Question 249164 in Javascript Question posted in
A very good W3school tutorial can be found here.

Javascript – How to get second unique date from array object

How to find second unique date using JS, e.g. output should return 2019-12-25T02:34:00Z

const data = [
  { date: '2019-12-24T03:24:00Z' },
  { date: '2019-12-24T04:56:00Z' },
  { date: '2019-12-25T02:34:00Z' },
  { date: '2019-12-26T01:23:00Z' }
]
Tags:

2

Answers


  1. 0

    Collect dates with counts and find the first date with count === 1 (unique):

    const data = [
  { date: '2019-12-24T03:24:00Z' },
  { date: '2019-12-24T04:56:00Z' },
  { date: '2019-12-25T02:34:00Z' },
  { date: '2019-12-26T01:23:00Z' }
];

const result = Object.values(data.reduce((r, {date: item}) => {
  const date = item.slice(0, 10);
  date in r ? r[date].count++ : r[date] = {count:1, item};
  return r;
}, {})).find(({count}) => count === 1)?.item || null;

console.log(result);
    Login or Signup to reply.
  2. 0

    You could reduce (right) all the dates and build a lookup may by unique date-only. Reducing right will keep the first value as it appears in the original array, but will reverse the output, so you should reverse at the very end.

    const data = [
  { date: '2019-12-24T03:24:00Z' },
  { date: '2019-12-24T04:56:00Z' },
  { date: '2019-12-25T02:34:00Z' },
  { date: '2019-12-26T01:23:00Z' }
];

const uniqueByDate = (data) => data.reduceRight((lookup, { date }) =>
  lookup.set(new Date(date).toLocaleDateString(), date), new Map);

const uniqueTimestampsByDate = [...uniqueByDate(data).entries()].reverse();

const getNthByUniqueDate = (index) =>
  index > -1 && index < uniqueTimestampsByDate.length
    ? uniqueTimestampsByDate[index][1]
    : null;

for (let i = 0; i < uniqueTimestampsByDate.length; i++) {
  console.log(`Unique at index: ${i} = ${getNthByUniqueDate(i)}`);
}

    Output:

    Unique at index: 0 = 2019-12-24T03:24:00Z
Unique at index: 1 = 2019-12-25T02:34:00Z
Unique at index: 2 = 2019-12-26T01:23:00Z

    Here is a version without reversing. It requires a convenience function to only update the lookup map, if the key does not already exist.

    const data = [
  { date: '2019-12-24T03:24:00Z' },
  { date: '2019-12-24T04:56:00Z' },
  { date: '2019-12-25T02:34:00Z' },
  { date: '2019-12-26T01:23:00Z' }
];

const setIfNotPresent = (map, key, value) =>
  !map.has(key) ? map.set(key, value) : map;

const uniqueByDate = (data) => data.reduce((lookup, { date }) =>
  setIfNotPresent(lookup, new Date(date).toLocaleDateString(), date), new Map);

const uniqueTimestampsByDate = [...uniqueByDate(data).entries()];

const getNthByUniqueDate = (index) =>
  index > -1 && index < uniqueTimestampsByDate.length
    ? uniqueTimestampsByDate[index][1]
    : null;

for (let i = 0; i < uniqueTimestampsByDate.length; i++) {
  console.log(`Unique at index: ${i} = ${getNthByUniqueDate(i)}`);
}
    Login or Signup to reply.
Please signup or login to give your own answer.

Back To Top
Search