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prefer-type-fest-set-return-type

Prefer SetReturnType from type-fest over direct function-type wrappers built from Parameters.

This rule lives only in the experimental preset and reports without autofixing.

Targeted pattern scopeโ€‹

This rule focuses on direct function-type wrappers of the form:

  • (...args: Parameters<Function>) => Result

It intentionally skips the async-specific Promise<Awaited<ReturnType<Function>>> shape so the more specific prefer-type-fest-asyncify rule can handle that case.

What this rule reportsโ€‹

This rule reports function types when all of the following are true:

  • the function type has exactly one rest parameter
  • that rest parameter is typed as Parameters<Function>
  • the return type is not the original ReturnType<Function>
  • the return type is not the Asyncify shape Promise<Awaited<ReturnType<Function>>>

The rule is currently report-only. It does not autofix or suggest a replacement yet.

Why this rule existsโ€‹

SetReturnType<Function, Result> states the transformation directly.

  • Readers can see that only the return type changes.
  • The helper is easier to reuse consistently across wrapper types.
  • It avoids repeating the same Parameters<...> boilerplate in local helper types.

โŒ Incorrectโ€‹

type WithResult<Function_ extends (...arguments_: any[]) => any, Result> =
    (...arguments_: Parameters<Function_>) => Result;

โœ… Correctโ€‹

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

type WithResult<Function_ extends (...arguments_: any[]) => any, Result> =
    SetReturnType<Function_, Result>;

Behavior and migration notesโ€‹

  • This rule targets the narrow rest-parameter form (...args: Parameters<F>) => R.
  • It ignores ordinary handwritten function types with explicit parameters.
  • It intentionally defers asyncified wrappers to prefer-type-fest-asyncify.

ESLint flat config exampleโ€‹

import typefest from "eslint-plugin-typefest";

export default [typefest.configs.experimental];

When not to use itโ€‹

Disable this rule if your team prefers to spell out the wrapper function type explicitly or if you need a custom helper name for return-type rewrites.

Package documentationโ€‹

TypeFest package documentation:

Source file: source/set-return-type.d.ts

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

type Wrapped<Function_ extends (...arguments_: any[]) => any> =
    SetReturnType<Function_, string>;

Rule catalog ID: R092

Further readingโ€‹

Adoption resourcesโ€‹