Javascript array to array of object with count

Question

What would be the best way to convert an array of dates into an array of unique objects with a count for each date?

Example Array

['07Sep', '09Sep', '14Sep', '05Sep', '12Sep', '06Sep', '10Sep', '14Sep', '08Sep', '10Sep', '04Sep', '05Sep', '07Sep', '08Sep', '13Sep', '12Sep', '05Sep', '05Sep', '06Sep', '10Sep', '06Sep', '13Sep', '05Sep', '06Sep', '05Sep', '04Sep', '12Sep', '05Sep', '11Sep', '10Sep', '11Sep', '06Sep', '04Sep', '10Sep']

Expected output with this structure: (vales are made up)

[
{04SEP:4},
{06SEP:2},
{07SEP:2},
{09SEP:4},
...
]

Robby Cornelissen · Accepted Answer · 2022-09-18 09:22:16Z

2

You can do it with a single reduce() statement:

const result = array.reduce((a, v) => ({...a, [v]: (a[v] || 0) + 1}), {});

In each iteration, simply spread the already accumulated result, and then increment the existing value (or 0) for each element.

Snippet:

const array = ['07Sep', '09Sep', '14Sep', '05Sep', '12Sep', '06Sep', '10Sep', '14Sep', '08Sep', '10Sep', '04Sep', '05Sep', '07Sep', '08Sep', '13Sep', '12Sep', '05Sep', '05Sep', '06Sep', '10Sep', '06Sep', '13Sep', '05Sep', '06Sep', '05Sep', '04Sep', '12Sep', '05Sep', '11Sep', '10Sep', '11Sep', '06Sep', '04Sep', '10Sep'];

const result = array.reduce((a, v) => ({...a, [v]: (a[v] || 0) + 1}), {});

console.log(result);

answered Sep 18, 2022 at 9:22

Robby Cornelissen

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2 Comments

Olly Over a year ago

Thank you. What would be the best way to also sort these values?

Robby Cornelissen Over a year ago

Do you want a single object or an array of single-key objects? Objects are usually considered to be unordered in JavaScript.

Hassan Imam · Accepted Answer · 2022-09-18 09:24:02Z

1

You can use array#reduce to group the dates and convert to your desired format using Object.entries and array#map.

const input = ['07Sep', '09Sep', '14Sep', '05Sep', '12Sep', '06Sep', '10Sep', '14Sep', '08Sep', '10Sep', '04Sep', '05Sep', '07Sep', '08Sep', '13Sep', '12Sep', '05Sep', '05Sep', '06Sep', '10Sep', '06Sep', '13Sep', '05Sep', '06Sep', '05Sep', '04Sep', '12Sep', '05Sep', '11Sep', '10Sep', '11Sep', '06Sep', '04Sep', '10Sep'],
      result = input.reduce((r, date) => {
        r[date] = (r[date] || 0) + 1;
        return r;
      },{}),
      output = Object.entries(result).map(arr => Object.fromEntries([arr]));
console.log(output);

answered Sep 18, 2022 at 9:24

Hassan Imam

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