Typescript: Narrowing down type bypasses type checker

[bpasero]

TypeScript 3.3.1

This imho should not be allowed:

const foo = { foo: false } as { foo: boolean, bar: boolean };

As it bypasses the type checker from doing its job. I now have basically introduced a type with undefined as foo.bar even though it should be boolean | undefined.

If someone wants this type by all means, it is still possible by using any.

const foo = { foo: false } as any as { foo: boolean, bar: boolean };

That way at least one can find hacks easily.

I would like to see a warning for this, possibly via some kind of new strict rule ideally.

I actually use as or <> a lot in my code to get IntelliSense for an object literal I am writing and ensuring the values I give it are correct, but now I am not so sure anymore.

[jack-williams]

Type assertions have always been unsound because they let you move in the inverse direction of assignability. If you want type-checking then you need to write:

const foo: { foo: boolean, bar?: boolean } = { foo: false };

If you explicitly want expression level syntax then I think you're looking for #7481

[bpasero]

It would be great if that was easy to use like the as operator, e.g. something like

const foo = { foo: false } is { foo: boolean, bar: boolean };

would report as error.

[RyanCavanaugh]

See #7481

Are you aware that you can annotate variables?

const foo: { foo: boolean, bar: boolean } = { foo: false };

[bpasero]

@RyanCavanaugh sure that is a workaround but I often end up with a long expression (possibly ternary operators) where I might have multiple types on the way and I typically prefer to declare their types on the way and not all upfront for better readability.

[typescript-bot]

This issue has been marked 'Working as Intended' and has seen no recent activity. It has been automatically closed for house-keeping purposes.