eliminate a variable for an accumulation operation

May 23, 2022

The following code produces the desired result but is there a way to refactor it to eliminate the accumulator variable?

const data = [
  { id: "428", date: "2017-01-24" },
  { id: "526", date: "2022-01-01" },
  { id: "428", name: "George" },
  { id: "526", name: "Sam" },
  { id: "827", name: "Dan" }
];

const accumulator = {};

data.forEach(o => {
  accumulator[o.id] = { ...accumulator[o.id] , ...o } || o;
});

console.log(accumulator);

>Solution :

The || o part doesn’t make sense (objects are always truthy), and the repeated spread can become a serious performance trap. It’s also good practice to start with Object.create(null) when using an object as a map to avoid keys colliding with things on Object.prototype (even though it looks like you have numeric keys for the moment).

So, let’s restart from this more efficient and reliable version:

const data = [
  { id: "428", date: "2017-01-24" },
  { id: "526", date: "2022-01-01" },
  { id: "428", name: "George" },
  { id: "526", name: "Sam" },
  { id: "827", name: "Dan" }
];

const accumulator = Object.create(null);

data.forEach(o => {
  Object.assign(accumulator[o.id] ??= {}, o);
});

console.log(accumulator);

With that in mind, here’s how you would use Array.prototype.reduce:

const data = [
  { id: "428", date: "2017-01-24" },
  { id: "526", date: "2022-01-01" },
  { id: "428", name: "George" },
  { id: "526", name: "Sam" },
  { id: "827", name: "Dan" }
];

const accumulator = data.reduce(
  (accumulator, o) => {
    Object.assign(accumulator[o.id] ??= {}, o);
    return accumulator;
  },
  Object.create(null)
);

console.log(accumulator);