Given a sorted array arr[] and a number x, write a function that counts the occurrences of x in arr[]. Expected time complexity is O(Logn).

findSortedArrayOcurrence

//1,2,3,4,4,4,4,4,5,6,7

function findFrequency(array, target){
    var first, last;
    var result = -1;
    
    first = findFirst(array, target, 0, array.length-1);
    
    if (first !== -1){
       last = findLast(array, target, 0, array.length-1);
       result = last - first + 1;
    }
    
    return result;
}

function findFirst(array, target, startIndex, lastIndex){
    
    // base case
    if (startIndex ===lastIndex){
        if (target === array[startIndex]){
            return startIndex;
        } else{
            return -1;
        }
    }
    
    var mediumIndex = startIndex + Math.floor( (lastIndex - startIndex ) / 2 );
    
    if (target <= array[mediumIndex]){    
        return findFirst(array, target, startIndex, mediumIndex);
    } else {
        return findFirst(array, target, mediumIndex + 1, lastIndex);
    }
    
}

function findLast(array, target, startIndex, lastIndex){
    
    // base case
    if (startIndex ===lastIndex){
        if (target === array[startIndex]){
            return startIndex;
        } else{
            return -1;
        }
    }
    
    var mediumIndex = startIndex + Math.floor( (lastIndex - startIndex ) / 2 );
    
    if (target >= array[mediumIndex + 1]){    
        return findLast(array, target, mediumIndex + 1, lastIndex);
    } else {
        return findLast(array, target, startIndex, mediumIndex);
    }
    
}

console.log( findFrequency([1,2,3,4,4,4,4,5,5,6,7], 5) );