Debugging is an essential skill for any developer. In this section, we will explore various tools and techniques to debug Node.js applications effectively. By the end of this module, you should be able to identify and fix bugs in your Node.js code with confidence.

1. Understanding Debugging: The process of identifying, isolating, and fixing bugs in your code.
2. Common Debugging Tools: Tools that help you debug Node.js applications, such as the built-in debugger, Chrome DevTools, and third-party tools like nodemon.
3. Debugging Techniques: Strategies for effective debugging, including breakpoints, watch expressions, and logging.

Node.js comes with a built-in debugger that you can use to step through your code and inspect variables.

1. Start the Debugger: Run your Node.js application with the inspect flag.

   node inspect app.js
   

2. Breakpoints: Insert the debugger statement in your code where you want to pause execution.

   function add(a, b) {
       debugger; // Execution will pause here
       return a + b;
   }
   console.log(add(2, 3));
   

3. Commands: Use the following commands to control the debugger:

   • cont or c: Continue execution.
   • next or n: Step to the next line.
   • step or s: Step into a function.
   • out or o: Step out of the current function.
   • repl: Open a REPL to evaluate expressions.

Run the above code with the built-in debugger:

Chrome DevTools provides a powerful interface for debugging Node.js applications.

1. Start the Debugger: Run your Node.js application with the --inspect flag.

   node --inspect app.js
   

2. Open DevTools: Open Chrome and navigate to chrome://inspect. Click on "Open dedicated DevTools for Node".

3. Set Breakpoints: Click on the line number in the source code to set breakpoints.

4. Inspect Variables: Use the "Scope" pane to inspect variables and the "Watch" pane to monitor expressions.

Run the above code with Chrome DevTools:

Nodemon is a utility that automatically restarts your Node.js application when file changes are detected. It can be combined with the built-in debugger for a smoother debugging experience.

1. Install Nodemon:

   npm install -g nodemon
   

2. Run with Nodemon:

   nodemon --inspect app.js
   

Logging is a simple yet effective way to debug your code. Use console.log to print variable values and execution flow.

Watch expressions allow you to monitor the value of variables and expressions as you step through your code.

Breakpoints pause the execution of your code at specific lines, allowing you to inspect the current state.

1. Task: Debug the following code to find and fix the bug.

   function multiply(a, b) {
       return a * b;
   }
   
   function calculateArea(length, width) {
       debugger; // Pause execution here
       return multiply(length, width);
   }
   
   console.log(calculateArea(5, 10)); // Expected output: 50
   

2. Steps:

   • Run the code with the built-in debugger or Chrome DevTools.
   • Set breakpoints and inspect variables.
   • Identify and fix the bug.

1. Run the Debugger:

   node inspect app.js
   

2. Set Breakpoints: Insert debugger statements or use DevTools to set breakpoints.

3. Inspect Variables: Check the values of length and width in the calculateArea function.

4. Fix the Bug: Ensure the multiply function is correctly implemented.

In this module, we covered the basics of debugging Node.js applications using the built-in debugger, Chrome DevTools, and third-party tools like Nodemon. We also explored various debugging techniques, including logging, watch expressions, and breakpoints. By practicing these techniques, you can effectively identify and fix bugs in your Node.js applications.