const binarySearch = (list, item) => {
  let startIndex = 0;
  let endIndex = list.length - 1;

  while (startIndex <= endIndex) {
    let midIndex = Math.floor((startIndex + endIndex) / 2);
    const candidate = list[midIndex];

    if (candidate === item) {
      console.log(`Item ${item} found at index ${midIndex}`);
      return midIndex;
    }

    if (candidate > item) {
      endIndex = midIndex - 1;
    } else {
      startIndex = midIndex + 1;
    }
  };

  console.log(`Not found "${item}"`);
  return null;
};

const list = [10, 20, 30, 50, 80, 100, 500];
binarySearch(list, 20); // 1
binarySearch(list, 40); // null