fix: called spied on functions with 'this' (#463)

Author: ojeytonwilliams

packages/helpers/lib/index.ts

@@ -36,29 +36,39 @@ export class RandomMocker {
   }
 }
 
+// eslint-disable-next-line @typescript-eslint/no-unsafe-function-type
+function assertIsFunction(func: unknown): asserts func is Function {
+  if (!(typeof func === "function")) {
+    throw new TypeError("spyOn can only be called on function properties");
+  }
+}
+
 /** Calling spyOn creates a simple spy, inspired by Jest and Jasmine's spyOn functions.
  * @param obj - The object to spy on
  * @param method - The method to spy on
  * @returns A spy function that can be used to track calls to the original method
  */
 export function spyOn<Args extends unknown[], Return>(
-  obj: Record<string, (...args: Args) => Return>,
+  obj: Record<string, unknown>,
   method: string,
 ): {
   restore: () => void;
   calls: Args[];
   returns: Return[];
 } {
   const original = obj[method];
+  assertIsFunction(original);
+
   const calls: Args[] = [];
   const results: Return[] = [];
 
-  const fn = (...args: Args) => {
+  function fn(this: unknown, ...args: Args): unknown {
     calls.push(args);
-    const result = original(...args);
+    // If fn is called, original will be a function.
+    const result = (original as (...x: Args) => Return).apply(this, args);
     results.push(result);
     return result;
-  };
+  }
 
   obj[method] = fn;
 

packages/tests/curriculum-helper.test.tsx

@@ -132,6 +132,32 @@ describe("spyOn", () => {
     obj.method("arg");
     expect(spy.calls).toEqual([]);
   });
+
+  it("should call the original function, providing 'this'", () => {
+    const obj = {
+      value: "obj",
+      method(arg = "") {
+        return this.value + arg;
+      },
+    };
+
+    const spy = helper.spyOn(obj, "method");
+    const result = obj.method("arg");
+    expect(result).toBe("objarg");
+    expect(spy.calls).toEqual([["arg"]]);
+  });
+
+  it("should throw if the spied on property is not a function", () => {
+    const obj = {
+      notAFunction: "foo",
+    };
+
+    vitest.spyOn(console, "warn").mockImplementation(() => {});
+
+    expect(() => helper.spyOn(obj, "notAFunction")).toThrow(
+      "spyOn can only be called on function properties",
+    );
+  });
 });
 
 describe("spyOnCallbacks", () => {