[Feature]: Add utility for mocking a class #15723
Description
🚀 Feature Proposal
Add a utility jest.mockClass() for mocking a class.
Motivation
When testing classes in Jest, creating mocks is tedious and error-prone. You have to manually mock each method, which isn't type-safe and does not reflect source changes.
Pitch
Add a jest.mockClass() function that automatically creates a mock instance with all methods as jest.fn(). It should walk the prototype chain to include inherited methods.
Example
class SomeService {
constructor(private logger: Logger) {}
f() {
this.logger.info("calling f");
}
}
let someService: SomeService;
let logger: jest.Mocked<Logger>;
beforeEach(() => {
// Current way (tedious)
// This can expands to a hundred lines for a large test
logger = {
info: jest.fn(),
warn: jest.fn(),
error: jest.fn(),
} as unknown as jest.Mocked<Logger>;
// Proposed way (simple)
logger = jest.mockClass(Logger);
someService = new SomeService(logger);
});
test("should call logger.info", () => {
someService.f();
expect(logger.info).toHaveBeenCalled();
});Reference implementation
/** create a mock instance with each method being jest.fn() */
export function mockClass<T>(ctr: new (...args: never[]) => T): jest.Mocked<T> {
const instance: Record<string, jest.Mock> = {};
let currentProto = ctr.prototype;
while (currentProto && currentProto !== Object.prototype) {
for (const key of Object.getOwnPropertyNames(currentProto)) {
if (key === 'constructor') {
continue;
}
// Avoid overriding a method from a child class with one from a parent.
if (Object.prototype.hasOwnProperty.call(instance, key)) {
continue;
}
const prop = (currentProto as Record<string, unknown>)[key];
if (typeof prop === 'function') {
instance[key] = jest.fn();
}
}
currentProto = Object.getPrototypeOf(currentProto);
}
return instance as jest.Mocked<T>;
}