Javascript - Remove oldest elements of a map (object)

Question

I have an object:

let lastPosts = {
  id1: {
    date: new Date("10/10/2000 11:30:34"),
  },
  id2: {
    date: new Date("10/10/2000 10:00:00"),
  },
  id3: {
    date: new Date("10/10/2000 10:01:30"),
  },
  id4: {
    date: new Date("11/10/2000 13:09:30"),
  },
  id5: {
    date: new Date("8/10/2005 18:10:15"),
  },
  id6: {
    date: new Date("11/10/2000 10:00:11"),
  },
  id7: {
    date: new Date("10/10/2000 10:00:01"),
  },
  id8: {
    date: new Date("15/10/2000 11:30:34"),
  },
  id9: {
    date: new Date("7/10/2002 11:30:34"),
  },
  id10: {
    date: new Date("8/10/2005 18:10:14"),
  },
  id11: {
    date: new Date("11/10/2000 10:00:00"),
  },
};

and I need to ensure that "lastPosts" only contains 9 posts (elements), deleting the remaining oldest posts.

function deleteOldestLastPosts(lastPosts) {
    if (Object.keys(lastPosts).length > 9) {
       // What can I do here?
    }
}

So, that I have the following result:

{
  id1: {
    date: new Date("10/10/2000 11:30:34"),
  },
  id3: {
    date: new Date("10/10/2000 10:01:30"),
  },
  id4: {
    date: new Date("11/10/2000 13:09:30"),
  },
  id5: {
    date: new Date("8/10/2005 18:10:15"),
  },
  id6: {
    date: new Date("11/10/2000 10:00:11"),
  },
  id8: {
    date: new Date("15/10/2000 11:30:34"),
  },
  id9: {
    date: new Date("7/10/2002 11:30:34"),
  },
  id10: {
    date: new Date("8/10/2005 18:10:14"),
  },
  id11: {
    date: new Date("11/10/2000 10:00:00"),
  },
}

Any ideas?

Sort `Object.values(lastPosts)` by date. Then find the property with the first date and delete it. — Barmar, Oct 06 '21 at 18:27
How big `lastPosts` could be? Do you care about optimisation and efficiency in finding k largest items? — Hossein Dehnokhalaji, Oct 06 '21 at 18:34
@HosseinDehnokhalaji Its max size should be 10, but might be little bigger (not really big, something small) — Raul, Oct 06 '21 at 18:41

score 1 · Answer 1 · answered Oct 06 '21 at 18:38

Use a priority queue. Javascript does not have one in-built, but you can find a package that provides it, or write one from scratch.

Use a min priority queue to store the id_n objects, using date field as the criteria. As soon as size of queue becomes > 9, remove the element at top of priority queue. Then, take that removed element's id_n and remove it from the current object (lastPosts).

At the end, you will have latest 9 elements in priority queue. And your object (lastPosts) will have latest 9 id_n objects.

The time complexity will be O(n log k), n is size of lastPosts and k is 9.

score 1 · Accepted Answer · answered Oct 06 '21 at 18:39

Sort all the values by date. Remove the newest 9 elements from this sorted array, producing an array of the dates to delete. Then loop through the properties, removing it if it's in the array.

function deleteOldestLastPosts(lastPosts) {
  let values = Object.values(lastPosts);
  if (values.length > 9) {
    let datesToDelete = values.map(el => el.date).sort((a, b) => a - b);
    datesToDelete.splice(-9);
    Object.entries(lastPosts).forEach(([key, {
      date
    }]) => {
      if (datesToDelete.includes(date)) {
        delete lastPosts[key];
      }
    });
  }
}

let lastPosts = {
  id1: {
    date: new Date("10/10/2000 11:30:34"),
  },
  id2: {
    date: new Date("10/10/2000 10:00:00"),
  },
  id3: {
    date: new Date("10/10/2000 10:01:30"),
  },
  id4: {
    date: new Date("11/10/2000 13:09:30"),
  },
  id5: {
    date: new Date("8/10/2005 18:10:15"),
  },
  id6: {
    date: new Date("11/10/2000 10:00:11"),
  },
  id7: {
    date: new Date("10/10/2000 10:00:01"),
  },
  id8: {
    date: new Date("15/10/2000 11:30:34"),
  },
  id9: {
    date: new Date("7/10/2002 11:30:34"),
  },
  id10: {
    date: new Date("8/10/2005 18:10:14"),
  },
  id11: {
    date: new Date("11/10/2000 10:00:00"),
  },
};

deleteOldestLastPosts(lastPosts);
console.log(lastPosts);

user2314737 · Answer 3 · 2021-10-06T19:51:16.857

Here's a function getlastPosts that will:

sort the json object by date descending
return the first n elements of the json object using filter by the index of entries

The function uses sort, filter, and reduce on the array Object.entries() of the JSON object.

let lastPosts = {
  id1: {
    date: new Date("10/10/2000 11:30:34"),
  },
  id2: {
    date: new Date("10/10/2000 10:00:00"),
  },
  id3: {
    date: new Date("10/10/2000 10:01:30"),
  },
  id4: {
    date: new Date("11/10/2000 13:09:30"),
  },
  id5: {
    date: new Date("8/10/2005 18:10:15"),
  },
  id6: {
    date: new Date("11/10/2000 10:00:11"),
  },
  id7: {
    date: new Date("10/10/2000 10:00:01"),
  },
  id8: {
    date: new Date("15/10/2000 11:30:34"),
  },
  id9: {
    date: new Date("7/10/2002 11:30:34"),
  },
  id10: {
    date: new Date("8/10/2005 18:10:14"),
  },
  id11: {
    date: new Date("11/10/2000 10:00:00"),
  },
};

function getlastPosts(posts, n) {
  return Object.entries(posts)
    .sort(([, a], [, b]) => b.date - a.date)
    .filter((e, i) => i < n)
    .reduce((r, [k, v]) => ({ ...r, [k]: v }), {})
}

// get first 3 elements
console.log(getlastPosts(lastPosts, 3));

See also Sorting object property by values

Javascript - Remove oldest elements of a map (object)

3 Answers3