FirstBadVersion.js

function firstBadVersion(arr){
  let found = undefined, start = 0, end = arr.length;
  
  while (start < end && !found){
    let mid = Math.floor((start + end)/2);
    
    if (isBad(arr[mid])){
      if (arr[mid - 1] && isBad(arr[mid - 1])){
        end = mid - 1;
      }
      else{
        found = mid;
      }
    }
    else{
      start = mid + 1;
    }
  }
  
  return found;
}

function isBad(version){
  return version === 'B';
}