Extend

src/Types/CommonKeys.ts

@@ -0,0 +1,4 @@
+/**
+ * Extracts the common keys between two types
+ */
+export type CommonKeys<A, B> = Extract<keyof A, keyof B>

src/Types/Extend.ts

@@ -0,0 +1,66 @@
+import type { CommonKeys } from "./CommonKeys"
+
+
+export type Extend<T extends readonly object[]> = (
+    T extends readonly [
+        infer Super,
+        infer Self,
+        ...infer Rest extends readonly object[],
+    ]
+        ? Pick<Self, CommonKeys<Self, Super>> extends Pick<Super, CommonKeys<Self, Super>>
+            ? Extend<readonly [
+                Omit<Super, CommonKeys<Self, Super>> & Self,
+                ...Rest,
+            ]>
+            : never
+        : T extends readonly [infer Self]
+            ? Self
+            : {}
+)
+
+export type Override<T extends readonly object[]> = (
+    T extends readonly [
+        infer Super,
+        infer Self,
+        ...infer Rest extends readonly object[],
+    ]
+        ? Override<readonly [
+            Omit<Super, CommonKeys<Self, Super>> & Self,
+            ...Rest,
+        ]>
+        : T extends readonly [infer Self]
+            ? Self
+            : {}
+)
+
+export type Extendable<T extends readonly object[]> = (
+    T extends readonly [
+        infer Super,
+        infer Self,
+        ...infer Rest extends readonly object[],
+    ]
+        ? Pick<Self, CommonKeys<Self, Super>> extends Pick<Super, CommonKeys<Self, Super>>
+            ? Extendable<readonly [
+                Omit<Super, CommonKeys<Self, Super>> & Self,
+                ...Rest,
+            ]>
+            : false
+        : true
+)
+
+export type NonExtendableKeys<T extends readonly object[]> = (
+    T extends readonly [
+        infer Super   extends object,
+        infer Self    extends object,
+        ...infer Rest extends readonly object[],
+    ]
+        ? {[K in keyof Super & keyof Self]: Self[K] extends Super[K]
+            ? never
+            : K
+        }[keyof Super & keyof Self]
+        | NonExtendableKeys<readonly [
+            Super & Self,
+            ...Rest,
+        ]>
+        : void
+)

src/Types/index.ts

@@ -1 +1,2 @@
+export * from "./CommonKeys"
 export * from "./StaticType"