snip.js

      // Imagine we have an observable of key counts. Each message has a key and a count, like so
      // {
      //    key: "SomeKey",
      //    count: 16
      //  }
      // We use buffer to convert this to an observable of arrays of messages
      // Now we want to reduce each array, keeping only the latest count for each key
      
      var messages = getKeyCountsObservablefromSomewhere()
      .buffer(function () { return Rx.Observable.timer(50); })
      .select(function(messages) {
        if (messages.length === 0) return messages;
        
        var hash = {};
        messages.forEach(function(m) {
          hash[m.key] = m.count;
        })
        
        console.log('hash', hash);

        var uniqueMessages = [];
        Object.keys(hash).forEach(function(key) {
          uniqueMessages.push({key: key, count: hash[key]});
        })        
        
        console.log('uniqueMessages', uniqueMessages);
        
        return uniqueMessages;        
      });