prefer-type-fest-union-member

Require TypeFest UnionMember<T> over custom union-member extraction helpers based on UnionToIntersection.

Targeted pattern scope

This rule targets exact custom helper definitions that reproduce TypeFest UnionMember<T> semantics with UnionToIntersection, distributive function wrappers, and the optional IsNever<T> guard.

What this rule reports

UnionToIntersection<T extends any ? () => T : never> extends () => infer R ? R : never
The guarded variant IsNever<T> extends true ? never : ...

Why this rule exists

UnionMember<T> is the canonical TypeFest helper for extracting an arbitrary union member. Using it avoids re-declaring a brittle conditional-type helper and standardizes on the TypeFest utility name.

❌ Incorrect

type LastOfUnion<Union> = IsNever<Union> extends true
    ? never
    : UnionToIntersection<Union extends any ? () => Union : never> extends () => infer Last
      ? Last
      : never;

✅ Correct

import type { UnionMember } from "type-fest";

type LastOfUnion<Union> = UnionMember<Union>;

Behavior and migration notes

This rule intentionally matches only exact, high-confidence helper shapes.
Similar custom helpers that differ structurally are ignored.
It will not autofix when UnionMember is shadowed in the local type scope.

Additional examples

❌ Incorrect — Additional example

type MemberOfUnion<Union> = UnionToIntersection<Union extends any ? () => Union : never> extends () => infer Member
    ? Member
    : never;

✅ Correct — Additional example

import type { UnionMember } from "type-fest";

type MemberOfUnion<Union> = UnionMember<Union>;

✅ Correct — Non-targeted usage

type LastOfUnion<Union> = UnionToIntersection<Union extends unknown ? () => Union : never> extends () => infer Last
    ? Last
    : never;

ESLint flat config example

import typefest from "eslint-plugin-typefest";

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

When not to use it

Disable this rule if your project intentionally keeps local helper names for compatibility or if your custom helper intentionally differs from TypeFest UnionMember<T> semantics.

Package documentation

TypeFest package documentation:

Source file: source/union-member.d.ts

/**
Returns an arbitrary member of a union type.

Use-cases:
- Implementing recursive type functions that accept a union type.
*/
export type UnionMember<T> =
    IsNever<T> extends true
        ? never
        : UnionToIntersection<T extends any ? () => T : never> extends () => (infer R)
            ? R
            : never;

Rule catalog ID: R081

prefer-type-fest-union-member

Targeted pattern scope

What this rule reports

Why this rule exists

❌ Incorrect

✅ Correct

Behavior and migration notes

Additional examples

❌ Incorrect — Additional example

✅ Correct — Additional example

✅ Correct — Non-targeted usage

ESLint flat config example

When not to use it

Package documentation

Further reading

Adoption resources

Targeted pattern scope​

What this rule reports​

Why this rule exists​

❌ Incorrect​

✅ Correct​

Behavior and migration notes​

Additional examples​

❌ Incorrect — Additional example​

✅ Correct — Additional example​

✅ Correct — Non-targeted usage​

ESLint flat config example​

When not to use it​

Package documentation​

Further reading​

Adoption resources​

Targeted pattern scope

What this rule reports

Why this rule exists

❌ Incorrect

✅ Correct

Behavior and migration notes

Additional examples

❌ Incorrect — Additional example

✅ Correct — Additional example

✅ Correct — Non-targeted usage

ESLint flat config example

When not to use it

Package documentation

Further reading

Adoption resources