Super fast isEven in JavaScript - Reddit meme

super-fast-iseven.js

var redis = require('redis');
var redisClient = redis.createClient();

/**
 * Super fast cached magic for determining evenness of a number.
 *
 * @param {number} x Number to check (integer).
 * @param {function} callback Called with the result, or when our magic failed.
 */
function isEven(x, callback) {
    if(x == 0) {
        callback(null, true);
        return;
    }

    if(x < 0)
        x *= -1;

    redisClient.get('isEven:' + x, function(err, result) {
        if(err || result != null) {
            callback(err, result);
            return;
        }

        isEven(x - 1, function(err, odd) {
            if(err) {
                callback(err);
                return;
            }

            redisClient.set('isEven:' + x, !odd);
            callback(null, !odd);
        });
    });
}

module.exports = isEven;