Debugging in Back-end (LoopBack 4): A Step-by-Step Guide for Juniors

 

LoopBack 4 is a powerful framework for building APIs, but like any complex system, it can sometimes be tricky to debug. This guide will walk you through the process of setting breakpoints in your code, stepping through it using the debugger, and understanding the overall flow of data within your backend.

1. Setting Up Your Debugging Environment

Before we dive into the specifics of debugging, you'll need to ensure your environment is set up for debugging:

• VS Code (Recommended): Visual Studio Code is a popular choice for debugging Node.js applications like LoopBack.
• Debugger Extension: Make sure you have the Node.js debugging extension installed.
• Source Maps: LoopBack 4 uses TypeScript, so ensure you have source maps enabled in your tsconfig.json file:

Once you've set this up, you can start debugging your LoopBack 4 application.

2. Understanding the Big Picture of Data Flow in LoopBack 4

To effectively debug LoopBack 4, it's important to have a clear understanding of the overall data flow:

• Client Request: The process starts when a client makes an HTTP request (GET, POST, etc.) to one of your API endpoints.
• Controller: The request is handled by a specific controller that corresponds to the API endpoint. Here, you’ll typically find methods like create, find, update, etc.
• Service (Optional): The controller might delegate logic to a service class, especially for more complex operations.
• Repository: The controller (or service) interacts with the repository, which is responsible for performing database operations.
• Database: Finally, the repository communicates with the underlying database (e.g., MongoDB, MySQL) to fetch or store data.

Each of these steps can be debugged to find where something may be going wrong.

3. Setting Breakpoints in Controllers

Let's start at the controller level. Controllers in LoopBack 4 are the entry point for handling API requests.

• Open your controller file (e.g., user.controller.ts).
• Locate the method where you want to start debugging (e.g., find(), create()).
• Set a breakpoint by clicking in the gutter (left side) of the line number. A red dot will appear, indicating the breakpoint is set.

For example:

4. Running the Debugger

• In VS Code, go to the Run and Debug view (left panel).
• Select Node.js as the environment.
• Hit the Start Debugging button (F5).

The application will run, and as soon as the code hits your breakpoint, the debugger will pause execution.

5. Stepping Through the Code

When the debugger pauses at your breakpoint, you can use the following commands to step through your code:

• Step Over (F10): Move to the next line of code without going into any function calls.
• Step Into (F11): Move into the next function call, allowing you to dive deeper into the execution.
• Step Out (Shift + F11): Move out of the current function and back to the calling function.
• Continue (F5): Continue running the application until the next breakpoint or the end of execution.

For example, after the controller method is called, you might want to step into the repository’s create() method to see how data is being saved to the database.

6. Debugging Services and Repositories

After the controller, the next stops are typically service classes and repositories. Services contain business logic, while repositories handle the data access layer.

• To debug a service or repository, set breakpoints in the service class (if you're using one) or repository method (e.g., create(), find()).

When you step into the repository method from the controller, the debugger will stop here, allowing you to inspect the state of user or any other variables.

7. Checking Variables and Expressions

While the debugger is paused, you can hover over any variable to inspect its current value. This is useful for verifying that data is flowing as expected between the controller, service, and repository layers.

You can also open the Debug Console in VS Code to evaluate expressions and inspect complex objects.

8. Common Debugging Techniques

• Watch Variables: Use the Watch section to track specific variables and see how they change over time.
• Call Stack: The Call Stack pane shows the sequence of function calls leading to the current breakpoint. This helps you understand how data is flowing through your application.
• Conditional Breakpoints: If a breakpoint is being hit too often, you can make it conditional. Right-click the breakpoint and add a condition (e.g., user.id === 5).

9. Debugging Asynchronous Code

Most of the operations in LoopBack 4 are asynchronous, so you'll often encounter Promise chains or async/await patterns. The debugger in VS Code handles asynchronous code well, but here are a few tips:

• Await Breakpoints: Place breakpoints before and after await statements to see how data is fetched or processed.
• Error Handling: Check for unhandled promises and set breakpoints in catch blocks to debug error handling.

10. Debugging the Full Data Flow

Here’s how you can debug the full data flow in a typical request:

1. Controller: Set a breakpoint in your controller’s method (e.g., createUser).
2. Step Into: Step into the service (if applicable) or directly into the repository’s method (e.g., create()).
3. Repository: Check how the repository interacts with the database (set breakpoints in repository methods like find() or create()).
4. Database: Confirm that the correct query is being executed and that data is being returned as expected.

By stepping through the code in this way, you can see exactly where things might be going wrong, whether it’s incorrect input to the controller, failed validation in the service, or a faulty query in the repository.

Conclusion

Debugging in back-end systems like LoopBack 4 is a powerful way to understand and troubleshoot your application. By setting breakpoints in the controller, stepping through the service and repository layers, and carefully inspecting variables and the call stack, you can pinpoint issues quickly.

For junior developers, understanding the flow of data from client to database is key to becoming proficient at debugging. LoopBack’s structured architecture makes this easier, as the responsibilities of controllers, services, and repositories are clearly defined.

Master these steps, and debugging will become much less intimidating!