Object mistyped as Array when Array is spread into Object

[StefanDBTLabs]

🔎 Search Terms

destructure array in to object
array spread in to object

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about arrays and objects

⏯ Playground Link

https://www.typescriptlang.org/play/?ts=5.6.2#code/C4TwDgpgBAYg9nKBeKBvKAjAhgJwFxQDOwOAlgHYDmUAvgNwCwAUAGYCu5AxsKXOVC3IAKYAXhwA2gF0AlGig4IwNjn7AAdAFssYISGQA+KAEJjIOTWac+xBcnnrHE9NnxQARMTJV3tKbUZWYRwZOiA

💻 Code

type Foo = { bar: string };
function fn(t: Foo[]) { return t.map(y => !!y) }
const r = { ...[{ bar: "string" }] };
fn(r);

🙁 Actual behavior

Foo Allows me to pass an Object when it wants an Array. when fn is called with Object it throws an error because there is no map on an Object

🙂 Expected behavior

There should be a typescript error when passing an Object to function that wants an Array

Additional information about the issue

In the d.ts file you can see how it has created a type for r that looks like an array.

[RyanCavanaugh]

Enumerability/ownness is not tracked in the type system, so we don't have any mechanism to recognize that this won't work. See #9726

[RyanCavanaugh]

I'm curious what the actual bug you had was. It might worth erroring when spreading something sufficiently arraylike in an object spread position.

[StefanDBTLabs]

@RyanCavanaugh the bug was accidentally passing an Object to a function that wanted an array and that function tried to use the array methods which did not exist. that's the screenshot I attached above.

[RyanCavanaugh]

Were you calling assign or doing a spread?

[StefanDBTLabs]

@RyanCavanaugh I was doing the spread I have in the above example

const r = { ...[{ bar: "string" }] };
fn(r);

but this was a mistake on my part. I would have wanted to do

const r = [ ...[{ bar: "string" }] ];
fn(r);

or even

const r = [{ bar: "string" }];
fn(r);

but I over looked it and would have hoped typescript would have given me a type error.

[StefanDBTLabs]

though if you look at the generated JS from the playground it seems to be doing an Object.assign after generation.
https://www.typescriptlang.org/play/?ts=5.6.2#code/C4TwDgpgBAYg9nKBeKBvKAjAhgJwFxQDOwOAlgHYDmUAvgNwCwAUAGYCu5AxsKXOVC3IAKYAXhwA2gF0AlGig4IwNjn7AAdAFssYISGQA+KAEJjIOTWac+xBcnnrHE9NnxQARMTJV3tKbUZWYRwZOiA

[typescript-bot]

This issue has been marked as "Design Limitation" and has seen no recent activity. It has been automatically closed for house-keeping purposes.

[martijnarts]

Just to add to this: it's really easy to run into this when refactoring a property from an object into an array. This causes Typescript to generate Javascript that's broken (playground) without compilation errors:

type WithArr = { a: string; b: Array<{ c: string; }> };

const arr: Array<{ c: string; }> = [{ c: "test" }];
const test = { a: "foo", b: { ...arr } };

console.log(test.b.map(a => a));
// test.b.map is not a function

[jcalz]

dupe of #34780