Debugging with VS Code in Node.js - Quick Revision & Key Differences

Recall & Review

beginner

What is the purpose of the launch.json file in VS Code debugging?

The launch.json file configures how VS Code starts and attaches the debugger to your Node.js application. It defines settings like the program entry point, environment variables, and runtime arguments.

beginner

How do breakpoints help during debugging in VS Code?

Breakpoints pause the running program at specific lines of code. This lets you inspect variables, check program flow, and find bugs step-by-step.

beginner

What is the function of the Debug Console in VS Code?

The Debug Console shows output from your program and lets you run commands or evaluate expressions while paused in the debugger.

beginner

Which VS Code feature allows you to step over, step into, and step out of functions during debugging?

The stepping controls let you move through your code line-by-line or jump into and out of functions to understand program behavior.

beginner

How can you inspect the value of a variable while debugging in VS Code?

You can hover your mouse over the variable in the editor or look at the Variables panel in the Debug view to see current values.

What file do you edit to configure how VS Code launches your Node.js app for debugging?

A package.json

B launch.json

C settings.json

D tasks.json

The launch.json file tells VS Code how to start and attach the debugger to your app.

What happens when you set a breakpoint in VS Code?

A The program pauses at that line during debugging

B The program runs faster

C The program skips that line

D The program restarts

Breakpoints pause execution so you can inspect the program state.

Which panel shows variable values while debugging in VS Code?

A Variables

B Problems

C Terminal

D Explorer

The Variables panel displays current values of variables during debugging.

What does the 'Step Into' button do during debugging?

A Exits the current function

B Skips the current line

C Runs the program to the end

D Enters into the function call on the current line

'Step Into' moves the debugger inside the function being called.

Where can you type commands or expressions to evaluate during a paused debug session?

A Problems panel

B Output panel

C Debug Console

D Explorer

The Debug Console lets you run commands and see output while debugging.

Explain how to set up and start debugging a Node.js app in VS Code.

Think about the steps from configuration to running the debugger.

Describe how breakpoints and stepping controls help find bugs in your code.

Focus on how you control program flow during debugging.

Practice

1. What is the main purpose of setting a breakpoint in VS Code when debugging a Node.js application?

easy

A. To speed up the execution of the program

B. To automatically fix syntax errors in the code

C. To convert JavaScript code into machine code

D. To pause the program at a specific line to inspect variables and program flow

Solution

1. Step 1: Understand what a breakpoint does

   A breakpoint pauses the program execution at a chosen line so you can check what is happening.

2. Step 2: Identify the purpose in debugging

   Pausing lets you inspect variables and see how the program flows step-by-step.

3. Final Answer:

   To pause the program at a specific line to inspect variables and program flow -> Option D

4. Quick Check:

   Breakpoint = Pause and inspect [OK]

Hint: Breakpoints pause code to check variables and flow [OK]

Common Mistakes:

• Thinking breakpoints fix errors automatically
• Believing breakpoints speed up code
• Confusing breakpoints with code compilation

2. Which of the following is the correct way to start debugging a Node.js app in VS Code using the launch.json file?

easy

A. { "type": "node", "request": "run", "program": "${workspaceFolder}/app.js" }

B. { "type": "node", "request": "launch", "program": "${workspaceFolder}/app.js" }

C. { "type": "node", "request": "compile", "program": "app.js" }

D. { "type": "python", "request": "launch", "program": "app.js" }

Solution

1. Step 1: Check the correct debug type for Node.js

   The type must be "node" to debug Node.js applications.

2. Step 2: Verify the request and program fields

   "request" should be "launch" to start debugging, and "program" points to the main file with workspace variable.

3. Final Answer:

   { "type": "node", "request": "launch", "program": "${workspaceFolder}/app.js" } -> Option B

4. Quick Check:

   Node debug launch with correct program path [OK]

Hint: Use type 'node' and request 'launch' in launch.json [OK]

Common Mistakes:

• Using wrong type like 'python' for Node.js
• Using 'compile' or 'run' instead of 'launch'
• Not using workspaceFolder variable for program path

3. Consider this Node.js code snippet debugged in VS Code with a breakpoint on line 3:

1 const x = 5;
2 const y = 10;
3 const sum = x + y;
4 console.log(sum);

When paused at the breakpoint on line 3, what will be the value of sum in the debugger?

medium

A. undefined

B. 15

C. 5

D. Error: sum is not defined

Solution

1. Step 1: Understand when the breakpoint pauses execution

   The breakpoint on line 3 pauses before executing that line, so sum is not yet assigned.

2. Step 2: Determine the value of sum at pause

   Since line 3 hasn't run, sum is declared but not assigned, so its value is undefined.

3. Final Answer:

   undefined -> Option A

4. Quick Check:

   Breakpoint pauses before assignment, sum = undefined [OK]

Hint: Breakpoint pauses before line runs, variable not assigned yet [OK]

Common Mistakes:

• Assuming sum already has the calculated value
• Confusing declaration with assignment timing
• Expecting runtime error at breakpoint

4. You set a breakpoint in VS Code but the debugger never pauses when running your Node.js app. Which of these is the most likely cause?

medium

A. The Node.js version is too new

B. You forgot to save the file before running

C. The launch.json file is missing or misconfigured

D. Breakpoints only work in browser debugging

Solution

1. Step 1: Check debugger configuration

   If launch.json is missing or has errors, VS Code cannot attach the debugger properly.

2. Step 2: Understand breakpoint behavior

   Without correct config, breakpoints won't pause execution even if set.

3. Final Answer:

   The launch.json file is missing or misconfigured -> Option C

4. Quick Check:

   Debugger config missing = breakpoints ignored [OK]

Hint: Verify launch.json exists and is correct to enable breakpoints [OK]

Common Mistakes:

• Blaming Node.js version without checking config
• Assuming breakpoints need browser debugging
• Not saving files but that usually doesn't stop breakpoints

5. You want to debug a Node.js app that uses environment variables. How can you configure launch.json to pass an environment variable named API_KEY with value 12345 to your app during debugging?

hard

A. { "type": "node", "request": "launch", "program": "${workspaceFolder}/app.js", "env": { "API_KEY": "12345" } }

B. { "type": "node", "request": "launch", "program": "app.js", "args": ["API_KEY=12345"] }

C. { "type": "node", "request": "launch", "program": "app.js", "envFile": "API_KEY=12345" }

D. { "type": "node", "request": "launch", "program": "app.js", "env": "API_KEY=12345" }

Solution

1. Step 1: Use the correct property for environment variables

   The "env" property in launch.json accepts an object with key-value pairs for environment variables.

2. Step 2: Format the environment variable correctly

   Set "API_KEY" as key and "12345" as its string value inside the "env" object.

3. Final Answer:

   { "type": "node", "request": "launch", "program": "${workspaceFolder}/app.js", "env": { "API_KEY": "12345" } } -> Option A

4. Quick Check:

   Use "env" object with key-value pairs in launch.json [OK]

Hint: Set env variables as key-value pairs inside "env" in launch.json [OK]

Common Mistakes:

• Using args array instead of env object
• Passing env as a string instead of object
• Confusing envFile with direct env variable setting