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prefer-type-fest-has-writable-keys

Require TypeFest HasWritableKeys<T> over WritableKeysOf<T> emptiness checks.

Targeted pattern scopeโ€‹

This rule reports exact WritableKeysOf<T> extends never ? false : true checks when WritableKeysOf is imported from type-fest.

It supports direct, aliased, and namespace-qualified TypeFest imports.

What this rule reportsโ€‹

This rule reports manual writable-key existence checks that can be replaced by HasWritableKeys<T>.

Why this rule existsโ€‹

HasWritableKeys<T> makes writable-key existence intent explicit and avoids repeating TypeFest helper compositions in user code.

โŒ Incorrectโ€‹

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

type Result<T extends object> = WritableKeysOf<T> extends never ? false : true;

โœ… Correctโ€‹

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

type Result<T extends object> = HasWritableKeys<T>;

Behavior and migration notesโ€‹

  • The WritableKeysOf reference must resolve to a type-fest import.
  • The rule only reports the exact extends never ? false : true shape.
  • Inverted checks and custom fallback branches are ignored.
  • Autofix is skipped when HasWritableKeys is shadowed in the local scope.

Additional examplesโ€‹

โŒ Incorrect โ€” Namespace importโ€‹

import type * as TypeFest from "type-fest";

type Result<T extends object> = TypeFest.WritableKeysOf<T> extends never ? false : true;

โœ… Correct โ€” Namespace importโ€‹

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

type Result<T extends object> = HasWritableKeys<T>;

โœ… Correct โ€” Inverted checkโ€‹

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

type Result<T extends object> = WritableKeysOf<T> extends never ? true : false;

ESLint flat config exampleโ€‹

import typefest from "eslint-plugin-typefest";

export default [
    {
        plugins: { typefest },
        rules: {
            "typefest/prefer-type-fest-has-writable-keys": "error",
        },
    },
];

When not to use itโ€‹

Disable this rule if a public helper must keep the expanded TypeFest composition for compatibility or documentation reasons.

Package documentationโ€‹

TypeFest package documentation:

Source file: source/has-writable-keys.d.ts

Rule catalog ID: R120

Further readingโ€‹

Adoption resourcesโ€‹