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                        /前端開發
                        
                            /JavaScript
                        
                        /JavaScript進階篇

var result = '';這段代碼代表什么，起什么作用，為什么少了它程序錯誤！

<html>
	<head>
	<title> new document </title> ?
	<script type="text/javascript">
	function count(){
	var txt1 ? = parseInt( document.getElementById('txt1').value);//獲取第一個輸入框的值
	var txt2 ? = parseInt( document.getElementById('txt2').value);//獲取第二個輸入框的值
	var select = document.getElementById('select').value;//獲取選擇框的值
	var result = '';
	switch (select)
	{
	case '+':
	result = txt1 + txt2;
	break;
	case '-':
	result = txt1 - txt2;
	break;
	case '*':
	result = txt1 * txt2;
	break;
	case '/':
	result = txt1 / txt2;
	break;
	}
	document.getElementById('fruit').value = result;//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>