Mapping over same objects and overwriting values

[HenriBeck]

When mapping over the same objects, flow doesn't allow to overwrite the values of the objects even though they are the same.
I'm assuming that flow checks merely all of the values of an object and checks if there is only one type as the value and if it's the same.

I guess this is a particular case. I think flow should only compare the types of the same keys and the structure of the object.

type Test = {
	key1: boolean,
	key2: string,
	key4: number,
	key6: Array<{}>,
};

function merge(obj1: Test, obj2: Test) {
	const keys = Object.keys(obj2);

	for (const key of keys) {
		obj1[key] = obj2[key];
	}

	return obj1;
}

Try Flow

[lll000111]

What I find strange is that Object.keys' return value is defined as Array<string>, but when I change the assignment in your example to...

const keys: Array<string> = Object.keys(obj2);

..., i.e. just adding a type that is already there according to how the type of Object.keys() is declared, the errors disappear.

Does Flow have some invisible hard-coded logic for Object.keys?

[goodmind]

It has, for reference #6927

[lll000111]

@goodmind Could it be declared using a generic (object) type of the keys of the object? There are similar discussions and issues in TypeScript, and TS's ahejlsberg wrote that's not possible (but I don't buy the argument he gives, his example just looks to me like somebody used the wrong type and not a fundamental issue).

[goodmind]

I think it should work like this, but without two generics

[SamChou19815]

This pattern will not be supported as it's too dynamic