Datorama JSON - Group by First Dimension convert to dictionary (array of objects) with Javascript

Group and convert JSON string to dictionary (array of objects) (Javascript)

//Convert a Datorama JSON string to a dictionary (array of objects) and group by the first dimension (first key).
//Ideal for D3 charts. The output can be used to present the SUM of all Values (Second Dimension in this case) per Date (First Dimension)
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//JSON query 
var json_data = {"headers":["Month","Value","Number"],"rows":[["2018-10-01 00:00:00.0","one",209],["2018-09-01 00:00:00.0","one",274],["2018-09-01 00:00:00.0","five",183],["2018-10-01 00:00:00.0","five",164],["2018-09-01 00:00:00.0","four",214],["2018-10-01 00:00:00.0","four",192],["2018-09-01 00:00:00.0","three",128],["2018-10-01 00:00:00.0","three",125],["2018-09-01 00:00:00.0","two",199],["2018-10-01 00:00:00.0","two",169]]};


function groupBy(accumulator, item) {
  //Pick the key (values included in the key)
  const [date,extra,value] = item;
  
  //It returns the substring of the date (excludes the day in this case)
  const key = date.slice(0,7);

  if(!accumulator[key]){
    accumulator[key] = 0
  }
  accumulator[key] += value;
  return accumulator;
}

var damn = json_data.rows.reduce(groupBy,{});

//Creates the new JSON (dictionary)
damn = Object.keys(damn).map(function(key){
  return {date: key, Value: "Total", num: damn[key]};
})