/*
Flattens an array, e.g. [[1,2,[3]],4] -> [1,2,3,4]

Using Array.prototype.reduce() as a functional alternative to a for loop and
relying on recursion to handle nesting.

For those unfamiliar, the first argument to reduce() is a function to run for
every item of the array and the second argument is the initial value that we
want to add to, which will eventually be our result. In this case we start with
an empty array and add each nested item to it.
*/

const flatten = nestedArray =>
  nestedArray.reduce((flatArray, currentValue) => {
    if (Array.isArray(currentValue))
      flatArray.push(...flatten(currentValue));
    else
      flatArray.push(currentValue);
    return flatArray;
  },
  []);