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parseInt函數問題

<!DOCTYPE?html>????
<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>

文中最上面獲取txt文本的值，為什么不加parseInt函數，運行加法的時候，為什么是兩個值得相結合，比如1+1的值是11，而不是2