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  function binarySearch(arr, target) {
    let left = 0;
    let right = arr.length - 1;
  
    // empty arr, so we won't find anything
    if (!arr.length) {
      return -1;
    }
  
    // the target is less than the smallest number in arr
    if (target < arr[left]) {
      return -1;
    }
  
    // the target is greater then the smallest number in arr
    if (arr[right] < target) {
      return -1;
    }
  
    // These two `if` are needed for the case when there are only 2 elements
    // in the array.
    if (arr[left] === target) {
      return left;
    }
    if (arr[right] === target) {
      return right;
    }
  
    while (left + 1 !== right) {
      const middle = left + Math.floor((right - left) / 2);
      if (arr[middle] < target) {
        left = middle;
      } else {
        right = middle;
      }
    }
  
    if (arr[right] === target) {
      return right;
    }
    
    return -1;
  }