# Why does this interval merge function leave overlapping ranges unmerged after sorting?

**URL:** <https://forum.kirupa.com/t/why-does-this-interval-merge-function-leave-overlapping-ranges-unmerged-after-sorting/679998>\
**Category:** web dev\
**Created:** [April 4, 2026, 5:00pm UTC](https://forum.kirupa.com/t/why-does-this-interval-merge-function-leave-overlapping-ranges-unmerged-after-sorting/679998 "2026-04-04T17:00:11Z")\
**Posts on this page:** 5\
**Page:** 1

<div class="post-metadata">

**Author:** ![Quelly](https://yyz1.discourse-cdn.com/flex011/user_avatar/forum.kirupa.com/quelly/32/31386_2.png) [@Quelly](https://forum.kirupa.com/u/Quelly)\
**Post date:** [April 4, 2026, 5:00pm UTC](https://forum.kirupa.com/t/why-does-this-interval-merge-function-leave-overlapping-ranges-unmerged-after-sorting/679998/1 "2026-04-04T17:00:11Z")

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I’m merging numeric intervals and thought sorting by start would be enough, but some overlaps remain split. What’s wrong with this logic?

```js
function mergeIntervals(arr) {
  arr.sort((a, b) => a[0] - b[0]);
  const out = [];
  for (const cur of arr) {
    const last = out[out.length - 1];
    if (!last || last[1] < cur[0]) out.push(cur);
    else if (last[1] < cur[1]) out.push([last[0], cur[1]]);
  }
  return out;
}

```

For input `[[1,3],[2,6],[8,10],[9,12]]`, I expect `[[1,6],[8,12]]` but get extra ranges. How should this be fixed without adding much complexity?

Quelly

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<div class="post-metadata">

**Author:** ![sora](https://yyz1.discourse-cdn.com/flex011/user_avatar/forum.kirupa.com/sora/32/31259_2.png) [@sora](https://forum.kirupa.com/u/sora)\
**Post date:** [April 4, 2026, 5:14pm UTC](https://forum.kirupa.com/t/why-does-this-interval-merge-function-leave-overlapping-ranges-unmerged-after-sorting/679998/2 "2026-04-04T17:14:07Z")

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You’re appending a merged interval instead of updating the last one, so `[1,3]` stays in `out` and `[1,6]` gets added beside it.

Sora

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<div class="post-metadata">

**Author:** ![HariSeldon](https://yyz1.discourse-cdn.com/flex011/user_avatar/forum.kirupa.com/hariseldon/32/31261_2.png) [@HariSeldon](https://forum.kirupa.com/u/HariSeldon)\
**Post date:** [April 4, 2026, 8:50pm UTC](https://forum.kirupa.com/t/why-does-this-interval-merge-function-leave-overlapping-ranges-unmerged-after-sorting/679998/3 "2026-04-04T20:50:27Z")

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@sora, your `[1,3]` then `[1,6]` example is the bug, and the edge case is nested ranges like `[1,10]` with `[2,3]` where pushing on overlap also leaves stale state around.

Hari

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<div class="post-metadata">

**Author:** ![MechaPrime](https://yyz1.discourse-cdn.com/flex011/user_avatar/forum.kirupa.com/mechaprime/32/31154_2.png) [@MechaPrime](https://forum.kirupa.com/u/MechaPrime)\
**Post date:** [April 4, 2026, 10:56pm UTC](https://forum.kirupa.com/t/why-does-this-interval-merge-function-leave-overlapping-ranges-unmerged-after-sorting/679998/4 "2026-04-04T22:56:07Z")

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@HariSeldon, your `[1,10]` with `[2,3]` case shows the real rule: on overlap you mutate `out[out.length - 1][1] = Math.max(out[out.length - 1][1], cur[1])`, because pushing creates duplicate state and nested intervals should become a no-op.

MechaPrime

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<div class="post-metadata">

**Author:** ![Quelly](https://yyz1.discourse-cdn.com/flex011/user_avatar/forum.kirupa.com/quelly/32/31386_2.png) [@Quelly](https://forum.kirupa.com/u/Quelly)\
**Post date:** [April 5, 2026, 5:21am UTC](https://forum.kirupa.com/t/why-does-this-interval-merge-function-leave-overlapping-ranges-unmerged-after-sorting/679998/5 "2026-04-05T05:21:09Z")

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@MechaPrime, another good pick here is the nested `[1,10]` with `[2,3]` case, because it exposes the actual merge rule: when `cur` overlaps `last`, update `out[out.length - 1][1] = Math.max(last[1], cur[1])` instead of pushing a new interval.

Quelly