Flow: Union types & Default params

[boopathi]

Search terms: destructuring, default arguments, default parameters, union

eg:

declare type Options = {
    a: null | Object,
    b: bool | Object,
}

function fn ({
    a = null, // errors here
    b = {} // errors here
} :Options = {}) {
    console.log(a, b);
}

let x :Options = {
    a: null, // No error
    b: {} // No error
};

It's strange that it happens only for default value assignment in object destructuring - fn and not an object definition - x.

This does NOT happen in Flow 0.28. But flow 0.29 throws errors.

[echenley]

@samwgoldman Any update on this? Similar issue in Flow v0.41.0:

// BlockTitle.js
// ...
type Props = {
  children?: React$Element<any>,
  className?: string,
  headerClassName?: string,
  size?: number | string,
  theme: Theme,
}

const BlockTitle = ({
  children,
  className = '',
  headerClassName = '',
  size = 3,
  theme,
}: Props) => {
// ...

And the error:

src/components/common/BlockTitle.js:18
 18:   size = 3,
       ^^^^ number. This type is incompatible with
 18:   size = 3,
       ^^^^ string

src/components/common/BlockTitle.js:18
 18:   size = 3,
       ^^^^ string. This type is incompatible with
 18:   size = 3,
       ^^^^ number

[arahansen]

I think I'm seeing this same issue. Is there any progress being made on this? or any solid workarounds?

Here's my example I'm running into: tryflow example

[trsh]

Same here, also with state.

[FireyFly]

I'm running into this issue as well, to me it looks like an inconsistency in how Flow handles union types and destructuring with default values:

type Obj = { foo?: string | number };

// Error: "Cannot assign `{...}.foo` to `foo` because string [1] is incompatible with number [2]."
const { foo = 1 }: Obj = {};

// Works!
const obj: Obj = {};
const { foo: bar = 1 } = obj;

// Works!
const baz: $PropertyType<Obj, 'foo'> = 1;

(flow.org/try)

The first example exhibits the problem in question. The second problem is what I feel the first one ought to be equivalent to (unless I'm misunderstanding how the type syntax applies to the left-hand side of the assignment). The third example demonstrates that it isn't a problem with assignments to the union type itself, but only when the assignment happens as a default value when destructuring.

This is also seen in https://github.com/facebook/flow/issues/183#issuecomment-467164769. Possibly more issues as well.

I'm surprised this hasn't been addressed since it seems like a common case that a lot of people run into, and an interaction of fairly basic JS features. Perhaps it's just not a code style that's used much at facebook?

[goodmind]

/cc @samwgoldman was this fixed?

[goodmind]

Fixed in https://github.com/facebook/flow/commit/de994b684b3b0561df4e596e0a46b63a93d70a33