options-types.js

// @flow
type Options = {
  getId: (passedProps: {}) => string | string[]
}

const Entities = (options: Options) => /* ...code ... */

Entities({ getId: ({ contactId }) => Immutable.List([contactId]) })
// => throws a compiler error, return type of getId must be string | string[]
Entities({ getId: ({ contactId }) => contactId && [contactId] })
// => throws a compiler error, contactId might be undefined, since it is not typed
Entities({ getId: ({ contactId }) => [contactId] })
// => all good, compiler compiles