                        課程
                    
                        /前端開發
                        
                            /JavaScript
                        
                        /JavaScript進階篇

很簡單，在編輯器里多試試就可以了

<!DOCTYPE?html>
<html>
<head>
????<title>?事件</title>
????<script?type="text/javascript">
????????function?count(){
????????????//獲取第一個輸入框的值
????????????var?x?=?document.getElementById("txt1").value;
????????????//獲取第二個輸入框的值
????????????var?y?=?document.getElementById("txt2").value;
????????????//獲取選擇框的值
????????????var?z?=?document.getElementById("select").value;
????????????//獲取通過下拉框來選擇的值來改變加減乘除的運算法則
????????????switch(z){
????????????????case?"+":
????????????????????document.getElementById("fruit").value=Number(x)+Number(y);
????????????????????break;
????????????????case?"-":
????????????????????document.getElementById("fruit").value=Number(x)-Number(y);
????????????????????break;
????????????????case?"*":
????????????????????document.getElementById("fruit").value=Number(x)*Number(y);
????????????????????break;
????????????????case?"/":
????????????????????document.getElementById("fruit").value=Number(x)/Number(y);
????????????????????break;
????????????}
????????????//設置結果輸入框的值

????????}
????</script>
</head>
<body>
<input?type='text'?id='txt1'?/>
<select?id='select'>
????<option?value='+'>+</option>
????<option?value="-">-</option>
????<option?value="*">*</option>
????<option?value="/">/</option>
</select>
<input?type='text'?id='txt2'?/>
<input?type='button'?value='?=?'??onclick="?count()"/>
<input?type='text'?id='fruit'?/>
</body>
</html>