binarySearch

function binarySearch(ar, elem) {
    let start = 0;
    let end = ar.length -1;
    let mid = Math.floor((start + end) / 2);

    while(ar[mid] !== elem && start < end ) {
        if (elem < ar[mid]) end = mid - 1;
        else start = mid + 1;
        mid = Math.floor((start + end) / 2);
        
    }
    return ar[mid] === elem ? mid : -1;
    
}

binarySearch([1,2,5,6,7,9,10], 2);