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Learn how to effectively spy on default exported functions using Jest, ensuring accurate unit tests for your JavaScript applications.
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How to Spy on Default Exported Functions with Jest

In the realm of JavaScript testing, particularly with frameworks such as React, ensuring that your functions are called correctly during various operations is paramount. This includes critical utility functions such as logging, which might not return a value but serves vital diagnostic purposes. In this guide, we’ll delve into a common problem many developers face: how to spy on a default exported function using Jest when testing React components.

The Problem

Let’s set the stage with a simple overview of the situation. Your JavaScript application consists of various components, and among them is a logger function designed to log error messages to the console whenever certain operations fail. You are tasked with testing an addCampus method. This method attempts to create a new campus entry, and in the event of an error, it should log that error using the logger function. However, the challenge arises when you attempt to verify whether the logger was called during your tests.

Here's the relevant code snippet for context:

Logger.js

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AddCampusList.jsx

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Your test file (AddCampusList.test.js) sets up various spies and mocks:

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The issue arises where Jest offers the following error message: "Cannot spy the default property because it is not a function; undefined given instead." This indicates that the approach to spying is not set up correctly.

The Solution

Understanding the Default Export

The logger function is exported as a default function. With default exports, there’s no named property called "default" that you can spy on directly. Should we approach the problem correctly, here's how to do it:

Step 1: Use the Correct Import Syntax

You will want to modify how you import and spy on the logger function. Instead of attempting to spy directly on the default export, you can import the module in a way that allows you to access the default export properly:

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However, because ES modules can have read-only properties, this might not always function as intended depending on your environment setup.

Step 2: Use Jest Mock for Modules

A much more reliable method is to mock the module directly, allowing you to define what the logger’s default function should be. Here’s how it can be achieved:

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Step 3: A Simpler Approach

In many instances, especially for modules like the logger that do not have complicated side effects, it may be even more effective to just mock the logger entirely:

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This method is straightforward and avoids possible pitfalls related to spying on function calls when dealing with default exports.

Conclusion

Testing your JavaScript code effectively, especially with utilities like logging functions, can be tricky as you've seen. Understanding how to work with default exports in Jest is crucial for successful unit testing. By either utilizing the correct import method or mocking the modules, you'll ensure your tests run smoothly and accurately reflect the intended behavior of your application.

Now you're armed with a strategy to not only spy on default exported functions in Jest but also enhance the overall quality of your unit tests. Happy testing!