How can i find the oldest date in an array of Date objects in JavaScript?

Question

I have an array of strings corresponding to Date objects. like this:

const timestamps =  [
  '2023-03-15T10:47:38.878Z',
  '2023-03-15T10:46:51.375Z',
  '2023-03-15T10:46:39.645Z',
  '2023-03-15T10:47:19.072Z',
  '2023-03-15T10:46:20.395Z'
]

var convertedTimestamps = [];
for (var time in timestamps) {
  var timeAsDate = Date.parse(time);
  console.log(time, timeAsDate);
  convertedTimestamps.push(timeAsDate);
}
const min = Math.min(...convertedTimestamps);

They are Date objects parsed into strings for other purposes This can be any length really, this is just one instance of it. I want to find the oldest of the timestamps.

In the example above, these are the converted timestamps, and the min value is correct based on the numbers, but not the date objects. The top one is the one that is removed

946681200000
978303600000
980982000000
983401200000
986076000000
min: 946681200000
'2023-03-15T10:47:38.878Z'

It might just be a math problem, but the way i understand it, a string parsed like this returns an int representing how much time has passed since 1970 etc, and so the oldest should be closest to that time should it not? this min value is in fact the most recent of the timestamps. When i try the max value it is not correct either.

Does this answer your question? Loop (for each) over an array in JavaScript — derpirscher
– derpirscher, Commented Mar 15, 2023 at 11:43
I converted your script into a runable example. Have a look what values time actually has and what is parsed to a date. Use a for .. of loop instead of a for .. in. Otherwise your code is fine ... — derpirscher
– derpirscher, Commented Mar 15, 2023 at 11:44

boppy · Accepted Answer · 2023-03-15 11:19:33Z

0

If all dates are following the format mentioned, it's enough to string-sort that array and get the first entry, since the string is "sorted" from longest time-unit to shortest.

So to get the oldest entry:

const timestamps =  [
  '2023-03-15T10:47:38.878Z',
  '2023-03-15T10:46:51.375Z',
  '2023-03-15T10:46:39.645Z',
  '2023-03-15T10:47:19.072Z',
  '2023-03-15T10:46:20.395Z'
];
const lowest = timestamps.sort()[0];

answered Mar 15, 2023 at 11:19

boppy

1,93813 silver badges12 bronze badges

Sign up to request clarification or add additional context in comments.

4 Comments

InSync Over a year ago

To readers: Note that .sort's time complexity is O(n log n) while that of a simple loop is O(n).

derpirscher Over a year ago

Sorting to find the maximum/minimum is turing an O(n) task into O(n logn). May not be relevant for small datasets, but still not best practice ...

boppy Over a year ago

Question is, if that results in longer runtime, since I think .sort might be faster (better optimized) than a self-implemented sort as mentioned. And will surely be faster than parsing each entry on its own to a Date-object...

derpirscher Over a year ago

There is no "sorting" necessary. Just iterate over the collection once and remember the current smallest element. This surely is faster than any (even if builtin and optimized) sorting algorithm ...

41 72 6c · Accepted Answer · 2023-03-15 11:56:38Z

0

Think you can solve this problem by adapting the loop in your case. The loop should look like this:

for (var i = 0; i < timestamps.length; i++) {
    var timeAsDate = Date.parse(timestamps[i]); # just adapted this to the loop
    ...
}

This should do the work for your case since the the loop before iterates over the keys of the array rather than the elements themselves which leads to a incorrect Date parsing because it tries to parse the index instead of the timestamp in that case.

edited Mar 15, 2023 at 11:56

answered Mar 15, 2023 at 11:14

41 72 6c

1,7786 gold badges25 silver badges37 bronze badges

3 Comments

derpirscher Over a year ago

While that answer solves the problem, you should also state why using a for .. in loop is wrong here ... (or just close it as a duplicate)

derpirscher Over a year ago

But your explanation is wrong. A for .. in loop does not iterate over the elements of an array (if it was, OP's code would work), but over the keys of an object, ie the time in OP's code gets the values "0", "1","2","3","4" which is obviously wrong

41 72 6c Over a year ago

mistaken that. what is meant with indexes is, that the loop is iterating over an objects property, which in the case of an array would be the indexes (key) and not the elements.

0stone0 · Accepted Answer · 2023-03-15 12:05:47Z

0

Since you're dealing with ISO 8601 timestamps, you can compare any string against it

Set lowest to a, an high value string
Loop over timestamps
If the current on is lower, overwrite
Log

const timestamps = [
  '2023-03-15T10:47:38.878Z',
  '2023-03-15T10:46:51.375Z',
  '2023-03-15T10:46:39.645Z',
  '2023-03-15T10:47:19.072Z',
  '2023-03-15T10:46:20.395Z'
];
let lowest = 'a';

for (let timestamp of timestamps) {
    if (lowest > timestamp) {
        lowest = timestamp;
    }
}

console.log('Lowest timestamp: ' + lowest)
// Lowest timestamp: 2023-03-15T10:46:20.395Z

Thanks to @Derpirscher for commenting this.

Useful reads:

Is direct comparison of Javascript ISO Date Strings safe?

edited Mar 15, 2023 at 12:05

answered Mar 15, 2023 at 11:16

0stone0

45.5k6 gold badges53 silver badges82 bronze badges

2 Comments

derpirscher Over a year ago

Actually, with ISO-Timestamps there is no need to parse. you can just compare them by string comparison. And it would also be more efficient to initialize the lower with a value like Number.MAX_SAFE_INTEGER, you you won't need to check for 0 in every iteration.

derpirscher Over a year ago

Sorry if my comment was unclear, but with your usage of MAX_SAFE_INTEGER.ToString() it's just pure coincidence that this code works, because MAX_SAFE_INTEGER is 9007199254740991 ... What if it was 19007199254740991? I meant if you compare by numeric value (ie with parsing) initialize it with MAX_SAFE_INTEGER. If you compare as string, just use let lower = '9' or to be completely sure (if you have to deal with year 9999) let lower = 'a'

rjax · Accepted Answer · 2023-03-15 12:23:47Z

0

for..in is used to iterate over the properties of an object, not an array. If you add console.log(time) in your loop you will see that time is a string representing each index of the array. If you change to for..of your code will work.

const timestamps = [
  '2023-03-15T10:47:38.878Z',
  '2023-03-15T10:46:51.375Z',
  '2023-03-15T10:46:39.645Z',
  '2023-03-15T10:47:19.072Z',
  '2023-03-15T10:46:20.395Z'
]

for (var time in timestamps) {
  console.log(time);
}

for (var time of timestamps) {
  console.log(time);
}

Also, unless you need the convert timestamps for something else, you might want to simply get the minimum in your loop.

const timestamps = [
  '2023-03-15T10:47:38.878Z',
  '2023-03-15T10:46:51.375Z',
  '2023-03-15T10:46:39.645Z',
  '2023-03-15T10:47:19.072Z',
  '2023-03-15T10:46:20.395Z'
];

let dateTime;
let minDateTime = Date.parse(timestamps[0]);
for (var time of timestamps) {
  dateTime = Date.parse(time);
  if (dateTime < minDateTime)
    minDateTime = dateTime;
}

console.log((new Date(minDateTime)).toISOString());

answered Mar 15, 2023 at 12:23

rjax

495 bronze badges

1 Comment

derpirscher Over a year ago

You have to be a bit careful with your initialization of let minDateTime = Date.parse(timestamps[0]); Your code will throw an error if the timestamps array is empty ...

Collectives™ on Stack Overflow

How can i find the oldest date in an array of Date objects in JavaScript?

4 Answers 4

4 Comments

3 Comments

2 Comments

1 Comment

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

4 Answers 4

4 Comments

3 Comments

2 Comments

1 Comment

Your Answer

Sign up or log in

Post as a guest

Linked

Related