Implementation using mixins

src/tests-inheritance.ts

@@ -0,0 +1,45 @@
+import { extendsAndExpresses, trait } from "./trait-inheritance"
+
+
+const Identified = <ID>() =>
+    trait(Parent => {
+        abstract class Identified<ID> extends Parent {
+            abstract id: ID
+
+            equals(el: Identified<ID>) {
+                return this.id === el.id
+            }
+        }
+
+        return Identified<ID>
+    })
+
+
+const Permissible = trait(Parent => {
+    abstract class Permissible extends Parent {
+        static readonly defaultPermissions: string[] = []
+        permissions: string[] = []
+
+        constructor() {
+            super()
+            console.log("Permissible constructor")
+        }
+    }
+
+    return Permissible
+})
+
+
+const UserProto = extendsAndExpresses(class {}, [
+    Identified<bigint>(),
+    Permissible,
+] as const)
+
+class User extends UserProto {
+    constructor(readonly id: bigint) {
+        super()
+    }
+}
+
+const user1 = new User(1n)
+user1.equals(user1)

src/trait-inheritance.ts

@@ -0,0 +1,64 @@
+import { AbstractClass, Opaque, UnionToIntersection } from "type-fest"
+
+
+/**
+ * Represents the static members of a class.
+ *
+ * @template C - The type of the class for which static members are extracted.
+ * @typeparam The static members of the class.
+ */
+export type StaticMembers<C> = Pick<C, keyof C>
+
+
+export type Trait<C extends AbstractClass<any, any>> =
+    Opaque<(Parent: AbstractClass<any, any[]>) => C, "Trait">
+
+export type GetTraitC<T> =
+    T extends Trait<infer C>
+        ? C
+        : never
+
+
+export function trait<
+    C extends AbstractClass<any, any[]>
+>(
+    trait: (Parent: AbstractClass<any, any[]>) => C
+) {
+    return trait as Trait<C>
+}
+
+
+export function extendsAndExpresses<
+    C      extends AbstractClass<any, any>,
+    Traits extends readonly Trait<any>[],
+>(
+    extend: C,
+    traits: Traits,
+) {
+
+    return extend as unknown as (
+        AbstractClass<
+            InstanceType<C> &
+            UnionToIntersection<
+                InstanceType<
+                    GetTraitC<
+                        Traits[number]
+                    >
+                >
+            >,
+
+            ConstructorParameters<C>
+        > &
+
+        StaticMembers<C> &
+
+        StaticMembers<
+            UnionToIntersection<
+                GetTraitC<
+                    Traits[number]
+                >
+            >
+        >
+    )
+
+}