Bug: [no-base-to-string] generate error with type filter above

[KuSh]

Before You File a Bug Report Please Confirm You Have Done The Following...

• I have tried restarting my IDE and the issue persists.
• I have updated to the latest version of the packages.
• I have searched for related issues and found none that matched my issue.
• I have read the FAQ and my problem is not listed.

Playground Link

https://typescript-eslint.io/play/#ts=5.8.2&fileType=.tsx&code=MYewdgzgLgBApgJwSBBZOEIEMDmcYC8MAFIsggFwwCuYA1mCAO5gCUV0CAlmDoQHwwA3gCgYMLgDMSUAJ4AHOCGlkUhAkQDkIAEYArOMCibWMBHCjUEYGACkAygHkAcgDpOPHFNmkkKVgDcYmYWVjb2UNy8vuSBIgC%2BQUA&eslintrc=N4KABGBEBOCuA2BTAzpAXGUEKQAIBcBPABxQGNoBLY-AWhXkoDt8B6Jge1oCMBDZRLXxdk%2BKkwDm6KImjQO0SODABfECqA&tsconfig=N4KABGBEDGD2C2AHAlgGwKYCcDyiAuysAdgM6QBcYoEEkJemy0eAcgK6qoDCAFutAGsylBm3TgwAXxCSgA&tokens=false

Repro Code

export const errorMessage = (error: unknown): string => {
  if (typeof error === 'object') return JSON.stringify(error);
  return String(error);
};

ESLint Config

module.exports = {
  "rules": {
    "@typescript-eslint/no-base-to-string": "error"
  }
}

tsconfig

{
  "compilerOptions": {
    "strictNullChecks": true
  }
}

Expected Result

No error

Actual Result

'error' may use Object's default stringification format ('[object Object]') when stringified. 3:17 - 3:22

Additional Info

Could be related to the fact that typescript consider the object to be {} | undefined after the test

[bradzacher]

Why do you expect there to be no error here?
As far as the types are declared -- there is no toString method declared on the type {} | undefined -- so it's going to use the default toString.
This is working as expected.

[KuSh]

Why do you expect there to be no error here? As far as the types are declared -- there is no toString method declared on the type {} | undefined -- so it's going to use the default toString. This is working as expected.

But {} falls in the typeof {} === "object" case so this can never happen, see console.log when running this typescript playground. no [object Object] output

So in that particular case, there's no toString method involved at all, only string coercion

[bradzacher]

Hover over the type and you'll see what TS narrows it to and why my comment will apply.
The rule is solely powered by types.

[KuSh]

So, if I read between the lines, I'd rather should report that to typescript?

[kirkwaiblinger]

So, if I read between the lines, I'd rather should report that to typescript?

You can try but likely this would be considered working as intended on their part. In theory maybe something like string | number | bigint | symbol | undefined | Function | boolean would be a better type but it's a little hazy because functions can be objects. Also there's the whole {} can of worms (see also microsoft/TypeScript#60582)

In any case, for our part, there's no generalizable way to distinguish between {} | undefined which is intended literally, and therefore, to which any object might be assigned, and a {} | undefined which is the result of a TS narrowing that (almost certainly) wouldn't actually be reachable at runtime by any non-toString() objects. Likely an eslint-disable comment is your best bet here.

[Josh-Cena]

functions can be objects

Functions always have typeof === "function".

[kirkwaiblinger]

functions can be objects

Functions always have typeof === "function".

Oh, yeah. I couldn't remember whether it was "technically not all functions are typeof === 'function'" or "technically not all functions are instanceof Function" (it's the latter). Regardless I also meant to be vague enough to allude to the opposite direction, that (runtime value) () => {} is assignable to (type) {}.

[KuSh]

Thanks @kirkwaiblinger for all the explanations and the context.

For the record a simple fix is to test both for nullness AND typeof.
With that error stays unknown after the condition and the linting error goes out

export const errorMessage = (error: unknown): string => {
  if (error !== null && typeof error === 'object') return JSON.stringify(error);
  return String(error);
};