javascript-algorithms/src/algorithms/search/binary-search/binarySearch.js

import Comparator from '../../../utils/comparator/Comparator';

/**
 * Binary search implementation.
 *
 * @param {*[]} sortedArray
 * @param {*} seekElement
 * @param {function(a, b)} [comparatorCallback]
 * @return {number}
 */

export default function binarySearch(sortedArray, seekElement, comparatorCallback) {
  // Let's create comparator from the comparatorCallback function.
  // Comparator object will give us common comparison methods like equal() and lessThen().
  const comparator = new Comparator(comparatorCallback);

  // These two indices will contain current array (sub-array) boundaries.
  let startIndex = 0;
  let endIndex = sortedArray.length - 1;

  // Let's continue to split array until boundaries are collapsed
  // and there is nothing to split anymore.
  while (startIndex <= endIndex) {
    // Let's calculate the index of the middle element.
    const middleIndex = startIndex + Math.floor((endIndex - startIndex) / 2);

    // If we've found the element just return its position.
    if (comparator.equal(sortedArray[middleIndex], seekElement)) {
      return middleIndex;
    }

    // Decide which half to choose for seeking next: left or right one.
    if (comparator.lessThan(sortedArray[middleIndex], seekElement)) {
      // Go to the right half of the array.
      startIndex = middleIndex + 1;
    } else {
      // Go to the left half of the array.
      endIndex = middleIndex - 1;
    }
  }

  // Return -1 if we have not found anything.
  return -1;
}