From d263c15393d9e4ef0553770234b09258c85cd645 Mon Sep 17 00:00:00 2001
From: Kir_Antipov <kp.antipov@gmail.com>
Date: Sat, 3 Dec 2022 05:12:11 +0000
Subject: [PATCH] Added a possibility to make objects callable

---
 src/utils/functions/callable.ts             | 47 ++++++++++++++++++++
 tests/unit/utils/functions/callable.spec.ts | 49 +++++++++++++++++++++
 2 files changed, 96 insertions(+)
 create mode 100644 src/utils/functions/callable.ts
 create mode 100644 tests/unit/utils/functions/callable.spec.ts

diff --git a/src/utils/functions/callable.ts b/src/utils/functions/callable.ts
new file mode 100644
index 0000000..a079002
--- /dev/null
+++ b/src/utils/functions/callable.ts
@@ -0,0 +1,47 @@
+/**
+ * A symbol representing the `call` function of a {@link Callable} object.
+ */
+export const CALL = Symbol.for("call");
+
+/**
+ * Represents an object, which can be converted into a {@link Callable} one.
+ */
+interface SemiCallable {
+    /**
+     * A method that should be invoked, when an object is used as a function.
+     */
+    [CALL](...args: unknown[]): unknown;
+}
+
+/**
+ * Represents an object, which can be called like a function.
+ *
+ * @template T - The type of the underlying object.
+ */
+export type Callable<T extends SemiCallable> = T & {
+    /**
+     * Redirects a call to the {@link CALL} function.
+     */
+    (...args: Parameters<T[typeof CALL]>): ReturnType<T[typeof CALL]>;
+};
+
+/**
+ * Makes an object callable.
+ *
+ * @template T - The type of the object.
+ * @param obj - The object to make callable.
+ *
+ * @returns A new {@link Callable} object with the same properties as the original one, but which can be called like a function.
+ */
+export function makeCallable<T extends SemiCallable>(obj: T): Callable<T> {
+    /**
+     * Redirects a call to the {@link CALL} function.
+     */
+    function call(...args: unknown[]): unknown {
+        return (call as unknown as T)[CALL](...args);
+    }
+
+    Object.assign(call, obj);
+    Object.setPrototypeOf(call, Object.getPrototypeOf(obj));
+    return call as unknown as Callable<T>;
+}
diff --git a/tests/unit/utils/functions/callable.spec.ts b/tests/unit/utils/functions/callable.spec.ts
new file mode 100644
index 0000000..938bf4d
--- /dev/null
+++ b/tests/unit/utils/functions/callable.spec.ts
@@ -0,0 +1,49 @@
+import { CALL, makeCallable } from "@/utils/functions/callable";
+
+describe("makeCallable", () => {
+    test("makes an object callable", () => {
+        const obj = {
+            [CALL]: (a: number, b: number) => a + b,
+        };
+
+        const callable = makeCallable(obj);
+
+        expect(callable(1, 2)).toBe(3);
+    });
+
+    test("preserves object properties", () => {
+        const obj = {
+            foo: 42,
+
+            [CALL](): number {
+                return this.foo;
+            },
+        };
+
+        const callable = makeCallable(obj);
+
+        expect(callable()).toBe(42);
+        expect(callable.foo).toBe(42);
+    });
+
+    test("preserves object prototype", () => {
+        class FooClass {
+            foo: number;
+
+            constructor(foo: number) {
+                this.foo = foo;
+            }
+
+            [CALL](): number {
+                return this.foo;
+            }
+        }
+
+        const obj = new FooClass(42);
+        const callable = makeCallable(obj);
+
+        expect(callable).toBeInstanceOf(FooClass);
+        expect(callable.foo).toBe(42);
+        expect(callable()).toBe(42);
+    });
+});