const base54 = (function(){
    var DIGITS = “abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ$_”;
    return function(num) {
    var ret = “”;
    do {
    ret = DIGITS.charAt(num % 54) + ret;
    num = Math.floor(num / 54);
    } while (num > 0);
    return ret;
    };
    })();
    base54（uid++）