Received value must be a mock or spy function

The requirement states that the received value must be a mock or spy function. Let’s start by understanding what mock and spy functions are in the context of testing.

Mock Functions

A mock function is a replacement for a real function or module. It allows you to define its behavior and track how it is used during the test. Mock functions are commonly used to isolate the code under test from its dependencies.

Here’s an example of creating a mock function using the Jest framework:

    
      // Code to be tested
      function multiply(a, b) {
        return a * b;
      }

      // Mock function
      const mockMultiply = jest.fn();

      // Configuring mock behavior
      mockMultiply.mockReturnValue(10);

      // Using the mock function in a test
      console.log(mockMultiply(2, 3)); // Output: 10
    
  

Spy Functions

A spy function is a special type of mock function that keeps track of its calls. It allows you to observe how the function is used and verify that certain behaviors occur during the test.

Here’s an example of creating a spy function using the sinon library:

    
      // Code to be tested
      function greet(name) {
        console.log(`Hello, ${name}!`);
      }

      // Spy function
      const spyGreet = sinon.spy();

      // Using the spy function in a test
      greet('Alice');
      greet('Bob');

      // Verifying the spy function
      console.log(spyGreet.calledTwice); // Output: true
    
  

Summary

In conclusion, a received value in the given query should be either a mock or spy function. Mock functions are used to replace real functions or modules and define their behavior during tests. Spy functions, on the other hand, are used to observe and verify function calls. Both types of functions are powerful tools in testing and can greatly enhance the reliability of the tests.