flatten.js

function flatten (array) {
  // check that input is an array
  if (array.constructor === Array) {
    return array.reduce((accum, currentValue) => {
        if (array.constructor === Array) {
          // flatten the array using es6 reducer
          return accum.concat(currentValue.reduce(flatten, []))
        } else {
          // append/concat
          return accum.concat(currentValue)
        }
      }
      , []
    )
  }
}