Convert extend util to TypeScript (#2928)

* Allow using file extension in core compat imports

Necessary for extend imports to have proper typings as we also have an unrelated extend/index.js file

* Add .ts file extension to extend imports for typings

* Fix changes to proxifyCompat regex breaking non-core import paths

* Move utility types to global types

Co-authored-by: David Wheatley <hi@davwheat.dev>

js/src/common/extend.ts

@@ -0,0 +1,91 @@
+/**
+ * Extend an object's method by running its output through a mutating callback
+ * every time it is called.
+ *
+ * The callback accepts the method's return value and should perform any
+ * mutations directly on this value. For this reason, this function will not be
+ * effective on methods which return scalar values (numbers, strings, booleans).
+ *
+ * Care should be taken to extend the correct object – in most cases, a class'
+ * prototype will be the desired target of extension, not the class itself.
+ *
+ * @example <caption>Example usage of extending one method.</caption>
+ * extend(Discussion.prototype, 'badges', function(badges) {
+ *   // do something with `badges`
+ * });
+ *
+ * @example <caption>Example usage of extending multiple methods.</caption>
+ * extend(IndexPage.prototype, ['oncreate', 'onupdate'], function(vnode) {
+ *   // something that needs to be run on creation and update
+ * });
+ *
+ * @param object The object that owns the method
+ * @param methods The name or names of the method(s) to extend
+ * @param callback A callback which mutates the method's output
+ */
+export function extend<T extends object, K extends KeyOfType<T, Function>>(
+  object: T,
+  methods: K | K[],
+  callback: (this: T, val: ReturnType<T[K]>, ...args: Parameters<T[K]>) => void
+) {
+  const allMethods = Array.isArray(methods) ? methods : [methods];
+
+  allMethods.forEach((method: K) => {
+    const original: Function | undefined = object[method];
+
+    object[method] = function (this: T, ...args: Parameters<T[K]>) {
+      const value = original ? original.apply(this, args) : undefined;
+
+      callback.apply(this, [value, ...args]);
+
+      return value;
+    } as T[K];
+
+    Object.assign(object[method], original);
+  });
+}
+
+/**
+ * Override an object's method by replacing it with a new function, so that the
+ * new function will be run every time the object's method is called.
+ *
+ * The replacement function accepts the original method as its first argument,
+ * which is like a call to `super`. Any arguments passed to the original method
+ * are also passed to the replacement.
+ *
+ * Care should be taken to extend the correct object – in most cases, a class'
+ * prototype will be the desired target of extension, not the class itself.
+ *
+ * @example <caption>Example usage of overriding one method.</caption>
+ * override(Discussion.prototype, 'badges', function(original) {
+ *   const badges = original();
+ *   // do something with badges
+ *   return badges;
+ * });
+ *
+ * @example <caption>Example usage of overriding multiple methods.</caption>
+ * extend(Discussion.prototype, ['oncreate', 'onupdate'], function(original, vnode) {
+ *   // something that needs to be run on creation and update
+ * });
+ *
+ * @param object The object that owns the method
+ * @param methods The name or names of the method(s) to override
+ * @param newMethod The method to replace it with
+ */
+export function override<T extends object, K extends KeyOfType<T, Function>>(
+  object: T,
+  methods: K | K[],
+  newMethod: (this: T, orig: T[K], ...args: Parameters<T[K]>) => void
+) {
+  const allMethods = Array.isArray(methods) ? methods : [methods];
+
+  allMethods.forEach((method) => {
+    const original: Function = object[method];
+
+    object[method] = function (this: T, ...args: Parameters<T[K]>) {
+      return newMethod.apply(this, [original.bind(this), ...args]);
+    } as T[K];
+
+    Object.assign(object[method], original);
+  });
+}