大小写字母的转换

        var str = ‘tracy‘;

        toUpper( str );

        // 转为大写

        function toUpper( str ){
            var result = str.toUpperCase();
            console.log(result);
        }

        // 转为小写

        var sstr = ‘LEE‘;

        toLower( sstr );

        function toLower( sstr ){
            var result = sstr.toLowerCase();
            console.log(result);
        }

        // 将字符串中的每一个单词的首字母都转为大写

        var words = "i am a student.";

        changeFirstLetter( words );        

        function changeFirstLetter( words ){
            var arr = words.split(" ");
            for( var i = 0; i < arr.length; i++ ){
                arr[i] = arr[i].charAt(0).toUpperCase() + arr[i].slice(1);
            }
            return console.log(arr.join(" "));
        }