recursive, call outer vs inner function

flatten.js

var array = [1, 2, [3, 4, [5, 6, [7, 8, [9]]]]]

// call outer function
function flatten(array) {
  return array.reduce((prev, cur)=> {
    return Array.isArray(cur)
      ? [...prev, ...flatten(cur)]
      : [...prev, cur]
    }, [])
}

// call inner function
function flatten(array) {
  return array.reduce(function inner(prev, cur) {
    return Array.isArray(cur)
      ? [...cur.reduce(inner, prev)]
      : [...prev, cur]
  }, [])
}

flatten(array)