Unlocking Private Methods in TypeScript Classes for Unit Testing

Description

Discover how to effectively test private methods in TypeScript classes using rewire for your Node.js modules. Learn step-by-step solutions and best practices. --- This video is based on the question https://stackoverflow.com/q/65846584/ asked by the user 'Wyctus' ( https://stackoverflow.com/u/14758800/ ) and on the answer https://stackoverflow.com/a/67741151/ provided by the user 'MajinDageta' ( https://stackoverflow.com/u/3706647/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions. Visit these links for original content and any more details, such as alternate solutions, latest updates/developments on topic, comments, revision history etc. For example, the original title of the Question was: Rewire private Typescript class methods Also, Content (except music) licensed under CC BY-SA https://meta.stackexchange.com/help/licensing The original Question post is licensed under the 'CC BY-SA 4.0' ( https://creativecommons.org/licenses/by-sa/4.0/ ) license, and the original Answer post is licensed under the 'CC BY-SA 4.0' ( https://creativecommons.org/licenses/by-sa/4.0/ ) license. If anything seems off to you, please feel free to write me at vlogize [AT] gmail [DOT] com. --- Unlocking Private Methods in TypeScript Classes for Unit Testing When developing applications in TypeScript, especially with frameworks like Node.js, it’s common to face the challenge of testing private methods within your classes. This can become particularly frustrating when using tools such as Mocha and rewire, as they do not easily allow access to private methods. In this guide, we will discuss how to effectively rewire private TypeScript class methods to enhance your unit testing efforts. The Problem Imagine you have a TypeScript class as shown below: [[See Video to Reveal this Text or Code Snippet]] Now, you want to write tests for the private method somePrivateMethod(), but upon attempting to use rewire, you encounter the error: [[See Video to Reveal this Text or Code Snippet]] This issue arises because private class methods are hidden from outside access, which makes them inaccessible even through rewire, whether you try it on the TypeScript file or the compiled JavaScript file. The Solution To address this issue, here's a systematic approach that you can follow to successfully test private methods in your TypeScript classes. Step 1: Export Your Class Properly Make sure to export your class correctly. Here’s how it looks in your own file: [[See Video to Reveal this Text or Code Snippet]] Step 2: Use Rewire in Your Test File In your test file, you need to import the rewire module. Here’s a step-by-step guide: Import Your Class Normally for Public Method Testing [[See Video to Reveal this Text or Code Snippet]] Rewire to Access Private Methods For testing the private methods, use rewire as shown below: [[See Video to Reveal this Text or Code Snippet]] Step 3: Instantiate and Test the Private Method Within your test case, instantiate the rewired class and call the private method: [[See Video to Reveal this Text or Code Snippet]] Considerations TypeScript Typing: One downside of this method is that the returned object from rewire is typed as any, which means you won’t get type safety or IntelliSense support from TypeScript. Debugging: You can still debug your private methods effectively if you have your development environment set up correctly. Conclusion By utilizing rewire in conjunction with well-structured TypeScript classes, you can overcome the challenges of testing private methods. This approach not only provides access to your private functionality but also enhances the robustness of your unit tests. Happy testing!