.mockreturnvalue is not a function

Charlotte

3 min read

·

01-03-2025

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The error ".mockReturnValue is not a function" in JavaScript testing often arises when using mocking libraries like Jest. This usually indicates a problem with how you're using the mockReturnValue method or a misunderstanding of the library's functionality. Let's dive into the common causes and solutions.

Understanding mockReturnValue

Before troubleshooting, let's clarify the purpose of mockReturnValue. This method is used within mocking libraries to define the return value of a mocked function. It allows you to control the output of a function during testing without actually executing its real implementation. This is crucial for isolating your unit tests and ensuring predictable results.

Common Causes and Solutions

The primary reason for the ".mockReturnValue is not a function" error is attempting to use mockReturnValue on something that isn't a mocked function. Let's break down the most frequent scenarios:

1. Incorrect Mocking Setup

The most frequent mistake is failing to properly mock the function before calling mockReturnValue. You need to use the mocking library's mock method (or similar) first. Here's how it should look using Jest:

// myModule.js (the module you're testing)
export const myFunction = () => {
  // ... some logic ...
  return "real value";
};


// myModule.test.js (your test file)
import { myFunction } from './myModule';

test('mocks the return value correctly', () => {
  myFunction.mockReturnValue('mocked value'); // Correct usage, now mocked
  expect(myFunction()).toBe('mocked value');
});

Solution: Ensure you've mocked the function correctly before calling mockReturnValue. Always check your import statements and the order of your mocking and assertion commands.

2. Mocking the Wrong Thing

You might be accidentally trying to apply mockReturnValue to a variable, object, or something else that isn't a function. The method only works on functions.

// Incorrect usage:
let myVariable = 5;
myVariable.mockReturnValue(10); // Error: myVariable is not a function

Solution: Verify that you are calling mockReturnValue on a function and not another data type. Carefully review your code to identify the exact object you're attempting to mock.

3. Incorrect Import or Module Setup

If you're importing the function incorrectly, or if the module isn't set up correctly, the mocking might not work as expected. Check for typos in your import statements and make sure the path to your module is accurate.

Solution: Double-check your import statement. Ensure the module containing the function you are attempting to mock is correctly imported. Consider using a relative path to avoid potential issues.

4. Version Conflicts or Library Issues

Occasionally, version conflicts between your testing libraries or unexpected library bugs can lead to this error.

Solution:

• Update your testing dependencies (Jest, etc.) to their latest versions.
• Check for any known issues related to your specific testing libraries in their GitHub repositories.
• Try creating a minimal reproducible example to isolate the issue.

5. Asynchronous Functions

When dealing with asynchronous functions (using async/await or Promises), you need to handle mocking differently. You often use mockResolvedValue or mockRejectedValue instead of mockReturnValue.

// myModule.js
export const asyncFunction = async () => {
  return Promise.resolve("async result");
};

// myModule.test.js
import { asyncFunction } from './myModule';

test('mocks async function', async () => {
  asyncFunction.mockResolvedValue('mocked async result'); // Correct for async
  const result = await asyncFunction();
  expect(result).toBe('mocked async result');
});

Solution: For asynchronous functions, use mockResolvedValue for successful promises or mockRejectedValue for rejected promises.

Debugging Tips

• Console Logging: Add console.log(myFunction) before using mockReturnValue to verify if the function is actually being mocked.
• Breakpoints: Use breakpoints in your debugger to step through your code and see exactly where the error occurs.
• Simplify: Create a minimal, reproducible example to isolate the problem.

By systematically investigating these potential causes and applying the suggested solutions, you can effectively resolve the ".mockReturnValue is not a function" error and ensure your tests run smoothly. Remember to always carefully check your mocking setup and the type of the object you're working with.