Andrew Seaford

In this article we are demostrate how to mock a function that returns a value.

Procedure

• Create the react app npc create-react-app demo-mocking-return-value.
• Change directory cd demo-mocking-return-value

• Create the file DemoMockReturnValue.js in the src directory. A referance to the function f is passed to the component as a prop. The function f is called and the result from the function is used to update the div with the test id myresult.
  DemoMockReturnValue.js

• Create the file DemoMockReturnValue.test.js in the src directory. The unit test will create a mocked function f using the jest library const f = jest.fn();. The function is confirmed to return a value using the code f.mockReturnValue(expectedValue);. The assert confirms that the div with the test id myresult contains the expected data.
  DemoMockReturnValue.test.js

• Run the tests npm test