In case you've always wanted to create your own Numbering System, here you go.

NumberingSystem.js

// Your own numbering system
function NumberingSystem(digits) {
  this.setSystem(digits);
}
NumberingSystem.prototype = {
	setSystem: function (digits) {
		this.digits = typeof digits === "string" ? digits : "0123456789";
		this.base = this.digits.length;
	},
	toDecimal: function (nr) {
		if (!nr.split) return NaN;
		var nrDigits = nr.split(""), digit, res = 0, conversion, isValid = true;
		while (isValid && (digit = nrDigits.shift()) != undefined) {
			res *= this.base;
			res += (conversion = this.digits.indexOf(digit)) === -1 ? (isValid = false) : conversion;
		}
		return isValid ? res : NaN;
	},
	fromDecimal: function (nr) {
		var division, integer = parseInt(nr), remainder, res = [];
		if (integer === 0) {
			return this.digits[0];
		}
		while (integer) {
			division = integer/this.base;
			integer = parseInt(division);
			remainder = division%1;
			res.unshift(this.digits[Math.round(remainder*this.base)]); // Sometimes it's 4.99999 instead of 5 or the like. That's why we need to Math.round the number
		}
		return res.join("");
	},
	to: function (nr, nrSystem) {
		if (!nrSystem instanceof NumberingSystem) return NaN;
		return nrSystem.fromDecimal(this.toDecimal(nr));
	},
	from: function (nr, nrSystem) {
		if (!nrSystem instanceof NumberingSystem) return NaN;
		return this.fromDecimal(nrSystem.toDecimal(nr));
	}
};

/**
 * Usage:
 * 
 * var myNumberingSystem = new NumberingSystem("What the fuck! A numbering System?! Why that?");
 * 
 * myNumberingSystem.fromDecimal(7533554402803);
 * 
 * "because!"
 * 
 */