How to use (or simulate) object spread with overriding fields in Flow?

Question

This doesn't compile on the "Try Flow":

/* @flow */

type A = { a: number, b: string};
type B = { a: string, b: string };

const x: A = { a:1, b:'2' };
const y: B = { ...x, a: x.a.toString() }

Error is:

const y: B = { ...x, a: x.a.toString() }
                ^ Cannot assign object literal to `y` because number [1] is incompatible with string [2] in property `a`.

    References:

    3: type A = { a: number, b: string};
                     ^ [1]

    4: type B = { a: string, b: string };
                     ^ [2]

Note that this code works in TypeScript (when I remove field override it fails to compile as it is supposed to).

How to achieve same behavior in Flow without enumerating all fields of original object?

Answer 1

Some major improvements to spreads are coming out in Flow v0.111.0, scheduled for release next week. This snippet will now typecheck. You can see it in action now in flow.org/try by switching to the "master" version.

Answer 2

Short answer: you can't do this, and it's a "known" bug in Flow. It is "known" but I didn't see any indication that someone is actually working on it.

You can:

• declare B.a to be a union of number | string.

• declare a mapping function like this:

  const mapfn = ({ a, ...rest }: A): B => ({ ...rest, a: a.toString() });
  
  const x: A = { a: 1, b: '2' };
  const y: B = mapfn(x);
  

EDIT: it seems you can now do this with the newest version of Flow. See the above issue for details, they've fixed this bug. Upgrade your Flow!