const binarySearch = (arr, x) => {
  
  // Initialise left and right bounds of search to start and end of arr
  let left_index = 0;
  let right_index = arr.length - 1;
  
  // While left_index and right_index have yet to overlap
  while (left_index <= right_index) {
    
  // Find the midpoint between left_index and right_index
    let mid_index = Math.floor((left_index + right_index) / 2);

    console.log("middle index", mid_index);
    
   // If the element at mid_index is x, return mid_index
    // Use Math.floor to ensure that the index position is an integer not a decimal
    if (arr[mid_index] === x) {
      return mid_index;
    }
    
    // Otherwise, if x is greater than elem at mid_index,
    // update left_index to be 1 above mid_index
    else if (arr[mid_index] < x) {
      left_index = mid_index + 1;
    }
    
 // Otherwise, if x is less than elem at mid_index,
    // update right_index to be 1 below mid_index
    else {
      right_index = mid_index - 1;
    }
  }
  // If x is not found, return -1
  return -1;
};

const myList = [2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12];
const result = binarySearch(myList, 6); // 4
console.log(result);