Maybe false positive from TS2801 (Promise<...> appears to always be defined)

[plantain-00]

Bug Report

🔎 Search Terms

TS2801

🕗 Version & Regression Information

• This changed between versions 4.4.2 and 4.3.5

⏯ Playground Link

Playground link with relevant code

💻 Code

const a: { [key: string]: Promise<string> } = {}
if (a['a']) {
  
}

🙁 Actual behavior

This condition will always return true since this 'Promise' is always defined.

🙂 Expected behavior

No error.

[MartinJohns]

You want to enable noUncheckedIndexedAccess. Without this compiler flag it's assumed that indexed access returns a valid value.

[Alexsey]

Unfortunately, due to #10530 noUncheckedIndexedAccess is not handy here:

if (a['a']) {
  a['a'] // Promise<string> | undefined
}

And I assume it will be even harder to narrow for index value 'a' to be variable and not literal

So, the workaround here is to use in instead of index access:

if ('a' in a) {
  a['a'] // Promise<string>
}