narrowing object by property doesn't work if key isn't a `const`

[DetachHead]

Bug Report

🔎 Search Terms

narrow property const

🕗 Version & Regression Information

4.9.0-dev.20221025

⏯ Playground Link

Playground link with relevant code

💻 Code

interface Data {
 a?: number
}

declare const data: Data

let key = 'a' as const

if (data.a !== undefined) {
 key // "a"
 const a = data[key] // number | undefined
 const b = data['a'] // number
}

🙁 Actual behavior

data[key] is number | undefined

🙂 Expected behavior

data[key] is narrowed to number

[KotlinIsland]

I managed to minify this issue a little bit, take a look below:

declare const data: {
 a?: number
}

if (data.a === undefined) throw "up"

const a = data['a' as 'a']
// ^? const a: number | undefined
const b = data['a']
// ^? const b: number

[fatcerberus]

Duplicate of #10530. Only certain patterns of property access induce narrowing, specifically only the forms obj.foo and obj['foo'] (i.e. the key must be a naked literal). obj[key] won't be narrowed, even if key is literal-typed.

[DetachHead]

the example in that issue seems to have been fixed ages ago #10530 (comment)

[fatcerberus]

Yeah, foo["bar"] used to not be narrowable. Now it is, but the larger issue of foo[keyVar] not narrowing (which is a much bigger can of worms, implementation-wise) is still present.

[typescript-bot]

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