How to count Total number of ID in JSON

I have same data but it's many. I have to calculate total number of ID 'v1' in my list what i have to do.

{
    "_id": "5416cb72603b9c3c7b630d76",
    "V1": "34c2cc7b-a650-6d53-9fd2-eb0dd77c4b85",
    "V2": "122.170.60.1741410780012332",
    "V3": 1234,
    "V4": "122.170.60.174",
    "V5": "1280*899",
    "V6": "Window",
    "V7": "",
    "V9": 23.039568000000003,
    "V10": 72.56600399999999,
    "V11": "Chrome:37.0.2062.120",
    "PubisherId": "53d919366ee580a63185572f",
    "PublicationID": "53ce129671b91d9e2c5907e7",
    "IssueID": "1408703918",
    "timestamp": 1410780018
},


var idCount = 0;
arr = JSON.parse('[{...},{...},{...}]');//json
arr.map(function(val){
  if(val.v1) {
    idCount++ 
  }
});
console.log(idCount);

Maybe something like this? I count DISTINCT V1 values.

function count_v1(in_obj){
    var obj = JSON.parse(in_obj);
    var found_id = {};
    var cnt = 0;
    for (var i = 0; i < obj.length; i++){
      if (obj[i]['V1'] != undefined)
        if (found_id[obj[i]['V1']] == undefined){
          found_id[obj[i]['V1']] = 1;
          cnt++
        }
    }
    return cnt;
}