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Comment by Dylan16807 | Hacker News Reader

This doesn't need magic powers. All you have to do is treat literal values inside type descriptions differently.

Inside of type descriptions, literal values are always treated as literals. The problem only appears when you declare a variable _without_ an explicit type like `const foo = {k: 'x'}`, which the compiler then infers to be of type `foo: {k: string}` and not `foo: {k: 'x'}`.

Both of these are valid inferences, and it's impossible to know which one the user wants in the general case, which is why you have to explicitly say `const foo = {k: 'x' as const}`.

So the example of `false | 'x' | 'y' | undefined` turning into string is invalid?

I feel like I'm missing something.

here's a full example. If you do this:

    function test(p: {k: false | 'x' | 'y' | undefined}) {
        // do stuff
    }
    const p = {k: 'x'} // inferred type: {k: string}
    //...
    test(p); // type error : Type 'string' is not assignable to type 'false | "x" | "y" | undefined'

The problem is that the type of p.k is inferred to `string` not `x`. The compiler can't know this is fine because you could call `foo(p)` in between the declaration and the `test` call, that mutates p.k.

If you add `as const` or an explicit type to p it is fine.

(I think this is what the GGP was referring to)

Didn't realize "as const" worked; will have to try that next time.

Thanks!

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