[flow]Multiple type check

Question

I use flowType to write express router code, and let flow to check the req.body data, body data have multiple type, such as UserData(bbb type) or LoginData(ccc type). But when the body transform data to function, flow always says something wrong. Is there any good idea to support body have multiple type check? Here is a demo wrong code.

 /* @flow */ declare type ccc = { ccc: string; } declare type bbb = { bbb: string; } declare type aaa = { body: bbb | ccc } function test(aaa: aaa) { const body = aaa.body test2(body) } function test2(body: ccc): string { return body['ccc'] } 

Answer 1

The error

Here's the error you get when running the supplied code with flow :

 34:   test2(body)
       ^^^^^^^^^^^ function call
 37: function test2(body: ccc): string {
                          ^^^ property `ccc`. Property not found in
 34:   test2(body)
             ^^^^ object type

The solution

The main issue here is test2 . aaa can have a body value of either the bbb or the ccc type, which means that when you call

const body = aaa.body

  test2(body)

body can either be a match for type bbb or for type ccc , we don't know. Meanwhile, test2 explicitly only accepts an argument of type ccc . Flow is seeing the possibility for bbb type to be passed, which will of course, not have the ccc property. At least one solution is to make the following change:

function test2(body: bbb | ccc): string {
  if (typeof body.ccc === 'string') {
    return body.ccc
  }

  return ''
}

We need bbb | ccc bbb | ccc because it can in fact be either option. Next, we need the typeof check to ensure that we don't return undefined in the case where we passed in a bbb type. This removes the error.