split() 按指定分隔符将字符串分割为数组
var s = "This is a dog."
var arr = [];
arr = s.split(' ');
arr : ["This","is","a","dog."]
join() 把数组转换成字符串，里面的每一个元素可以用你指定的连接符来连接起来，这个连接符就是你要传入的参数。
var arr = ["This","is","a","dog."] ;
var s = arr.join(' ');
s :"This is a dog."
