prefer-type-fest-is-null
Require TypeFest IsNull<T> over manual tuple-wrapped null conditional type guards.
Targeted pattern scopeโ
This rule reports exact conditional type guards shaped like [T] extends [null] ? true : false.
It does not report distributive T extends null ? true : false checks.
What this rule reportsโ
This rule reports manual non-distributive null checks that can be replaced by IsNull<T>.
[T] extends [null] ? true : false
Why this rule existsโ
IsNull<T> makes null detection intent explicit and keeps type-guard helpers aligned with TypeFest.
โ Incorrectโ
type Result<T> = [T] extends [null] ? true : false;
โ Correctโ
type Result<T> = IsNull<T>;
Behavior and migration notesโ
- Only the canonical tuple-wrapped form is reported.
undefinedandneverguards are handled by their own rules.- Autofix is skipped when
IsNullis shadowed in the local scope.
Additional examplesโ
โ Incorrect โ Generic helperโ
type Nullable<T> = [T] extends [null] ? true : false;
โ Correct โ Generic helperโ
type Nullable<T> = IsNull<T>;
โ Correct โ Distributive conditionalโ
type Nullable<T> = T extends null ? true : false;
ESLint flat config exampleโ
import typefest from "eslint-plugin-typefest";
export default [
{
plugins: { typefest },
rules: {
"typefest/prefer-type-fest-is-null": "error",
},
},
];
When not to use itโ
Disable this rule if a public helper must keep a hand-written null conditional for teaching or compatibility reasons.
Package documentationโ
TypeFest package documentation:
Source file: source/is-null.d.ts
Rule catalog ID: R105