feat: add assertType.hasProperty() (#50)

README.md

@@ -100,6 +100,7 @@ assertType.isFalse(assignability<YourType>()(subject))
 - `assertType<T>(subject)`: assert `subject` satisfies type `T`.
 - `assertType.isXXX(value)`: ensure typeof `value` is `XXX`
 - `assertType.noXXX(value)`: ensure typeof `value` does not contain `XXX`. i.e. cannot assign `XXX` to `value`.
+- `assertType.hasProperty(value, prop)`: assert `value` has property `prop`. This will pick the correct union type.
 - `typeAssert.*` (deprecated) replaced by `assertType`.
 - `typeAssertion<T>()`: creates a type assertion function of type `T`
 

src/assertType.spec.ts

@@ -168,3 +168,18 @@ describe('noString()', () => {
     // typeAssert.noString('a' as string | undefined)
   })
 })
+
+describe('hasProperty', () => {
+  type X = { name: string } & ({ a: 1 } | { b: 2 })
+
+  const x: X = { name: 'n', a: 1 } as any
+
+  assertType.hasProperty(x, 'a')
+
+  expect(x.a).toBe(1)
+
+  const y: X = { name: 'n', b: 2 } as any
+  assertType.hasProperty(y, 'b')
+
+  expect(y.b).toBe(2)
+});

src/assertType.ts

@@ -1,3 +1,5 @@
+import { UnionKeys } from './UnionKeys'
+
 /**
  * assert the subject satisfies the specified type T
  * @type T the type to check against.
@@ -17,6 +19,10 @@ assertType.isFalse = noop as (value: false) => void
 assertType.isString = noop as (value: string) => void
 assertType.noString = noop as <T>(value: Exclude<T, string>) => void
 assertType.isNever = noop as (value: never) => void
+
+// eslint-disable-next-line @typescript-eslint/no-unused-vars
+assertType.hasProperty = function hasProperty<T, P extends UnionKeys<T>>(value: T, prop: P): asserts value is T & Record<P, T[P]> { }
+
 // assertType.hasProperty = <T>(value: T, propertyName: KeyTypes)
 function noop() { return }