Unions seem to sidestep `--noUncheckedIndexedAccess` and lose the possibility of undefined
#61.225 aberto em 19 de fev. de 2025
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Description
🔎 Search Terms
noUncheckedIndexedAccess, union, undefined
🕗 Version & Regression Information
- This is the behavior in every version I tried, and I reviewed the FAQ for entries about
--noUncheckedIndexedAccess
⏯ Playground Link
💻 Code
const nums: { [k: string]: number } = Math.random() < 0.5 ? { a: 1 } : { b: 2 };
const str = { a: "hello" }
// with --noUncheckedIndexedAccess on
const hmm = (Math.random() < 0.5 ? nums.a : str.a) // string | number | undefined
const wha = (Math.random() < 0.5 ? nums : str).a // string | number <-- 😕
🙁 Actual behavior
The type of wha is string | number, completely ignoring the possibility that it might be undefined, even though --noUncheckedIndexedAccess is enabled. Looks like unions of types with index signatures and known keys lose the --noUncheckedIndexedAccess behavior.
🙂 Expected behavior
wha should be of type string | number | undefined, just like hmm, since indexed access into a union should look like a union of indexed accesses.
Additional information about the issue
This is related to #50474. It’s also related to #47531, which had specifically to do with never[] and so the focus was on avoiding never[] as opposed to investigating what happened to the indexed access. ( https://github.com/microsoft/TypeScript/issues/47531#issuecomment-1229522810 )
Ran into this when looking at a Stack Overflow question