This line stubs the getRandom function to always return 1 … functions don't keep their reference so stubbing it will change the ref and the original inclusion won't be updated. The goal is to mock fetchData call in feature.js when writing functional tests. Setup Module mock using jest.mock() Function mock using jest.fn() # The simplest and most common way of creating a mock is jest.fn() method. jest.mock does this automatically for all functions in a module jest.spyOn does the same thing but allows restoring the original function Mock a module with jest.mock afaik. A mock is a mixture between a spy and a stub, so it implements the API of both of them. What I do here is wrap function's exposed by modules inside another module that wraps the said function in an object that can be stubbed easily. Ideally our test should look something like this: QUnit . To do this we need a way to Mock ChildA and ChildB and intercept the properties passed to them. create ( ) ; //Set up the spec helper. Fakes, In Sinon, a fake is a Function that records arguments, return value, the value of To plug the fakes into the system under test, you can use the sinon.replace* Sinon stubs the propertyof the object, not the function itself. In our example, we will use Sinon.JS, but Jasmine can be used as well. // feature.js module import { fetchData } from './backend'; export function doSomething() { // some code which calls fetchData } feature.js imports fetchData function from backend.js. sandbox = Sinon . Basically to mock a method on Helper class just get the reference of the function through class prototype and stub the same. If no implementation is provided, it will return the undefined value. There is plenty of helpful methods on returned Jest mock to control its input, output and implementation. This is done at the outer-most scope of our test suite so that this whole collection of tests can use mocked function. We can check what arguments were passed to a function using sinon.assert.calledWith, or by accessing the call directly using spy.lastCall or spy.getCall(). In addition to Sinon.js, we will be using Mocha and Chai to setup and run the tests. You can read our guide our guide to learn more about them before continuing. A stub is a spy with predetermined behavior.. We can use a stub to: Take a predetermined action, like throwing an exception; Provide a predetermined response; Prevent a specific method from being called directly (especially when it triggers undesired behaviors like HTTP requests) Using Sinon.js to Create a Mock. We use Sinon to mock Typescript modules by using mockModule to create a function that can mock the given module. sandbox . Let’s have a look at a few examples. module ( "Test a parent component" , { beforeEach : function ( ) { this . In addition to spies and stubs, Sinon has another element called mock which may be useful in our unit tests. 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