Base62 encode & decode (alphanumeric only) in JavaScript

Base62.js

var Base62 = (function () {

    var ALPHA = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789';

    var Base62 = function () {};

    var _encode = function (value) {

        if (typeof(value) !== 'number') {
            throw 'Value is not number!';
        }

        var result = '', mod;
        do {
            mod = value % 62;
            result = ALPHA.charAt(mod) + result;
            value = Math.floor(value / 62);
        } while(value > 0);

        return result;
    };

    var _decode = function (value) {

        var result = 0;
        for (var i = 0, len = value.length; i < len; i++) {
            result *= 62;
            result += ALPHA.indexOf(value[i]);
        }

        return result;
    };

    Base62.prototype = {
        constructor: Base62,
        encode: _encode,
        decode: _decode
    };

    return Base62;

})();

var base62 = new Base62();

base62.encode(3270397159014518); // > "O8pK4ZIns"

base62.decode('O8pK4ZIns'); // > 3270397159014518