Binary search in ES6

binarySearch.js

/**
 * Binary search
 * @param {array} list - Array of numbers for search
 * @param {number} elem - Looking item
 * @returns {number|null} - Found index or null if not found
 */
const binarySearch = (list, elem) => {
  let startIndex = 0;
  let lastIndex = list.length - 1;

  while (startIndex <= lastIndex) {
    let index = Math.floor((startIndex + lastIndex) / 2);

    if (list[index] === elem) {
      return index;
    }

    if (list[index] < elem) {
      startIndex = index + 1;
    } else {
      lastIndex = index - 1;
    }
  }

  return null;
};