How to aggregate objects that exist in two array?

April 20, 2022

if I have two (large) arrays that specify a key and a numeric value

var a = [
  { gtin: 'a1', quantity: 1 },
  { gtin: 'a3', quantity: 1 },
];
var b = [
  { gtin: 'a1', quantity: 1 },
  { gtin: 'a4', quantity: 1 },
];

what is the easiest way to get a single array that sums the quantities?
(Lodash ok, fewest iterations over array preferred)

 [
  { gtin: 'a1', quantity: 2 },
  { gtin: 'a3', quantity: 1 },
  { gtin: 'a4', quantity: 1 },
];

>Solution :

You could use a Map to keep track of gtin and its accumulated quantity, and then transform it back to array

const a = [
  { gtin: 'a1', quantity: 1 },
  { gtin: 'a3', quantity: 1 },
];

const b = [
  { gtin: 'a1', quantity: 1 },
  { gtin: 'a4', quantity: 1 },
];

const res = Array.from(
  a
    .concat(b)
    .reduce(
      (map, el) => map.set(el.gtin, (map.get(el.gtin) || 0) + el.quantity),
      new Map()
    )
).map(([gtin, quantity]) => ({ gtin, quantity }));

console.log(res);