feat: spy() support having no callback

Author: apademide

.changeset/funky-comics-shave.md

@@ -0,0 +1,5 @@
+---
+"@auaust/toolkit": patch
+---
+
+`spy()` gracefully handles missing callbacks

src/functions/spy.ts

@@ -1,14 +1,42 @@
-export function spy<A extends any[], R, This>(
-  fn: (this: This, ...args: A) => R,
-  before?: ((this: This, ...args: A) => void) | null,
-  after?: ((this: This, result: R, ...args: A) => void) | null
-): (this: This, ...args: A) => R {
-  return function (this: This, ...args: A): R {
+export type SafeReturnType<F> = F extends (...args: any[]) => infer R
+  ? R
+  : undefined;
+
+export type SafeParameters<F> = F extends (...args: infer P) => any ? P : [];
+
+export function spy<F extends (this: This, ...args: any[]) => any, This>(
+  fn: F,
+  before?: ((this: This, ...args: Parameters<F>) => void) | null,
+  after?:
+    | ((this: This, result: ReturnType<F>, ...args: Parameters<F>) => void)
+    | null
+): (this: This, ...args: Parameters<F>) => ReturnType<F>;
+export function spy<
+  F extends ((...args: any[]) => any) | null | undefined,
+  This
+>(
+  fn: F,
+  before?: ((this: This, ...args: SafeParameters<F>) => void) | null,
+  after?:
+    | ((
+        this: This,
+        result: SafeReturnType<F>,
+        ...args: SafeParameters<F>
+      ) => void)
+    | null
+): (this: This, ...args: SafeParameters<F>) => SafeReturnType<F>;
+export function spy(
+  this: any,
+  fn: any,
+  before?: any,
+  after?: any
+): (...args: any[]) => any {
+  return function (this: any, ...args: any[]): any {
     if (typeof before === "function") {
       before.apply(this, args);
     }
 
-    const result = fn.apply(this, args);
+    const result = typeof fn === "function" ? fn.apply(this, args) : undefined;
 
     if (typeof after === "function") {
       after.call(this, result, ...args);

src/index.ts

@@ -33,7 +33,11 @@ export {
   type PipeEntry,
 } from "~/functions/pipe.js";
 export { sleep } from "~/functions/sleep.js";
-export { spy } from "~/functions/spy.js";
+export {
+  spy,
+  type SafeParameters,
+  type SafeReturnType,
+} from "~/functions/spy.js";
 export { tap } from "~/functions/tap.js";
 export { throttle } from "~/functions/throttle.js";
 export { value, type Value } from "~/functions/value.js";

tests/functions/spy.test.ts

@@ -69,4 +69,17 @@ describe("spy()", () => {
 
     expect(result).toBe(10);
   });
+
+  test("handles null or undefined closure gracefully", () => {
+    const before = vi.fn();
+    const after = vi.fn();
+
+    const spiedFunction = spy(null, before, after);
+
+    const result = spiedFunction();
+
+    expect(result).toBeUndefined();
+    expect(before).toHaveBeenCalledWith();
+    expect(after).toHaveBeenCalledWith(undefined);
+  });
 });