.mockreturnvalue is not a function

Daniel Parker

3 min read

·

27-02-2025

1

0

Share

The error ".mockReturnValue is not a function" in Jest testing typically arises when you're attempting to use the mockReturnValue method on something that isn't a mock function. This means Jest doesn't recognize the object you're trying to manipulate as a mocked function created by Jest. Let's explore the common causes and how to resolve this issue.

Understanding the Problem

Jest's mocking capabilities allow you to replace real functions with mock functions that return predefined values or exhibit specific behaviors during testing. The .mockReturnValue method is crucial for setting the return value of a mocked function. The error appears when you call this method on an object that hasn't been properly mocked by Jest.

Common Causes and Solutions

Here are the most frequent reasons for the ".mockReturnValue is not a function" error and how to fix them:

1. Forgetting to Mock the Function

The most basic cause is simply not mocking the function you intend to test. Jest needs to know it should replace the real function with a mock before you can use methods like mockReturnValue.

Incorrect:

// myModule.js
export function myFunction() {
  return someExternalAPI(); 
}

// myModule.test.js
import { myFunction } from './myModule';
myFunction.mockReturnValue('mocked value'); // Error!

Correct:

// myModule.test.js
import { myFunction } from './myModule';
jest.mock('./myModule'); // Mock the entire module

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

//or, if you only need to mock a specific function:
import { myFunction } from './myModule';

jest.mock('./someExternalAPI', () => ({
  __esModule: true,
  default: jest.fn(() => "mocked external API response")
}));

test('myFunction returns mocked value', () => {
    expect(myFunction()).toBe("mocked external API response")
});

In the corrected version, jest.mock('./myModule') tells Jest to replace the myModule with a mocked version before the test runs. Alternatively, you can selectively mock functions within a module.

2. Incorrect Mocking Syntax

Ensure you are using the correct syntax to mock your function, especially when dealing with imported modules or ES6 modules. Incorrect module imports can lead to attempting to use .mockReturnValue on the original function instead of the mock.

3. Mocking an Object, Not a Function

.mockReturnValue only works on mock functions. If you're trying to use it on a plain object, class instance, or something else that's not a function, you'll get this error.

Incorrect:

const myObject = { someMethod: () => {} };
myObject.someMethod.mockReturnValue('test'); // Error!

Correct:

jest.mock('./myModule', () => ({
    __esModule: true,
    default: jest.fn(() => ({ someMethod: jest.fn(() => 'test') }))
  }));
  
  const myObject = require('./myModule').default();
  expect(myObject.someMethod()).toBe('test');

Here, we mock the someMethod within the object.

4. Asynchronous Functions and Promises

When dealing with asynchronous functions that return promises, you need to use .mockResolvedValue or .mockRejectedValue instead of .mockReturnValue.

Incorrect:

const myAsyncFunction = jest.fn(() => Promise.resolve('async value'));
myAsyncFunction.mockReturnValue('wrong'); // Error, or incorrect behavior!

Correct:

const myAsyncFunction = jest.fn(() => Promise.resolve('async value'));
myAsyncFunction.mockResolvedValue('resolved value'); 

5. Typographical Errors

Double-check for any typos in your code. A simple misspelling of mockReturnValue can cause this error.

Debugging Tips

• Console Logging: Use console.log to inspect the object you're calling .mockReturnValue on. This helps you determine its type and whether it's a mock function.
• Jest Watch Mode: Run Jest in watch mode (jest --watch) to automatically rerun tests when you make changes. This makes debugging faster.
• Simplify Your Tests: Isolate the problematic part of your test to pinpoint where the error originates.

By carefully reviewing these points and using the debugging tips, you should be able to effectively resolve the ".mockReturnValue is not a function" error in your Jest tests. Remember to always ensure you're mocking the correct object and using the appropriate mocking method based on the function's return type.