title: how to sum object array with same key value
body: <p>i have an array of object like this :</p>
  
  <pre><code>var original_db = [
              {date_diff:"2016/10/31", date_number:1,hangup_cause:"NORMAL_CLEARING"},
  {date_diff:"2016/10/31", date_number:1,hangup_cause:"NORMAL_CLEARING"},
  {date_diff:"2016/10/31", date_number:1,hangup_cause:"NO_ANSWER"},
  {date_diff:"2016/10/31", date_number:1,hangup_cause:"ORIGINATOR-CANCEL"},
  {date_diff:"2016/10/30", date_number:2,hangup_cause:"NORMAL_CLEARING"},
  {date_diff:"2016/10/30", date_number:2,hangup_cause:"NO_ANSWER"},
  {date_diff:"2016/10/30", date_number:2,hangup_cause:"NO_ANSWER"},
  {date_diff:"2016/10/30", date_number:2,hangup_cause:"NO_ANSWER"},
  {date_diff:"2016/10/29", date_number:3,hangup_cause:"NORMAL_CLEARING"},
  {date_diff:"2016/10/29", date_number:3,hangup_cause:"ORIGINATOR-CANCEL"},
  {date_diff:"2016/10/29", date_number:3,hangup_cause:"ORIGINATOR-CANCEL"},
          ];
  </code></pre>
  
  <p>and here is my expected result:</p>
  
  <pre><code>result : 
  [
  {date_diff : "2016/10/31", NORMAL_CLEARING : 2, NO_ANSWER : 1, ORIGINATOR-CANCEL : 1},
  {date_diff : "2016/10/30", NORMAL_CLEARING : 1, NO_ANSWER : 3},
  {date_diff : "2016/10/29", NORMAL_CLEARING : 1, ORIGINATOR-CANCEL : 2}
  ]
  </code></pre>
  
  <p>Is there an easy way to do this?</p>