prefer-type-fest-is-unknown

Require TypeFest IsUnknown<T> over manual unknown conditional type guards.

Targeted pattern scope

This rule reports exact conditional type guards shaped like unknown extends T ? [T] extends [null] ? false : true : false.

What this rule reports

This rule reports manual unknown detection helpers that can be replaced by IsUnknown<T>.

• unknown extends T ? [T] extends [null] ? false : true : false

Why this rule exists

IsUnknown<T> documents the intent directly and avoids repeating a brittle conditional type trick across a codebase.

❌ Incorrect

type Result<T> = unknown extends T ? [T] extends [null] ? false : true : false;

✅ Correct

type Result<T> = IsUnknown<T>;

Behavior and migration notes

• The rule intentionally requires the exact tuple-wrapped null exclusion.
• Simpler unknown extends T ? true : false helpers are ignored because they also match any.
• Autofix is skipped when IsUnknown is shadowed in the local scope.

Additional examples

✅ Correct — Existing utility

type Result<T> = IsUnknown<T>;

✅ Correct — Simpler but not equivalent

type Result<T> = unknown extends T ? true : false;

ESLint flat config example

import typefest from "eslint-plugin-typefest";

export default [
    {
        plugins: { typefest },
        rules: {
            "typefest/prefer-type-fest-is-unknown": "error",
        },
    },
];

When not to use it

Disable this rule if a public type alias must preserve a hand-written conditional for compatibility documentation.

Package documentation

TypeFest package documentation:

Source file: source/is-unknown.d.ts

Rule catalog ID: R111

Further reading

• type-fest README
• IsUnknown source
• TypeScript Handbook: Conditional Types

Adoption resources

• Rule adoption checklist
• Rollout and fix safety