as const does not narrow when object literal defined with type

[jisaacks]

Bug Report

🔎 Search Terms

Searched:

typeof doesn't narrow, ignores as const when object literal defined with type

🕗 Version & Regression Information

Example in playground

💻 Code

interface A {
    [k: string]: 'foo' | 'bar' | 'baz' | 'other'
}

const foo:A = {
    a: 'foo',
    b: 'bar',
} as const

type FooKeys = keyof typeof foo

🙁 Actual behavior

Since foo is defined as A the typeof ignores the as const and FooKeys ends up being string | number

🙂 Expected behavior

I would expect foo being defined with as const would narrow to a type with only a and b keys.
So I would expect FooKeys to be "a" | "b"

---

Workaround

interface A {
    [k: string]: 'foo' | 'bar' | 'baz' | 'other'
}

const bar = {
    a: 'foo',
    b: 'bar',
} as const

const foo:A = bar

type FooKeys = keyof typeof bar

If this is not a bug

If this is actually expected, can this issue be transferred into a feature request?

[RyanCavanaugh]

See #7481

[jcalz]

This is working as intended and doesn't have much to do with as const.

A is not a union type, so the assignment of a value of type { readonly a: "foo"; readonly b: "bar"; } to a variable annotated as type A does not result in any narrowing via control flow analysis. The variable foo is of type A; it is not narrowed by assignment; its keys are keyof A.

If you're asking that control flow analysis take place on non-union types, then this might be a duplicate of #16976.

[jisaacks]

@jcalz gotcha thanks, I'll close it then.