Object access reports error after upgrading to TypeScript 5.5

[jkillian]

🔎 Search Terms

"noUncheckedIndexedAccess", "Object is possibly 'undefined'."

🕗 Version & Regression Information

• This changed between versions 5.4.5 and 5.5.3

⏯ Playground Link

https://www.typescriptlang.org/play/?noUncheckedIndexedAccess=true&ts=5.5.3#code/DYUwLgBAzmBOBc04EsB2BzCAfCBXVAJiAGZogEDcAUMsRABQCyAhmABYB0szhA9gLb0AlBAA8EAAwcAjCIDeVCElgQAvBABEzDVQC+VKgGNeqGBGaHDiORADaqZvxCIYsNOgC6iVLn4AjEBVdNQg5XWpaBlcIAEJ1H2BgeUVzS1tXDwgAfiz1CWolC0N0uA8AajLqfSA

💻 Code

// @strict: true
let str: string | undefined;
if (Math.random() < 0.1) {
  str = "a"
}

const acc: { [name: string]: number } = {};
if (str != null) {
  acc[str] ??= 0;
  acc[str]++; // In TS 5.5: Object is possibly 'undefined'. (2532)
}

🙁 Actual behavior

In TS 5.5.x, with noUncheckedIndexedAccess enabled, acc[str]++; reports an error "Object is possibly 'undefined'. (2532)".

🙂 Expected behavior

In TS 5.4.x and before, no errors were reported.

Additional information about the issue

I admittedly didn't do a very deep dive on the root cause here, but wanted to file an issue because intuitively it felt unexpected when upgrading to TS 5.5 that the above code now fails type-checking. Apologies if I missed an existing issue for this or something in the release notes!

[typescript-bot]

The change between v5.4.5 and main occurred at 247a983.

[andrewbranch]

I highly doubt that, @typescript-bot

[Andarist]

5.4 incorrectly always allowed acc[str]++:

let str: string | undefined;
if (Math.random() < 0.1) {
  str = "a"
}

const acc: { [name: string]: number } = {};
if (str != null) {
  // acc[str] ??= 0;
  acc[str]++; // no error in 5.4
}

This has been fixed in 5.5 and that affects ur repro. TS 5.5 introduces Control Flow Narrowing for Constant Indexed Accesses but we can read there that:

TypeScript is now able to narrow expressions of the form obj[key] when both obj and key are effectively constant.

Since your str isn't effectively constant... this doesn't work in a form that u'd like it to. Notice how this version works alright:

let str = Math.random() < 0.1 ? "a" : undefined;

const acc: { [name: string]: number } = {};

if (str != null) {
  acc[str] ??= 0;
  acc[str]++;
}

export {};

[jkillian]

Thanks for some of the context @Andarist! Perhaps that makes this a feature request instead of a bug report then. It does seem like for the duration of these three lines, str is effectively constant enough that TS could allow the code regardless of how str is assigned above:

if (str != null) {
  acc[str] ??= 0;
  acc[str]++;
}

Does that seem reasonable, or am I missing something here?

[Andarist]

From what I understand, it was a performance concern to allow further analysis to handle cases like those. But yes - it is a potential feature request.