js如何实现一个UUID的生成（代码）

function uuid(len, radix) {
 var chars = '0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz'.split('');
 var uuid = [], i;
 radix = radix || chars.length;
 
 if (len) {
 // Compact form
 for (i = 0; i < len; i++) uuid[i] = chars[0 | Math.random()*radix];
 } else {
 // rfc4122, version 4 form
 var r;
 
 // rfc4122 requires these characters
 uuid[8] = uuid[13] = uuid[18] = uuid[23] = '-';
 uuid[14] = '4';
 
 // Fill in random data. At i==19 set the high bits of clock sequence as
 // per rfc4122, sec. 4.1.5
 for (i = 0; i < 36; i++) {
 if (!uuid[i]) {
 r = 0 | Math.random()*16;
 uuid[i] = chars[(i == 19) ? (r & 0x3) | 0x8 : r];
 }
 }
 }
 
 return uuid.join('');
}