Promise sequence fallback (first non-empty) – with Flow

fallback-promise.js

// @flow
type Task<T> = (...args: any[]) => Promise<?T>

type Options<T> = {
  args?: any[],
  initial?: T,
  isEmpty?: (value: ?T) => boolean
}

export default <T> (
  tasks: Task<T>[],
  options: Options<T> = {}
): Promise<?T> => {
  const {
    args = [],
    initial,
    isEmpty = (x) => !x
  } = options

  return tasks.reduce(
    (prev, next) => prev.then((value) =>
      isEmpty(value) ? next(...args) : value
    ),
    Promise.resolve(initial)
  )
}