js拼接字符串,字符串轉數組

想要把字符串按一定的規則拼起來如 1,2,3

var a = [];

a.push(1);
a.push(2);
a.push(3);

a.join(',');

=>> 1,2,3

想要把'1,2,3' 轉成字符串 ["1", "2", "3"] 字符串分割

var a = '1,2,3';

a.split(',')

=>> ["1", "2", "3"]