How to most efficiently aggregate array of objects by ID?

Question

My starting data set is an array of objects containing metrics, each containing an ID. I need to convert this data set into an array of aggregates, by ID. For example:

const startingArray = [
  { id: 1, metricA: 5, metricB: 8, metricC: 1 }
  { id: 2, metricA: 4, metricB: 0, metricC: 7 }
  { id: 1, metricA: 9, metricB: 8, metricC: 2 }
  { id: 3, metricA: 1, metricB: 8, metricC: 2 }
  { id: 3, metricA: 6, metricB: 6, metricC: 1 }
  { id: 2, metricA: 3, metricB: 1, metricC: 9 }
  { id: 1, metricA: 3, metricB: 9, metricC: 8 }
]

const aggregates = {};

startingArray.forEach((item) => {
  if (!aggregates[item.id]) {
    aggregates[item.id] = {
      id: item.id,
      metricA: item.metricA, 
      metricB: item.metricB, 
      metricC: item.metricC
    }
  } else {
    aggregates[item.id].metricA += item.metricA,
    aggregates[item.id].metricB += item.metricB,
    aggregates[item.id].metricC += item.metricC
  }
});

// convert to flat array using lodash toArray() method
const endingArray = toArray(aggregates);

// results:
// [
//   { id: 1, metricA: 17, metricB: 25, metricC: 11 }
//   { id: 2, metricA: 5, metricB: 1, metricC: 16 }
//   { id: 3, metricA: 5, metricB: 8, metricC: 3 }
// ]

The arrays can be massive, what is the most efficient way of processing this data set?

Answer 1

You could destructure id from the object and collect the rest for getting all entries and create a new object and sum all rest properties.

 const data = [{ id: 1, metricA: 5, metricB: 8, metricC: 1 }, { id: 2, metricA: 4, metricB: 0, metricC: 7 }, { id: 1, metricA: 9, metricB: 8, metricC: 2 }, { id: 3, metricA: 1, metricB: 8, metricC: 2 }, { id: 3, metricA: 6, metricB: 6, metricC: 1 }, { id: 2, metricA: 3, metricB: 1, metricC: 9 }, { id: 1, metricA: 3, metricB: 9, metricC: 8 }], result = Object.values(data.reduce((r, { id, ...o }) => { Object.entries(o).forEach(([k, v]) => { r[id]??= { id }; r[id][k] = (r[id][k] || 0) + v; }); return r; }, {})); console.log(result);

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