Deep flatten a JavaScript array using Array.prototype.reduce

javascript-deep-flatten.js

function flatten(array) {
  return array.reduce( (acc, e) => {
    if(Array.isArray(e)) {
      // if the element is an array, fall flatten on it again and then take the returned value and concat it.
      return acc.concat(flatten(e));
    } else {
      // otherwise just concat the value.
      return acc.concat(e);
    }
  }, [] ) // initial value for the accumulator is []
};

flatten([1,2,3, [4,5, [6,7,[9,8]]]])