Convert number to it's base-64 representation, and reverse it. It is NOT THE SAME as Base64 encoding!

Math.toString(64).js

var Base64 = function(){};
Base64._rixits = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz-_";
Base64.toHash = function( n ) {
    if( isNaN(Number(n)) || n===null || n===Number.POSITIVE_INFINITY || n<0 ) throw "The input is not valid";
    n = Math.floor( n );

    var result = '';
    do result = this._rixits.charAt(n%64) + result;
    while( n=Math.floor(n/64) );
    return result; // String
};
Base64.toNumber = function( h ) {
    for(var i=0, result=0; i<h.length; i++) result = (result*64) + this._rixits.indexOf(h.charAt(i));    
    return result; // Integer
};