/*
  Let's set up some example arrays with arbitrarily
  nested arrays within
*/
const array1 = ['a', 'b', ['c', 'd']]
const array2 = ['a', ['b', 'c'], 'd']
const array3 = ['a', ['b', ['c', 'd']], 'e']

/*
  We'll use the prototype reduce() function.
  It takes a callback and an initial value.
  It will establish an accumulator with the initial value.
  It will then execute the callback on each item in the array.

  We will make a function that will take an array,
  use the reduce() method to run a callback that appends
  each value to the accumulator. If the value is another
  array, it will call itself recursively against that array.
  This will allow it to flatten an infinite number of nested
  arrays into a single flat array
*/
const flatten = (array) => array.reduce(
  (acc, val) => acc.concat(
    Array.isArray(val) ? flatten(val) : val
  ), []
)

/*
  Let's test it out on our arrays
*/
const flatArray1 = flatten(array1);
const flatArray2 = flatten(array2);
const flatArray3 = flatten(array3);

/*
  And log our results to the console
*/
console.log(flatArray1);  // logs '["a", "b", "c", "d"]'
console.log(flatArray2);  // logs '["a", "b", "c", "d"]'
console.log(flatArray3);  // logs '["a", "b", "c", "d", "e"]'