feat: Add PublicInterface type (#197)

Add a type for deriving the "public interface" of a type, excluding
private properties. Excluding private properties can be useful in making
our classes more easily testable (types with private properties are more
difficult to mock).

src/misc.test-d.ts

@@ -1,8 +1,28 @@
 import { expectAssignable, expectNotAssignable, expectType } from 'tsd';
 
-import type { RuntimeObject } from './misc';
+import type { PublicInterface, RuntimeObject } from './misc';
 import { isObject, hasProperty, getKnownPropertyNames } from './misc';
 
+//=============================================================================
+// PublicInterface
+//=============================================================================
+
+class ClassWithPrivateProperties {
+  #foo: string;
+
+  bar: string;
+
+  constructor({ foo, bar }: { foo: string; bar: string }) {
+    this.#foo = foo;
+    this.bar = bar;
+  }
+}
+
+// Private properties not required
+expectAssignable<PublicInterface<ClassWithPrivateProperties>>({ bar: 'bar' });
+// Public properties still required
+expectNotAssignable<PublicInterface<ClassWithPrivateProperties>>({});
+
 //=============================================================================
 // isObject
 //=============================================================================

src/misc.ts

@@ -17,6 +17,15 @@ export type Mutable<
   [Key in keyof Omit<ObjectValue, TargetKey>]: ObjectValue[Key];
 };
 
+/**
+ * Get a type representing the public interface of the given type. The
+ * returned type will have all public properties, but will omit private
+ * properties.
+ *
+ * @template Interface - The interface to return a public representation of.
+ */
+export type PublicInterface<Interface> = Pick<Interface, keyof Interface>;
+
 /**
  * Useful for representing some value that _might_ be present and / or complete.
  *