Friendly Date Ranges（freecodecamp高级算法5）

让日期区间更友好！

把常见的日期格式如：YYYY-MM-DD 转换成一种更易读的格式。

易读格式应该是用月份名称代替月份数字，用序数词代替数字来表示天 (1st 代替 1).

记住不要显示那些可以被推测出来的信息: 如果一个日期区间里结束日期与开始日期相差小于一年，则结束日期就不用写年份了；在这种情况下，如果月份开始和结束日期如果在同一个月，则结束日期月份也不用写了。

另外, 如果开始日期年份是当前年份，且结束日期与开始日期小于一年，则开始日期的年份也不用写。

例如:

包含当前年份和相同月份的时候，makeFriendlyDates(["2017-01-02", "2017-01-05"]) 应该返回 ["January 2nd","5th"]

不包含当前年份，makeFriendlyDates(["2003-08-15", "2009-09-21"]) 应该返回 ["August 15th, 2003", "September 21st, 2009"]。

请考虑清楚所有可能出现的情况，包括传入的日期区间是否合理。对于不合理的日期区间，直接返回 undefined 即可

function makeFriendlyDates(arr) {
                    var currYear = (new Date()).getFullYear();//2017，当前年份
                
                    var fir = arr[0].split("-");
                    var las = arr[1].split("-");
　　　　　　　　　　　　var firStr,lasStr;
                    var arrStr = [];//返回数组
                    
                    var month = ["January","February","March","April","May","June","July","August","September"
                    ,"October","November","December"];
                    
                    firStr = month[fir[1]-1] + " " + changeDate(fir[2]) + ", " + fir[0];
                    
                    //是否同一年
                    var flag = (las[0] == fir[0]) || 
                            ((las[0] == Number(fir[0])+1) && (fir[1] > las[1])) ||
                            ((las[0] == Number(fir[0])+1) && (fir[1] == las[1]) && (fir[2] > las[2]));
                    
                    if(las[0] == fir[0] && las[1] == fir[1] && las[2] == fir[2]){//结束日期和开始日期相同，返回的结束日期为空
                        lasStr = "";
                    }else if(las[0] == fir[0] && las[1] == fir[1] ){
                        //如果一个日期区间里结束日期与开始日期相差小于一年，则结束日期就不用写年份了；
                        //在这种情况下，如果月份开始和结束日期如果在同一个月，则结束日期月份也不用写了。
                        lasStr = changeDate(las[2]);
                    }else if(flag){//同一年，结束日期不写年
                        lasStr = month[las[1]-1] + " " + changeDate(las[2]);
                    }else{
                        lasStr = month[las[1]-1] + " " + changeDate(las[2]) + ", " + las[0];
                    }
                    
                    if(fir[0] == currYear && flag){//如果开始日期年份是当前年份，且结束日期与开始日期小于一年，则开始日期的年份也不用写。
                        firStr = firStr.substr(0,firStr.length-6);//如：", 2017"
                    }
                    
                    arrStr[0] = firStr;
                    if(lasStr != ""){
                        arrStr[1] = lasStr;
                    }
                    
                    if(fir[0] > las[0]){
                        return undefined;
                    }
                    
                    return arrStr;
                    
                }
                
                function changeDate(str){//返回对应日期
                    var nub = Number(str);//"01"-->1；也可以使用parseInt(str)
                    switch(nub){
                        case 1:
                        case 21:
                        case 31:
                            nub += "st";break;
                        case 2:
                        case 22:
                            nub += "nd";break;
                        case 3:
                        case 23:
                            nub += "rd";break;
                        default:
                            nub += "th";break;
                    }
                    return nub;
                    
                }

　　　　　　　　　//alert(makeFriendlyDates(['2017-01-02', '2017-01-05']));//["January 2nd","5th"].
                //alert(makeFriendlyDates(["2017-02-01", "2017-03-03"]));//["February 1st","March 3rd"].
                //alert(makeFriendlyDates(["2016-05-11", "2017-04-04"]));//["May 11th, 2016","April 4th"].
                //alert(makeFriendlyDates(["2017-07-12", "2018-06-13"]));//["July 12th","June 13th"]
                            
                alert(makeFriendlyDates(["2016-05-11", "2017-04-04"]));//["May 11th, 2016","April 4th"].
                alert(makeFriendlyDates(["2017-07-12", "2018-06-13"]));//["July 12th","June 13th"]
                alert(makeFriendlyDates(["2003-08-15", "2009-09-21"]));//["August 15th, 2003", "September 21st, 2009"].
                alert(makeFriendlyDates(["2010-10-23", "2011-10-22"]));//["October 23rd, 2010","October 22nd"].
                alert(makeFriendlyDates(["2008-10-31", "2009-10-31"]));//["October 31st, 2008","October 31st, 2009"].
                alert(makeFriendlyDates(["2004-11-17", "2005-12-25"]));//["November 17th, 2004","December 25th, 2005"].
                alert(makeFriendlyDates(["2001-12-20", "2001-12-20"]));//["December 20th, 2001"].