javascript 38 lines · 8 steps

Merging overlapping date ranges

Sort intervals by start date, then sweep once to fold overlapping or adjacent ranges into a compact list.

Explained by highlit

Intermediate intervals sorting dates greedy merging

1function mergeDateRanges(ranges) {
2  const parsed = ranges
3    .map(({ start, end }) => ({
4      start: new Date(start),
5      end: new Date(end),
6    }))
7    .filter(({ start, end }) => start <= end)
8    .sort((a, b) => a.start - b.start);
9 
10  const merged = [];
11 
12  for (const range of parsed) {
13    const last = merged[merged.length - 1];
14 
15    if (last && range.start <= addDay(last.end)) {
16      if (range.end > last.end) {
17        last.end = range.end;
18      }
19    } else {
20      merged.push({ ...range });
21    }
22  }
23 
24  return merged.map(({ start, end }) => ({
25    start: toISODate(start),
26    end: toISODate(end),
27  }));
28}
29 
30function addDay(date) {
31  const next = new Date(date);
32  next.setDate(next.getDate() + 1);
33  return next;
34}
35 
36function toISODate(date) {
37  return date.toISOString().slice(0, 10);
38}

01 / 01

STEP 01

Walkthrough

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Three takeaways

1Sorting intervals by start turns merging into a single left-to-right pass.
2You only ever need to compare each range against the most recent merged one.
3Normalizing input early (parsing, filtering) keeps the core algorithm clean and correct.

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