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.

TypeScript Playground のリンクと f および g の最小再現から始め、typeof チェック後の推論された型を比較します。違いの原因となっている control-flow narrowing と推論された predicate の動作を追跡します。完了条件は、predicate のケースで a が null なしの object に絞り込まれ、報告された例に対するリグレッションカバレッジが追加されていることです。