add type decoration utilities

Author: ruxxzebre

types/utility.d.ts

@@ -40,4 +40,26 @@ type IfEquals<T, U, Y = true, N = false> =
     (<G>() => G extends T ? 1 : 0) extends
     (<G>() => G extends U ? 1 : 0) ? Y : N;
 
+/**
+ * @summary Extracts 'this' parameter from a function, if it exists. Otherwise, returns fallback.
+ * @param {T} T Function type to extract 'this' parameter from.
+ * @param {F} F Fallback type to return if 'this' parameter does not exist.
+ */
+type ThisParameter<T, F> = T extends { (this: infer This): void }
+  ? This
+  : F;
+
+/**
+ * @summary Decorates all functions in an object with 'this' parameter.
+ * @param {T} T Object with functions as values to add 'D' parameter to as 'this'. {@link D}
+ * @param {D} D The type to be added as 'this' parameter to all functions in {@link T}.
+ */
+type AddThisParameter<T, D> = {
+  [K in keyof T]: T[K] extends (...args: infer A) => infer R
+    ? ThisParameter<T[K], unknown> extends unknown
+      ? (this: D, ...args: A) => R
+      : T[K]
+    : T[K];
+};
+
 }