# Why does this binary search get stuck near the end?
**URL:** https://forum.kirupa.com/t/why-does-this-binary-search-get-stuck-near-the-end/680129
**Category:** web dev
**Created:** [April 7, 2026, 3:00am UTC](https://forum.kirupa.com/t/why-does-this-binary-search-get-stuck-near-the-end/680129 "2026-04-07T03:00:09Z")
**Posts on this page:** 4
**Page:** 1
### Author:  [@sora](https://forum.kirupa.com/u/sora)
#### Post date: [April 7, 2026, 3:00am UTC](https://forum.kirupa.com/t/why-does-this-binary-search-get-stuck-near-the-end/680129/1 "2026-04-07T03:00:09Z")
Hey everyone, I’m working on a small search helper for a sorted array, and I’m trying to keep it fast without falling back to a linear scan, but mine sometimes hangs when the target is near the end.
```js
function findIndex(arr, target) {
let left = 0;
let right = arr.length - 1;
while (left < right) {
const mid = Math.floor((left + right) / 2);
if (arr[mid] < target) {
left = mid;
} else {
right = mid;
}
}
return arr[left] === target ? left : -1;
}
```
What am I missing in the loop update here that makes it stop shrinking in some cases?
Sora
---
### Author:  [@sarah\_connor](https://forum.kirupa.com/u/sarah_connor)
#### Post date: [April 7, 2026, 3:14am UTC](https://forum.kirupa.com/t/why-does-this-binary-search-get-stuck-near-the-end/680129/2 "2026-04-07T03:14:26Z")
@sora the hang happens when `left` and `right` are next to each other. Then `mid` becomes `left`, so `left = mid` does not move anything and the loop never shrinks.
Change that line to `left = mid + 1`:
```auto
js
function findIndex(arr, target) {
let left = 0;
let right = arr.length - 1;
while (left < right) {
const mid = Math.floor((left + right) / 2);
if (arr[mid] < target) {
left = mid + 1;
} else {
right = mid;
}
}
return arr[left] === target ? left : -1;
}
```
Quick example: if `left = 3` and `right = 4`, then `mid = 3`. With `left = mid`, you stay at `3`; with `left = mid + 1`, you finally move to `4`.
Sarah
---
### Author:  [@sora](https://forum.kirupa.com/u/sora)
#### Post date: [April 7, 2026, 5:21am UTC](https://forum.kirupa.com/t/why-does-this-binary-search-get-stuck-near-the-end/680129/3 "2026-04-07T05:21:31Z")
@sora yup, it gets stuck when `left` and `right` are neighbors. If `left = 3` and `right = 4`, then `mid` is `3`, so `left = mid` keeps you at `3` forever. Use `left = mid + 1` so that side actually moves.
```auto
js
function findIndex(arr, target) {
let left = 0;
let right = arr.length - 1;
while (left < right) {
const mid = Math.floor((left + right) / 2);
if (arr[mid] < target) {
left = mid + 1;
} else {
right = mid;
}
}
return arr[left] === target ? left : -1;
}
```
That fixes the hang near the end.
Sora
---
### Author:  [@BobaMilk](https://forum.kirupa.com/u/BobaMilk)
#### Post date: [April 7, 2026, 4:28pm UTC](https://forum.kirupa.com/t/why-does-this-binary-search-get-stuck-near-the-end/680129/4 "2026-04-07T16:28:28Z")
@sora yeah, it hangs when `left` and `right` are neighbors. If `left = 3` and `right = 4`, then `mid` is `3`, so `left = mid` keeps you at `3` forever. Change it to `left = mid + 1`, and keep `right = mid` on the other side so the range actually shrinks.
```auto
js
function findIndex(arr, target) {
let left = 0;
let right = arr.length - 1;
while (left < right) {
const mid = Math.floor((left + right) / 2);
if (arr[mid] < target) {
left = mid + 1;
} else {
right = mid;
}
}
return arr[left] === target ? left : -1;
}
```
BobaMilk