Debugging Node.js Express.js Applications with Visual Studio Code

If you're a Node.js developer, you know how important it is to be able to debug your applications effectively. Debugging can be a challenging task, especially when you're dealing with complex applications. Fortunately, Visual Studio Code (VS Code) provides powerful debugging tools that can make your life a lot easier. In this article, we'll take a look at how to debug Node.js Express.js applications with VS Code.

Prerequisites

Before we dive into debugging, let's make sure we have everything we need. In order to follow along with this tutorial, you should have the following:

• Basic knowledge of Node.js and Express.js
• Visual Studio Code installed on your computer
• An Express.js application that you want to debug

If you don't have an Express.js application to debug, you can create a simple one by following the Express.js Getting Started guide.

Setting Up Debugging in VS Code

The first step in debugging a Node.js Express.js application with VS Code is to set up debugging in your application. To do this, you'll need to create a launch configuration file.

1. Open your Express.js application in VS Code.
2. Click on the Debug icon on the left-hand side of the screen.
3. Click on the gear icon to open the launch.json file.
4. Click on the Add Configuration button and select Node.js from the list of options.
5. Modify the configuration as needed. Here is an example configuration:

{
    "type": "node",
    "request": "launch",
    "name": "Debug Express.js Application",
    "program": "${workspaceFolder}/app.js",
    "runtimeExecutable": "nodemon",
    "restart": true,
    "port": 9229
}

This configuration sets up VS Code to launch your Express.js application using Nodemon, which automatically restarts your application when changes are made. It also opens port 9229 for debugging.

Starting the Debugger

Now that we have our launch configuration set up, we can start the debugger.

1. Click on the Debug icon on the left-hand side of the screen.
2. Click on the green play button to start the debugger.
3. VS Code will start your Express.js application and attach the debugger.

Debugging Your Application

Once the debugger is attached, you can start debugging your application. Here are some common debugging scenarios:

Setting Breakpoints

Setting breakpoints allows you to pause the execution of your application at a specific point in your code. This can be helpful for understanding how your application works and identifying issues.

To set a breakpoint:

1. Click on the line number in your code where you want to set the breakpoint.
2. A red dot will appear on the line number to indicate that a breakpoint has been set.

Stepping Through Code

Once a breakpoint has been hit, you can step through your code to understand how it's working. Here are the different ways you can step through your code:

• Step Over: This will execute the current line and move to the next line.
• Step Into: This will execute the current line and move into any function calls on that line.
• Step Out: This will move out of the current function and back to the calling function.

You can access these options by clicking on the appropriate button in the Debug panel or by using the keyboard shortcuts (F10 for Step Over, F11 for Step Into, and Shift+F11 for Step Out).

Inspecting Variables

As you step through your code, you may want to inspect the values of variables to better understand how your code is working. You can do this by hovering over a variable in your code or by opening the Variables panel in the Debug view.

To inspect a variable:

1. Hover over the variable in your code.
2. A tooltip will appear with the value of the variable.
3. Click on the magnifying glass icon in the tooltip to open the variable in the Variables panel.

In the Variables panel, you can see the current value of all variables in scope.

Debugging Errors

If your application encounters an error, the debugger will pause execution and highlight the line where the error occurred. You can then use the tools we've discussed to step through your code and identify the source of the error.

Debugging Asynchronous Code

Debugging asynchronous code can be challenging, but VS Code provides some powerful tools to help. One of these tools is the Async Stack Trace feature, which allows you to see the entire call stack, even for asynchronous functions.

To enable the Async Stack Trace feature:

1. Open the Debug panel.
2. Click on the gear icon to open the Debug Configuration.
3. Add the following line to the configuration:

"outputCapture": "std"

This will enable the Async Stack Trace feature and allow you to see the call stack for asynchronous functions.

Conclusion

Debugging Node.js Express.js applications with Visual Studio Code can be a powerful tool for developers. With VS Code, you can easily set up debugging, set breakpoints, step through your code, inspect variables, and debug errors. Asynchronous code can be particularly challenging to debug, but VS Code provides powerful tools to help you understand how your code is working. With these tools, you can become a more effective developer and create more robust, reliable applications.