.map fn

README.md

@@ -428,6 +428,19 @@ const f = (a: number) => [a, a]
 expectTypeOf(f).toBeCallableWith('foo')
 ```
 
+Use `.map` to transform types:
+
+This can be useful for generic functions or complex types which you can't access via `.toBeCallableWith`, `.toHaveProperty` etc. The callback function isn't called at runtime, which can make this a useful way to get complex inferred types without worrying about running code.
+
+```typescript
+const capitalize = <S extends string>(input: S) =>
+  (input.slice(0, 1).toUpperCase() + input.slice(1)) as Capitalize<S>
+
+expectTypeOf(capitalize)
+  .map(fn => fn('hello world'))
+  .toEqualTypeOf<'Hello world'>()
+```
+
 You can also check type guards & type assertions:
 
 ```typescript

src/index.ts

@@ -498,6 +498,14 @@ export interface BaseExpectTypeOf<Actual, Options extends {positive: boolean}> {
    */
   toBeNullable: Scolder<ExpectNullable<Actual>, Options>
 
+  /**
+   * Transform that type of the value via a callback.
+   *
+   * @param fn A callback that transforms the input value. Note that this function is not actually called - it's only used for type inference.
+   * @returns A new type which can be used for further assertions.
+   */
+  map: <T>(fn: (value: Actual) => T) => ExpectTypeOf<T, Options>
+
   /**
    * Checks whether a function is callable with the given parameters.
    *
@@ -911,6 +919,7 @@ export const expectTypeOf: _ExpectTypeOf = <Actual>(
     toMatchTypeOf: fn,
     toEqualTypeOf: fn,
     toBeConstructibleWith: fn,
+    map: expectTypeOf,
     toBeCallableWith: expectTypeOf,
     extract: expectTypeOf,
     exclude: expectTypeOf,

test/usage.test.ts

@@ -305,6 +305,19 @@ test("You can't use `.toBeCallableWith` with `.not` - you need to use ts-expect-
   expectTypeOf(f).toBeCallableWith('foo')
 })
 
+/**
+ * This can be useful for generic functions or complex types which you can't access via `.toBeCallableWith`, `.toHaveProperty` etc.
+ * The callback function isn't called at runtime, which can make this a useful way to get complex inferred types without worrying about running code.
+ */
+test('Use `.map` to transform types', () => {
+  const capitalize = <S extends string>(input: S) =>
+    (input.slice(0, 1).toUpperCase() + input.slice(1)) as Capitalize<S>
+
+  expectTypeOf(capitalize)
+    .map(fn => fn('hello world'))
+    .toEqualTypeOf<'Hello world'>()
+})
+
 test('You can also check type guards & type assertions', () => {
   const assertNumber = (v: any): asserts v is number => {
     if (typeof v !== 'number') {