A binary search function on a sorted array of strings which is interspersed with empty strings.

findLocInInterspersedSortedArr

var findLocInInterspersedSortedArr = function(strArr, target, first, last){
    var mid = Math.floor( last - first / 2);
    
    if (strArr[mid] === ''){
        var left = mid - 1;        
        var right = mid + 1;
       
        while (true){
            if (left < first && right > last){
                return -1;
            } else if  (right <= last && strArr[right] !== ''){
                mid = right;
                break;
            } else if (left >= first && strArr[left] !== ''){
                mid = left;
                break;
            } else {
                left--;
                right++;
            }
            
        }
    }
    
    if (strArr[mid] === target){
        return mid;
    } else if (target < strArr[mid]) {
        return findLocInInterspersedSortedArr(strArr, target, first, mid-1);
    } else {
        return findLocInInterspersedSortedArr(strArr, target, mid+1, last);
    }
}

// var strArr = ["at", "", "", "", "ball", "", "", "car", "", "", "dad", "", ""];
// var result = findLocInInterspersedSortedArr(strArr, "dad", 0, strArr.length-1);
// console.log(result);