Add .pick and .omit (#51)

This PR:

- [X] Adds `.pick` and `.omit` methods which work similarly to
`.extract` and `.exclude`.

---------

Co-authored-by: Misha Kaletsky <15040698+mmkal@users.noreply.github.com>

README.md

@@ -263,6 +263,22 @@ expectTypeOf<Customer | Employee>().extract<{foo: string}>().toBeNever()
 expectTypeOf<Customer | Employee>().exclude<{name: string}>().toBeNever()
 ```
 
+Use `.pick` to pick a set of properties from an object:
+
+```typescript
+type Person = {name: string; age: number}
+
+expectTypeOf<Person>().pick<'name'>().toEqualTypeOf<{name: string}>()
+```
+
+Use `.omit` to remove a set of properties from an object:
+
+```typescript
+type Person = {name: string; age: number}
+
+expectTypeOf<Person>().omit<'name'>().toEqualTypeOf<{age: number}>()
+```
+
 Make assertions about object properties:
 
 ```typescript

src/index.ts

@@ -326,6 +326,8 @@ export interface BaseExpectTypeOf<Actual, Options extends {positive: boolean}> {
   toBeConstructibleWith: Options['positive'] extends true ? (...args: ConstructorParams<Actual>) => true : never
   extract: <V>(v?: V) => ExpectTypeOf<Extract<Actual, V>, Options>
   exclude: <V>(v?: V) => ExpectTypeOf<Exclude<Actual, V>, Options>
+  pick: <K extends keyof Actual>(v?: K) => ExpectTypeOf<Pick<Actual, K>, Options>
+  omit: <K extends keyof Actual>(v?: K) => ExpectTypeOf<Omit<Actual, K>, Options>
   parameter: <K extends keyof Params<Actual>>(number: K) => ExpectTypeOf<Params<Actual>[K], Options>
   parameters: ExpectTypeOf<Params<Actual>, Options>
   constructorParameters: ExpectTypeOf<ConstructorParams<Actual>, Options>
@@ -415,6 +417,8 @@ export const expectTypeOf: _ExpectTypeOf = <Actual>(
     /* eslint-enable mmkal/@typescript-eslint/no-unsafe-assignment */
     extract: expectTypeOf,
     exclude: expectTypeOf,
+    pick: expectTypeOf,
+    omit: expectTypeOf,
     toHaveProperty: expectTypeOf,
     parameter: expectTypeOf,
   }

test/usage.test.ts

@@ -169,6 +169,18 @@ test('`.extract` and `.exclude` return never if no types remain after exclusion'
   expectTypeOf<Customer | Employee>().exclude<{name: string}>().toBeNever()
 })
 
+test('Use `.pick` to pick a set of properties from an object', () => {
+  type Person = {name: string; age: number}
+
+  expectTypeOf<Person>().pick<'name'>().toEqualTypeOf<{name: string}>()
+})
+
+test('Use `.omit` to remove a set of properties from an object', () => {
+  type Person = {name: string; age: number}
+
+  expectTypeOf<Person>().omit<'name'>().toEqualTypeOf<{age: number}>()
+})
+
 test('Make assertions about object properties', () => {
   const obj = {a: 1, b: ''}