Array.find on Array<{...}> raises

[benlangfeld]

Everything below was tested with flow-bin 0.41.0, where the following declaration is in place:

declare class Array<T> extends $ReadOnlyArray<T> {
  ...
  find(callbackfn: (value: T, index: number, array: Array<T>) => any, thisArg?: any): T | void;
  ...
}

/* @flow */

type Foo = {|id: number|}
const foo: Array<Foo> = [{id: 1}, {id: 3}]
const bar: Foo = foo.find((val: Foo) => val.id > 1)

test.js:5
5: const bar: Foo = foo.find((val: Foo) => val.id > 3)
                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ undefined. Inexact type is incompatible with exact type
5: const bar: Foo = foo.find((val: Foo) => val.id > 3)
              ^^^ exact type: object type

In order to get this past Flow, I have to const bar: any. Curiously, this works:

const bar: Array<Foo> = foo.filter((val: Foo) => val.id > 3)
const baz: Foo = foo.pop()

If I use an inexact type, I get a more generic error:

/* @flow */

type Foo = {id: number}
const foo: Array<Foo> = [{id: 1}, {id: 3}]
const bar: Foo = foo.find((val: Foo) => val.id > 1)

test.js:117
5: const bar: Foo = foo.find((val: Foo) => val.id > 1)
                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ undefined. This type is incompatible with
5: const bar: Foo = foo.find((val: Foo) => val.id > 1)
              ^^^ object type

[nmn]

The error might be slightly misleading but this is a genuine type-error:
The return type of find is an optional.

Use this in your examples:

const bar: void | Foo = foo.find((val: Foo) => val.id > 1)

[benlangfeld]

@nmn Indeed that does resolve it. Thank you very much for the answer and for being lightning fast.

Now you point it out, it is indeed blindingly obvious. I was expecting to see : ?T and my brain ignored the | void.