# Binary search

JavaScript, Algorithm, Array · Dec 29, 2020

Finds the index of a given element in a sorted array using the binary search algorithm.

- Declare the left and right search boundaries,
`l`

and`r`

, initialized to`0`

and the`length`

of the array respectively. - Use a
`while`

loop to repeatedly narrow down the search subarray, using`Math.floor()`

to cut it in half. - Return the index of the element if found, otherwise return
`-1`

. **Note:**Does not account for duplicate values in the array.

```
const binarySearch = (arr, item) => {
let l = 0,
r = arr.length - 1;
while (l <= r) {
const mid = Math.floor((l + r) / 2);
const guess = arr[mid];
if (guess === item) return mid;
if (guess > item) r = mid - 1;
else l = mid + 1;
}
return -1;
};
```

```
binarySearch([1, 2, 3, 4, 5], 1); // 0
binarySearch([1, 2, 3, 4, 5], 5); // 4
binarySearch([1, 2, 3, 4, 5], 6); // -1
```