I encountered a 'not assignable type' error when I turned on the 'noUncheckedIndexedAccess' option.

[hotsoycandy]

Bug Report

🔎 Search Terms

• type guard
• noUncheckedIndexedAccess

🕗 Version & Regression Information

Typescript version that I tried: 4.9.5, 5.0.4, 5.1.0-beta

⏯ Playground Link

Make it sure that the "noUncheckedIndexedAccess" option is turned on.

Playground link

💻 Code

{
  "compilerOptions": {
    "noUncheckedIndexedAccess": true
  }
}

const fruits: Record<string, string> = { apple : 'best', orange: 'good' }

function getFruit (fruitName: string): string {
    if (typeof fruits[fruitName] === 'string') {
        // Type 'string | undefined' is not assignable to type 'string'.
        // Type 'undefined' is not assignable to type 'string'.(2322)
        return fruits[fruitName]
    }
    return 'nothing'
}

🙁 Actual behavior

Type guard doesn't work properly and typescript occurs not assignable type error when using the "noUncheckedIndexedAccess" option.

🙂 Expected behavior

The type guard works properly even the "noUncheckedIndexedAccess" option is turned on.

For example, fruits[fruitName] should be 'string', not 'string | undefined'

[MartinJohns]

It's working as intended / a known design limitation with many many issues opened already, and it really needs to be added to the FAQ.

Indexed access is not narrowed, unless the key is a literal.

[jcalz]

Tracked at #10530

[microsoft-github-policy-service]

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

[microsoft-github-policy-service]

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