Binary Search

Works only on sorted array

function binarySearch(arr, elem) {
    let start = 0
    let end = arr.length - 1
    let middle = Math.floor((start + end) / 2)
    while (arr[middle] !== elem && start <= end) {
        if (elem < arr[middle]) end = middle - 1
        else start = middle + 1
        middle = Math.floor((start + end) / 2)
    }
    return arr[middle] === elem ? middle : -1;
}

console.log(binarySearch([2,5,6,9,13,15,28], 28))