Wrong behavior after usage of object rest operator

[apsavin]

type TypeWithOptionalProps = {
    requiredProp: string,
    optionalProp?: string,
}

const obj: TypeWithOptionalProps = { requiredProp: 'string', optionalProp: 'string' };
const { optionalProp = '', ...clearedObj } = obj;

(clearedObj: TypeWithOptionalProps);

const objForCheck: TypeWithOptionalProps = { requiredProp: 'string' };

Expected result: no errors Actual result: error on line (clearedObj: TypeWithOptionalProps);

You can reproduce it here

Introduced in 0.60 (works correctly in 0.59)

[calebmer]

This is the expected behavior 😊

Consider:

const x: {a: number, b: boolean} = {a: 42, b: true};
const y: {a: number} = x;
const z: {a: number, b?: string} = y;

try-Flow

TypeScript allows this, but Flow does not because it is unsound!

It’s a little hard to see that behavior in this case since the rest object we know absolutely doesn’t have that property. Flow currently doesn’t have any special type for “object which never had property p.” We are working on some spread + rest refactors. We may introduce this functionality during that.

For now you can use $Diff<TypeWithOptionalProps, {optionalProp: string | void}> to create a type without the optional prop 😊

Going to leave this open. While it’s the expectedbehavior for now, we could improve object rest typing to support this 👍

[apsavin]

Thanks for the answer!

we could improve object rest typing to support this

If it's possible then it should be done: it's obvious for a developer that the code in my example can't lead to errors (while code in your example obviously can).

[anru]

@calebmer ok, but why this error is still happens on exact objects ?

See this example ( TRY )

// @flow

type A = {|
  a: number,
  b: number,
|}

type C = {
  a: number,
  c?: number,
}

let a: A = {a: 1, b: 2};

const { b, ...restA } = a; // restA == {| a: 1 |}

const c: C = restA; // Why error ?

As we can see, restA can't have more properties than just a, but flow complains about restA does not satisfy type C

[anru]

if anyone are interested, found a workaround for example above

[SamChou19815]

it now works