Is there a faster way of summing up values for keys across large array of objects?

Question

I have a large dataset in the form:

data = [{ a: 12, b: 8 }, { a: 2, c: 4, d: 14 }, { c: 2, e: 4, f: 14 }]

What I want is an object with all keys (here a-f) and the sum of their values across the data set, like so:

{ a: 14, b: 8, c: 6, d: 14, e: 4, f: 14 }

I can get the desired result like this:

function sum(a, b) { return a + b };

function countTotal(n) {
  let ndata = data.filter((i) => Object.keys(i).includes(n))
  let cnt = Object.assign(ndata.map((i) => i[n])).reduce(sum);  
  return {[n]:cnt};
};

let names = 'abcdef'.split('')
let res = Array.from(names).map((n) => countTotal(n))
res = Object.assign({}, ...res);

My problem is that this takes quite long for the actual data set I have (which is quite big). Is there a way to do the same more efficiently?

Below is some code do create a large dummy data set approximating the real data set.

let dummy_names = [];
for (let i = 0; i < 2000; i++) {
    dummy_names.push((Math.random() + 1).toString(36).slice(2,7));
};
dummy_names = [...new Set(dummy_names)];
names = new Set();

function makeResponses() {
  let responses = {};
  let idx = 0;
  for (let j = 0; j <= Math.floor(Math.random() * 7); j++) {
    idx = Math.floor(Math.random()*dummy_names.length);
    inam = dummy_names[idx];
    names.add(inam);
    responses[inam] = Math.floor(Math.random()*20);
  };
  return responses;
};

let data = [];
for (let i = 0; i < 20000; i++) {
  data.push(makeResponses());
    };

Answer 1

I'd use a helper object to keep track of the sums and loop through the objects in the array.

The most important thing is only look at each value once to keep the complexity (in terms of O notation) low. There are many ways to iterate, I'm not sure if for loops or .forEach is faster.

Here is a rough solution:

    const data = [{a: 12, b: 8}, {a: 2, c: 4, d: 14}, {c: 2, e: 4, f: 14}];
    const sums = {};
    data.forEach(object => {
        Object.entries(object).forEach(([key, value]) => {
            if (sums.hasOwnProperty(key)) {
                sums[key] += value;
            } else {
                sums[key] = value;
            }
        });
    });
    console.log(sums);