Bug: null can be excluded by !== null in flow before typeof === 'object', but not excluded by predicate function.

🔎 Search Terms

"null", "control flow", "narrowing", "narrow", "exclusion", "object", "typeof"

🕗 Version & Regression Information

Potentially, we could expect the behaviors of if (a === null) and if ((v => v !== null)(a)) (inferred as predicate since TS 5.5) should be the same, but it's actually not in specific cases.

If it's combined with typeof a === 'object' check in the same flow, first one is correctly omits the null possibility, while second one doesn't.

• This changed between versions:
  • I confirmed that the exclusion in flow can be done since TS4.5. It's not regression, but the new feature seems to have been partially applied.
  • TS4.4 https://www.typescriptlang.org/play/?ts=4.4.4#code/MYewdgzgLgBAlhAcgVwDapgXhgCgG4BcMyYA1mCAO5gCURe8EMYaGmAfDA5j86wNwAoAPTCY4mAD0A-INCRYAMyy4AhkRLkqtLJwDegifGU4EKdDlU0aMAE4BTKMltghRuCagBPAA72QyqowAIS8AOQgAEYAVvbAUGE2Dk4ubhKqbqJSsgC+QoJy4NAwAOYqlhpkFNQ2HDAG7iZBPNgs6EmOzq6GEh643n4BMEGh2BExcQkdKd1GGYZZMoJ5QA
  • TS4.5 https://www.typescriptlang.org/play/?ts=4.5.5#code/MYewdgzgLgBAlhAcgVwDapgXhgCgG4BcMyYA1mCAO5gCURe8EMYaGmAfDA5j86wNwAoAPTCY4mAD0A-INCRYAMyy4AhkRLkqtLJwDegifGU4EKdDlU0aMAE4BTKMltghRuCagBPAA72QyqowAIS8AOQgAEYAVvbAUGE2Dk4ubhKqbqJSsgC+QoJy4NAwAOYqlhpkFNQ2HDAG7iZBPNgs6EmOzq6GEh643n4BMEGh2BExcQkdKd1GGYZZMoJ5QA
• This changed in commit or PR (not sure for now, sorry)

⏯ Playground Link

https://www.typescriptlang.org/play/?ts=5.6.2#code/MYewdgzgLgBAlhAcgVwDapgXhgCgG4BcMyYA1mCAO5gCUWAfDHlptmGqgNwBQA9LzEEwAegH5uoSLABmWXAEMiJclVoMYAb25D4snAhToc8mnQBOAUyjIzYHjrh6oATwAOFkLPkwAhKxgA5CAARgBWFsBQAeZWNnbaQvL2-CLiAL483BLg0DAA5nLGSmQU1HSYjFoOet6sbBwx1rb2Qo64Lu6eMN5+2EFhEVGNcS2CSdopYtwZQA

💻 Code

const isNull = (v: unknown) => v === null;
//    ^?
const f = (a: unknown) => {
    if (isNull(a)) return;
    if (typeof a !== 'object') return;
    a;
 // ^?
};


const g = (a: unknown) => {
    if (a === null) return;
    if (typeof a !== 'object') return;
    a;
 // ^?
};

🙁 Actual behavior

Last a in f is inferred as object | null.

🙂 Expected behavior

Last a in f would be inferred as object.

Additional information about the issue

Here is important to check typeof === 'object' after the check for null, because type narrowing step is formally, unknown → object | null → object. === null alone cannot narrow anything on unknown.

If you swap the if-statements in above, you can see both is inferred as object.

Start with the TypeScript Playground links and the minimal reproductions for f and g, comparing the inferred types after the typeof check. Trace the control-flow narrowing and inferred predicate behavior responsible for the difference; done means the predicate case narrows a to object without null, with regression coverage for the reported example.