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ReturnType

Last Updated: 3/6/2026

.returnType()

The .returnType<SomeType>() method allows you to control the return type of all of your branches of code. It accepts a single type parameter that will be used as the return type of your match expression.

Signature

function returnType<TOutputOverride>(): Match<TInput, TOutputOverride>;

Type Arguments

TOutputOverride

The type that your match expression will return. All branches must return values assignable to it.

Example

import { match, P } from 'ts-pattern';
 
type State =
  | { status: 'idle' }
  | { status: 'loading' }
  | { status: 'success'; data: string };
 
const handleState = (state: State) =>
  match(state)
    .returnType<State>() // Force all branches to return State
    .with({ status: 'idle' }, () => ({ status: 'loading' }))
    .with({ status: 'loading' }, () => ({ status: 'success', data: 'done' }))
    .with({ status: 'success' }, (s) => s)
    .exhaustive();

Type Checking

All code branches must return values assignable to the specified type:

match(input)
  .returnType<string>()
  .with('hello', () => 'greeting') // ✅ OK
  .with(42, () => 'number') // ✅ OK
  .with(true, () => 123) // ❌ Error: number is not assignable to string
  .otherwise(() => 'default');

When to Use

Use .returnType() when:

  1. You want explicit type control: Ensure all branches return a specific type
  2. Working with complex types: Make sure transformations preserve type structure
  3. State machines: Guarantee state transitions return valid states
  4. Refactoring safety: Catch type errors early when changing return types

Optional Usage

Note that .returnType() is optional. TypeScript can usually infer the return type from your handler functions:

// TypeScript infers return type as string
const result = match(value)
  .with('a', () => 'letter a')
  .with('b', () => 'letter b')
  .otherwise(() => 'other');
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