express.js caching (middleware)

dejavu.js

/**
 * @category Application
 * @package  Middleware
 * @author   Kanstantsin A Kamkou ([email protected])
 */

// tiny caching engine
var Dejavu = function () {
  // default cache holder
  this._cache = [];

  /**
   * Returns the cached key
   *
   * @param {String} key
   * @return {Mixed|null}
   */
  this.get = function (key) {
    return this._cache[key] || null;
  };

  /**
   * Checks if cache has such key
   *
   * @param {String} key
   * @return {Boolean}
   */
  this.has = function (key) {
    return !!this._cache[key];
  };

  /**
   * Stores data to the cache set
   *
   * @param {String} key
   * @param {Mixed} data
   * @param {Number} seconds
   * @return {Dejavu}
   */
  this.set = function (key, data, seconds) {
    // cache set
    this._cache[key] = data;

    // cache cleanup
    seconds = parseInt(seconds, 10);
    if (seconds > 0) {
      setTimeout(function () { this.cleanup(key); }.bind(this), seconds * 1000);
    }

    // chaining
    return this;
  };

  /**
   * Key cleanup
   *
   * @param {String} key
   * @return {Dejavu}
   */
  this.cleanup = function (key) {
    // key cleanup
    if (!delete this._cache[key]) {
      this._cache[key] = null;
    }

    // chaining
    return this;
  };
};

// exporting stuff
module.exports = function (req, res, next) {
  // we have it
  if (req.app.get('dejavu')) {
    return next();
  }

  // adding the cache engine to the settings
  req.app.set('dejavu', new Dejavu());

  // jumping up
  next();
};

why not check if expires in get method, use setTimeout in set method will generate too much timeout if set too much keys