                        课程
                    
                        /前端开发
                        
                            /JavaScript
                        
                        /JavaScript进阶篇

var result = '';这段代码代表什么，起什么作用，为什么少了它程序错误！

<html>
	<head>
	<title> new document </title>
	<script type="text/javascript">
	function count(){
	var txt1 = parseInt( document.getElementById('txt1').value);//获取第一个输入框的值
	var txt2 = parseInt( document.getElementById('txt2').value);//获取第二个输入框的值
	var select = document.getElementById('select').value;//获取选择框的值
	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;//设置结果输入框的值
	}
	</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>