随机生成字符串，可用来当id

// 随机生成字符串
RandomNumb(n) {
  let str = 'abcdefghijklmnopqrstuvwxyz9876543210';
  let tmp = '',
    i = 0,
    l = str.length;
  for (i = 0; i < n; i++) {
    tmp += str.charAt(Math.floor(Math.random() * l));
  }
  return tmp;
}
var id = vm.RandomNumb(20)
console.log(id)