Javascript 生成 uuid 函数

这个函数用最短的代码生成了非常有效的uuid，非常巧妙。短小精悍，一个字符都不多。Returns a random v4 UUID of the form xxxxxxxx-xxxx-4xxx-yxxx-xxxxxxxxxxxx, where each x is replaced with a random hexadecimal digit from 0 to f, and y is replaced with a random hexadecimal digit from 8 to b.