Transform an Array into an Object using .reduce()

Question

I am trying to study Array.reduce. And I was given the following task:

Input data:

const report = [
  {
    dateOfReport: "11-01-2021",
    userId: "id1",
    userMetric: { first_metric: 10, second_metric: 15 },
  },
  {
    dateOfReport: "11-01-2021",
    userId: "id2",
    userMetric: { first_metric: 9, second_metric: 14 },
  },
  {
    dateOfReport: "12-01-2021",
    userId: "id1",
    userMetric: { first_metric: 11, second_metric: 14 },
  },
  {
    dateOfReport: "12-01-2021",
    userId: "id2",
    userMetric: { first_metric: 16, second_metric: 19 },
  },
];

And I need to get this data in the output

const output = [
  {
    dateOfReport: "11-01-2021",
    id1: { first_metric: 10, second_metric: 15 },
    id2: { first_metric: 9, second_metric: 14 },
  },
  {
    dateOfReport: "12-01-2021",
    id1: { first_metric: 11, second_metric: 14 },
    id2: { first_metric: 16, second_metric: 19 },
  },
];

I tried to write some code, but I have no idea how to do it correctly. How can I solve this problem?

Code:

 const result = report.reduce((acc, dataItem) => {
    let outputArray = [];

    if (dataItem) {
      outputArray.push({ ...dataItem, date: dataItem.dateOfReport, [dataItem.userId]: dataItem.userMetric });
    }

    return outputArray;
  });

  return result;

Does this answer your question? [How to group an array of objects by key](https://stackoverflow.com/questions/40774697/how-to-group-an-array-of-objects-by-key) — ponury-kostek, Nov 16 '21 at 11:31
With the reduce function, acc is the accumulator. Basicly an object, you can give an initial value and it is in your use through every iteration. You don't neeed temp arrays, like outputArray. — hade, Nov 16 '21 at 11:37

score 1 · Accepted Answer · answered Nov 16 '21 at 11:37

Corrected the logic

const report = [
  {
    dateOfReport: "11-01-2021",
    userId: "id1",
    userMetric: { first_metric: 10, second_metric: 15 },
  },
  {
    dateOfReport: "11-01-2021",
    userId: "id2",
    userMetric: { first_metric: 9, second_metric: 14 },
  },
  {
    dateOfReport: "12-01-2021",
    userId: "id1",
    userMetric: { first_metric: 11, second_metric: 14 },
  },
  {
    dateOfReport: "12-01-2021",
    userId: "id2",
    userMetric: { first_metric: 16, second_metric: 19 },
  },
];
const result = report.reduce((acc, dataItem) => {
  const node = acc.find(item => item.dateOfReport === dataItem.dateOfReport);
  if (node) {
    node[dataItem.userId] = dataItem.userMetric;
  } else {
    acc.push({ dateOfReport: dataItem.dateOfReport, [dataItem.userId]: dataItem.userMetric });
  }
  return acc;
}, []);

console.log(result);

Transform an Array into an Object using .reduce()

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