The jest object is automatically in scope within every test file. We’ll start with a fictional utils.js file that contains three methods that are all exported as named exports:. Instead of mocking out fetch which is a built-in browser API we simply create a wrapper around it. We call jest.mock('../request') to tell Jest to use our manual mock. export const method1 = => 'You have called Method 1' export const method2 = => 'You have called Method 2' export const method3 = => 'You have called Method 3' This API will allow more expressive mocks, extending on top of the idea of mockReturnValue … For Jest to mock the exports, the property __esModule must be enabled in the return value: Mock functions helps us make testing of links between code easy, by erasing the actual implementation of a function, capturing the calls to the function (and the parameters passed in those calls), capturing the instances of constructor functions when instantiated with the new keyword, and finally allowing test-time configuration of return values. First, let’s consider how we would test a module that only exports named exports. Now as soon as the verify function is called inside jwt, we mock the return value of jwt using jest’s mockReturnValue according to what we need. The core assertions we tend to use for spies and stubs are used to answer the following questions: was the stub/spy called? let mockFunction: jest.Mock it's helpful to look at it as . I would like to mock a function with Jest, but only if it is called with specific arguments, for example: function sum(x, y) { return x + y; } // mock sum(1, 1) to return 4 sum(1, 1) // returns 4 (mocked) sum(1, 2) // returns 3 (not mocked) There is a similar feature implemented in the Ruby's RSpec library: Mocking Named Exports. Looking at jest.mock, the jest.mock part stays. The methods in the jest object help create mocks and let you control Jest's overall behavior. 🚀Feature Proposal. ... where the only input are the parameters and the only output is the return value. The first value is what you plan on returning, while the second value is actually an array of the inputs. Assertions for a spy/mock/stub beyond Jest. If no implementation is provided, it will return the undefined value. 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