reduce_basics.js

arr.reduce((accumulator, currentValue, currentIndex, sourceArray) => {
  // accumulator: the current running "total"
  // currentValue: the current value of the arr (we are looping through them, one by one)
  // currentIndex: where you are in the loop (0 based, as usual)
  // sourceArray: access to the unmodified original arr array

  // Return a function here leveraging the accumulator or currentValue
}, initialValue);