I am writing this story to explain why it is not straight forward and what is the possible options or tools we can use to do that. If you need to check that certain functions are called in order, you can use spies or stubs together with sinon.assert.callOrder: Here's the code we will test. Benchmark.js simplifies micro-benchmark testing on different components and their methods. Wrapping a test with sinon.test() allows us to use Sinon’s sandboxing feature, allowing us to create spies, stubs and mocks via this.spy(), this.stub() and this.mock(). It is best practice to stub the method you expect to use on the same type of object you use in your code. Beware that this is inferred based on the value of the this object and the spy function’s prototype, so it may give false positives if you actively return the right kind of object. Let's say we have this function that updates the database: Sinon Stub API.returns (obj) - specify that whenever call Stub it will return the Object passed as param.throws - tells Sinon to throw general exception whenever given Stub is called.throws ("type") - tells Sinon to throw a particular type of exception whenever given Stub is called Test 2 fails with: AssertionError: returned value is the fakedata from the stub. Due to this fact it's not viable to make it accept property descriptors as values, because then we wouldn't be able to know whether the user wants to pass a property descriptor or an simple object to replace that property. sandbox = sinon.createSandbox(); fileStub = sandbox.stub(AwsEksDetector, 'fileAccessAsync' as any).resolves(true); Benchmark.js. If you need to support very old runtimes that have incomplete support for ES5.1 you might get away with using loading es5-shim in your test environment. This behaves the same as spy.alwaysCalledWith(sinon.match(arg1), sinon.match(arg2), ...). Sinon.JS used to stub properties and methods in a sandbox. Without it, the stub may be left in place and it may cause problems in other tests. Well, yes. There should not be any issues with using Sinon v9.2.2 in newer versions of the same runtimes. The sandbox stub method can also be used to stub any kind of property. Sinon will throw a very helpful error if the method you attempt to stub doesn’t exist. Mocking Time with sinon.js – The Chronicles of Nerdia, There are situations where new Date() or Date.now is used in to remember how to mock/stub or spy on Date.now / new Date with Jest. Methods and properties are restored after test(s) are run. Test 1 passes. However it is not straight forward . Sinon helps you create mocks, stubs and spies to help with unit testing. - stub-properties-and-methods-sinon.js Checking the Order of Function Calls or Values Being Set. spy.calledWithNew(); Returns true if spy/stub was called the new operator. This will avoid writing unit tests that pass if the code is using non-existent methods: I see sandboxing is an option but I do not see how you can use a sandbox for this. The log messages show that it has printed the data from the file rather than the fakedata. Sinon sandbox makes stubbing much easier. All we have to do is wrap the component around the declared object and call its methods. Is that possible? When I inspect the this.sandbox.loader function in the debugger it is correctly set to the stub. Component around the declared object and call its methods was called the new operator used to stub method... 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