.mockreturnvalue is not a function

Breiz Heurope

3 min read

·

10-03-2025

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The error ".mockReturnValue is not a function" is a common issue encountered when using mocking libraries like Jest in JavaScript testing. This comprehensive guide will help you understand the root causes and provide effective solutions to resolve this frustrating problem. We'll cover various scenarios and offer practical debugging techniques.

Understanding the Error

This error arises when you attempt to use the .mockReturnValue() method on something that isn't a mock function. Essentially, you're trying to apply a mocking function to something that isn't set up for mocking. This often happens due to incorrect setup or misunderstandings about how mocking works.

Common Causes and Solutions

Let's explore the most frequent reasons behind this error and how to fix them:

1. Incorrect Mocking of the Function

The most prevalent cause is failing to properly mock the function before attempting to use .mockReturnValue(). You need to use the jest.mock() function to set up your mock.

Incorrect:

// Incorrect - No mocking setup
myFunction(); // This will throw the error if you try to use .mockReturnValue after this line
myFunction.mockReturnValue('mocked value'); 

Correct:

jest.mock('./myModule'); // Assuming myFunction is in myModule.js

import { myFunction } from './myModule';

test('my test', () => {
  myFunction.mockReturnValue('mocked value');
  expect(myFunction()).toBe('mocked value');
});

Explanation: The jest.mock('./myModule') line is crucial. It tells Jest to create a mock for the myFunction imported from myModule.js. Without this, myFunction remains the original function, and .mockReturnValue() won't work.

2. Mocking the Wrong Object

Sometimes, you might be attempting to mock a variable or object instead of a function. .mockReturnValue() only applies to mocked functions.

Incorrect:

let myObject = { value: 5 };
myObject.mockReturnValue(10); // Error!

Correct: If you need to mock a property of an object, you'd typically mock the object's method or modify the object directly within the test.

let myObject = {
  getValue: () => 5,
};

test('testing object method', () => {
    myObject.getValue.mockReturnValue(10);
    expect(myObject.getValue()).toBe(10);
});

or

test('modifying object directly', () => {
    const myObject = {value: 5};
    myObject.value = 10;
    expect(myObject.value).toBe(10);
});

3. Incorrect Import or Module Resolution

Ensure your import statement correctly targets the module containing the function you are trying to mock. Double-check for typos and that the module path is accurate relative to your test file. If you're using a module bundler like Webpack, ensure it's configured correctly to resolve module paths for your tests.

4. Asynchronous Operations

If your function is asynchronous (uses async/await or promises), you might need to use mockResolvedValue or mockRejectedValue instead of mockReturnValue.

const myAsyncFunction = jest.fn();
myAsyncFunction.mockResolvedValue('async value');

test('Testing async function', async () => {
  await expect(myAsyncFunction()).resolves.toBe('async value');
});

5. Forgotten jest.fn()

For functions you are creating and immediately mocking within a test, you need to explicitly create a mock function using jest.fn().

const myFunction = jest.fn(); //Create a mock function
myFunction.mockReturnValue('mocked value');

test('testing simple mock', () => {
    expect(myFunction()).toBe('mocked value')
});

Debugging Tips

1. Console Logging: Add console.log(typeof myFunction) before using .mockReturnValue() to verify that myFunction is a function.
2. Check Jest Configuration: Confirm that Jest is configured correctly in your package.json or a Jest configuration file (jest.config.js).
3. Simplify Your Test: Isolating the problem by creating a minimal test case can often pinpoint the issue.
4. Read the Error Message Carefully: The error message itself often gives clues about the line of code causing the problem.

By carefully examining your code and following these troubleshooting steps, you can effectively resolve the ".mockReturnValue is not a function" error and ensure your tests run smoothly. Remember to always thoroughly mock your dependencies before making assertions in your test cases.