AI solution for JavaScript Coding Question on BFE.dev
50. search element right before target with Binary Search(possible duplicate array)

Solution:

We will use binary search to find the element right before first appearance of target number.

Here is the JavaScript code for the solution.

/**
 * @param {number[]} nums - sorted ascending array with duplicate numbers
 * @param {number} target
 * @return {number|undefined}
 */
function binarySearchLastBefore(nums, target) {
  
  let left = 0;
  let right = nums.length - 1;
  let index = -1;

  while (left <= right) {
  
    // middle index
    const mid = Math.floor((left + right) / 2);
    
    if (nums[mid] === target) {
      // if mid index is target and mid index is not first element of array AND
      // previous element to mid index is not target then return nums[mid-1]
      if (mid > 0 && nums[mid-1] !== target) {
        index = mid-1;
        break;
      } else {
        // Otherwise try for the left half
        right = mid-1;
      }
    } else if (nums[mid] < target) {
      // Try for right half
      left = mid+1;
    } else {
      // Try for left half
      right = mid-1;
    }
  }

  if (index !== -1) {
    return nums[index];
  }

  return undefined;
}