fix(match): properly type return type

src/function.test.ts

@@ -1,4 +1,4 @@
-import { expect, it } from 'vitest'
+import { expect, expectTypeOf, it } from 'vitest'
 import { asyncInvoke, invoke, match, tap, value } from './function'
 
 it('invoke', async() => {
@@ -67,4 +67,19 @@ it('match', () => {
 		bar: () => '2',
 		default: '1',
 	})).toBe('1')
+
+	expect(match(123, {
+		100: 'foo',
+		200: 'bar',
+		default: 'baz',
+	})).toBe('baz')
+
+	expectTypeOf(match('foo', { foo: () => 1, bar: () => 2 })).toBeNumber()
+	expectTypeOf(match('foo', { foo: () => 'a', bar: () => 'b' })).toBeString()
+	expectTypeOf(match('foo', { foo: () => 'a', bar: () => 'b', default: () => 'c' })).toBeString()
+	expectTypeOf(match('foo', { foo: 1, bar: 2 })).toBeNumber()
+	expectTypeOf(match('foo', { baz: 1, bar: 2, default: 3 })).toBeNumber()
+	expectTypeOf(match(123, { 123: 'foo', 200: 'bar' })).toBeString()
+	expectTypeOf(match(123, { 100: 'foo', 200: 'bar', default: 'baz' })).toBeString()
+	expectTypeOf(match('bar' as string | number, { foo: 'a', default: 'b' })).toMatchTypeOf<string | number>()
 })

src/function.ts

@@ -70,7 +70,9 @@ interface DefaultLookup<TReturnValue> {
 	default: MaybeCallable<TReturnValue>
 }
 
-type Lookup<TValue extends string | number | undefined = string, TReturnValue = unknown> = { [K in NonNullable<TValue>]: MaybeCallable<TReturnValue> }
+type Lookup<TValue extends string | number | undefined, TReturnValue> = {
+	[K in NonNullable<TValue>]: MaybeCallable<TReturnValue>
+}
 
 /**
  * Matches a value against a lookup table.
@@ -86,21 +88,25 @@ type Lookup<TValue extends string | number | undefined = string, TReturnValue =
  */
 export function match<
 	TValue extends string | number | undefined,
-	TReturnValue = unknown,
-	TLookup extends(Lookup<TValue, TReturnValue> | (Partial<Lookup<TValue, TReturnValue>> & DefaultLookup<TReturnValue>)) = Lookup<TValue, TReturnValue>,
+	TLookup extends(Lookup<TValue, TReturnValue> | (Partial<Lookup<TValue, TReturnValue>> & DefaultLookup<TReturnValue>)),
+	TReturnValue,
 >(
 	value: TValue,
 	lookup: TLookup,
 	...args: any[]
-): TReturnValue {
+): TLookup extends Lookup<TValue, infer U>
+		? U
+		: TLookup extends (Partial<Lookup<TValue, infer V>> & DefaultLookup<infer V>)
+			? V
+			: never {
 	if (value! in lookup) {
-		const returnValue = lookup[value!]
+		const returnValue = lookup[value!] as TLookup[NonNullable<TValue>]
 		return isFunction(returnValue) ? returnValue(...args) : returnValue
 	}
 
 	if ('default' in lookup) {
-		const returnValue = lookup.default
-		return isFunction(returnValue) ? returnValue(...args) : returnValue!
+		const returnValue = lookup.default as TLookup[NonNullable<TValue>]
+		return isFunction(returnValue) ? returnValue(...args) : returnValue
 	}
 
 	const handlers = Object.keys(lookup)