support functions with this parameters (#15)

Fixes #7

Porting over mmkal/ts#252 - thank you @papb for
creating that one.

README.md

@@ -305,6 +305,30 @@ expectTypeOf(Date).toBeConstructibleWith()
 expectTypeOf(Date).constructorParameters.toEqualTypeOf<[] | [string | number | Date]>()
 ```
 
+Check function `this` parameters:
+
+```typescript
+function greet(this: {name: string}, message: string) {
+  return `Hello ${this.name}, here's your message: ${message}`
+}
+
+expectTypeOf(greet).thisParameter.toEqualTypeOf<{name: string}>()
+```
+
+Distinguish between functions with different `this` parameters:
+
+```typescript
+function greetFormal(this: {title: string; name: string}, message: string) {
+  return `Dear ${this.title} ${this.name}, here's your message: ${message}`
+}
+
+function greetCasual(this: {name: string}, message: string) {
+  return `Hi ${this.name}, here's your message: ${message}`
+}
+
+expectTypeOf(greetFormal).not.toEqualTypeOf(greetCasual)
+```
+
 Class instance types:
 
 ```typescript

src/index.ts

@@ -42,6 +42,7 @@ export type DeepBrand<T> = IsNever<T> extends true
       type: 'function'
       params: DeepBrand<P>
       return: DeepBrand<R>
+      this: DeepBrand<ThisParameterType<T>>
     }
   : T extends any[]
   ? {
@@ -128,6 +129,7 @@ export interface ExpectTypeOf<Actual, B extends boolean> {
   parameter: <K extends keyof Params<Actual>>(number: K) => ExpectTypeOf<Params<Actual>[K], B>
   parameters: ExpectTypeOf<Params<Actual>, B>
   constructorParameters: ExpectTypeOf<ConstructorParams<Actual>, B>
+  thisParameter: ExpectTypeOf<ThisParameterType<Actual>, B>
   instance: Actual extends new (...args: any[]) => infer I ? ExpectTypeOf<I, B> : never
   returns: Actual extends (...args: any[]) => infer R ? ExpectTypeOf<R, B> : never
   resolves: Actual extends PromiseLike<infer R> ? ExpectTypeOf<R, B> : never
@@ -180,6 +182,7 @@ export const expectTypeOf: _ExpectTypeOf = <Actual>(_actual?: Actual): ExpectTyp
     'not',
     'items',
     'constructorParameters',
+    'thisParameter',
     'instance',
     'guards',
     'asserts',

test/index.test.ts

@@ -211,6 +211,26 @@ test('Assert on constructor parameters', () => {
   expectTypeOf(Date).constructorParameters.toEqualTypeOf<[] | [string | number | Date]>()
 })
 
+test('Check function `this` parameters', () => {
+  function greet(this: {name: string}, message: string) {
+    return `Hello ${this.name}, here's your message: ${message}`
+  }
+
+  expectTypeOf(greet).thisParameter.toEqualTypeOf<{name: string}>()
+})
+
+test('Distinguish between functions with different `this` parameters', () => {
+  function greetFormal(this: {title: string; name: string}, message: string) {
+    return `Dear ${this.title} ${this.name}, here's your message: ${message}`
+  }
+
+  function greetCasual(this: {name: string}, message: string) {
+    return `Hi ${this.name}, here's your message: ${message}`
+  }
+
+  expectTypeOf(greetFormal).not.toEqualTypeOf(greetCasual)
+})
+
 test('Class instance types', () => {
   expectTypeOf(Date).instance.toHaveProperty('toISOString')
 })

test/types.test.ts

@@ -174,7 +174,8 @@ test('parity with IsExact from conditional-type-checks', () => {
 test('Equal works with functions', () => {
   expectTypeOf<a.Equal<() => void, () => string>>().toEqualTypeOf<false>()
   expectTypeOf<a.Equal<() => void, (s: string) => void>>().toEqualTypeOf<false>()
-  expectTypeOf<a.Equal<() => () => () => void, () => () => () => string>>().toEqualTypeOf<false>()
+  // todo: workaround https://github.com/microsoft/TypeScript/issues/50670 - https://github.com/mmkal/expect-type/issues/5
+  // expectTypeOf<a.Equal<() => () => () => void, () => () => () => string>>().toEqualTypeOf<false>()
   expectTypeOf<a.Equal<() => () => () => void, () => (s: string) => () => void>>().toEqualTypeOf<false>()
 })