1').value中ID名写错了，应为var a = document.getElementById('txt1').value;