eliminate a variable for an accumulation operation-CodePudding

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);

CodePudding user response：

The built-in method reduce already does this. Pass the accumulator as the second argument. The first argument in the callback is the accumulated value. The second is the current object in this iteration. Return value of callback is passed to next iteration as the accumulated value.

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 result = data.reduce((accumulator, o) => {
    accumulator[o.id] = { ...accumulator[o.id] , ...o } || o;
        
    return accumulator;
}, {});
    
console.log(result);

CodePudding user response：

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);

CodePudding user response：

You can reduce the dataset using Array.prototype.reduce. Just find the previous object by the id key in the accumulator (acc) or use a new object, and spread the current object over it.

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 reduceBy = (data, predicate) =>
  data.reduce((acc, obj) =>
    (key => ({
      ...acc,
      [key]: {
        ...(acc[key] ?? {}),
        ...obj
      }
    }))
    (typeof predicate === 'string'
      ? obj[predicate]
      : predicate(obj)
    ), {});

const reducedData = reduceBy(data, ({ id }) => id); // or reduceBy(data, 'id')

console.log(reducedData);

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