text:     /**
       * Define the listeners for the sensors
       */
      const listeners = {
          temperature: [],
          humidity: [],
          humidityground: [],
          CO2: [],
          LDR: []
        }
    
        /**
         * The subscribe function takes two arguments, the
         callback function, and the type.
         * It pushes the provided listener to the array of 
         listeners (depending on the "type" of sensor
         provided)
         */
        const subscribe = (listener, type) => {
          listeners[type].push(listener)
        }
    
        /**
         * To remove a listener from our list, we call   
            the unsubscribe function.
         * This accepts the exact same types of arguments as the 
         subscribe function
         */
         const unsubscribe = (removedListener, type) => {
          /**
           * The list of listeners is sifted through and the 
          listener functions are filtered on
          * the basis of whether they are the same as the 
         listener provided
           * Those that are the same (which would be only one) 
         are removed
           */
           listeners[type] = listeners[type].filter(listener => 
        listener !== removedListener)
        }
    
        /**
         * The notify function is used by the cache when it 
         receives an update from the sensor
         * It takes a value and a type as an argument, and 
         notifies all the listeners of that type that a new value 
         has been received
         */
        const notify = (value, type) => {
          listeners[type].forEach(listener => {
            listener(value)
          })
        }
    
        /**
         * The functions that are to be used by other modules are 
        exported
         */
        module.exports = {
          subscribe, unsubscribe, notify
        }
  