Spy

General

// Definition

A test double that wraps real behaviour and records calls, so the test can both let the real code run and inspect how it was used.

// Code Example

SinonSpy on a method to verify calls

TypeScript

import sinon from 'sinon';
import { logger } from './logger';

const spy = sinon.spy(logger, 'info');

doImportantWork();

expect(spy.calledOnce).toBe(true);
expect(spy.firstCall.args[0]).toBe('work complete');
spy.restore();

// Related terms

Mock
A test double that records how it was called and lets the test assert on those interactions. Pre-programmed with expectations about which methods will be called and how.
Stub
A test double that returns canned responses to method calls, replacing real behaviour without recording or asserting on calls.
Test Doubles
Umbrella term for any object that stands in for a real dependency in a test. Includes dummies, fakes, stubs, spies, and mocks — each with distinct semantics.