CODE EXAMPLE FOR JAVASCRIPT

sum similar keys in array of objects

const arr = [ { 'name': 'P1', 'value': 150 }, { 'name': 'P1', 'value': 150 }, { 'name': 'P2', 'value': 200 }, { 'name': 'P3', 'value': 450 } ];

const sumByKey = (arr, key, value) => {
  const map = new Map();
  for(const obj of arr) {
    const currSum = map.get(obj[key]) || 0;
    map.set(obj[key], currSum + obj[value]);
  }
  const res = Array.from(map, ([k, v]) => ({[key]: k, [value]: v}));
  return res;
}

console.log(sumByKey(arr, 'name', 'value')); // 'name' = value to group by, 'value' = value to sum
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