let str = "January,February,March,April,May,June,July,August,September,October,November,December"
let arr = str.split(',');
это приведет к:
["January", "February", "March", "April", "May", "June", "July", "August", "September", "October", "November", "December"]
и если вы хотите преобразовать следующее в:
["January", "February", "March", "April", "May", "June", "July", "August", "September", "October", "November", "December"]
это:
"January,February,March,April,May,June,July,August,September,October,November,December";
использование:
str = arr.join(',')