group and sum json object javascript

Question

I would like to group json object by "topic_id" and sum "score" of each "topic_id".

I have json object like this

{
 "topic_id": {
   "0": 1,
   "1": 1,
   "6": 2,
   "8": 2,
   "12": 3,
   "13": 3,
   "18": 4,
   "20": 4,
},
   "score": {
   "0": 1,
   "1": 3,
   "6": 2,
   "8": 3,
   "12": 3,
   "13": 1,
   "18": 3,
   "20": 3,
}
}

My result should be

{
"topic_id" : {
   "0" : 1,
   "1" : 2,
   "2" : 3,
   "3" : 4,
},
"score" : {
   "0" : 4,
   "1" : 5,
   "2" : 4,
   "3" : 6,
}
}

Hope anyone can help me.

Thank so much.

Pranav C Balan · Accepted Answer · 2017-01-25 04:29:41Z

You can use Array#reduce method to generate the object.

var data = {"topic_id": {"0": 1, "1": 1, "6": 2, "8": 2,  "12": 3, "13": 3, "18": 4, "20": 4, },"score": { "0": 1,  "1": 3, "6": 2, "8": 3, "12": 3, "13": 1, "18": 3,  "20": 3, }};

// variable for holding the index reference of id
var ref = {},
  // counter variable for property name, you can avoid if you are using array  
  i = 0;

console.log(
  // get kesy of topic_id
  Object.keys(data.topic_id)
  // iterate over the property names
  .reduce(function(obj, k) {
    // check id refernce present already
    if (data.topic_id[k] in ref) {
      // if present then update the score
      obj.score[ref[data.topic_id[k]]] += data.score[k];
    } else {
      // if not present add reference in the ref object
      ref[data.topic_id[k]] = i++;
      // add entry in topic_id
      obj.topic_id[ref[data.topic_id[k]]] = data.topic_id[k];
      // add score value
      obj.score[ref[data.topic_id[k]]] = data.score[k];
    }
    // return the object reference
    return obj;
    // set initial value as our prefered object format
  }, {topic_id: {}, score: {}})
)

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