javascript限制文本框输入内容

<script type="text/javascript">
        function judgeNumber(code) {
            if (code >= 48 & code <= 57) {
                return true;
            }
            else {
                return false;
            }
        }

        function judgeChar(code) {
            if ((code >= 65 & code <= 90) || (code >= 97 & code <= 122)) {
                return true;
            }
            else {
                return false;
            }
        }
    </script>

html代码：

数字：<input type="text" name="userName" onkeypress="return judgeNumber(event.keyCode);" /><br />
        字母：<input type="text" name="password" onkeypress="return judgeChar(event.keyCode);" />