the callback function supplied to Array.prototype.map() takes three arguments:

currentValue: The current element being processed in the array.
index (Optional): The index of the current element being processed in the array.
array (Optional): The array map was called upon.

the tests pass even if you leave the second two out (see below) but I think the official solution is more complete with them

module.exports = function map(arr, fn) {
  return arr.reduce(function(acc, item, index) {
    acc[index] = (fn(item))
    return acc
  }, [])
}