Chrome DevTools for Node.js in Node.js - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

Understanding Chrome DevTools Debugger Behavior in Node.js

When you run a Node.js script with the --inspect-brk flag and open Chrome DevTools, what happens immediately after the script starts?

Node.js

node --inspect-brk script.js

A The script runs fully without pausing, and DevTools attaches after completion.

B The script pauses only if a breakpoint is manually set in DevTools after starting.

C The script pauses on the first line before executing any code, waiting for you to resume.

D The script throws an error because <code>--inspect-brk</code> is deprecated.

💡 Hint

Think about what the brk part in --inspect-brk means.

The --inspect-brk flag tells Node.js to start the debugger and pause execution on the first line of the script. This allows you to set breakpoints or inspect variables before any code runs.

📝 Syntax

intermediate

Correct Usage of the Node.js Inspector Protocol URL

Which of the following is the correct format of the WebSocket URL used by Chrome DevTools to connect to a running Node.js process with the inspector enabled?

A ws://127.0.0.1:9229/uuid

B http://127.0.0.1:9229/uuid

C wss://127.0.0.1:9229/uuid

D ftp://127.0.0.1:9229/uuid

💡 Hint

DevTools uses a WebSocket connection, not HTTP or FTP.

The Node.js inspector listens on a WebSocket URL starting with ws:// by default on port 9229. This is how Chrome DevTools communicates with the Node.js process.

🔧 Debug

advanced

Identifying the Cause of a Debugger Connection Failure

You start a Node.js process with node --inspect=localhost:9230 app.js but Chrome DevTools fails to connect. What is the most likely reason?

A The <code>--inspect</code> flag requires the port to be above 1024, and 9230 is invalid.

B The <code>localhost</code> hostname is not allowed; you must use <code>0.0.0.0</code>.

C The Node.js process must be started with <code>--inspect-brk</code> to allow any connection.

D Chrome DevTools only connects to port 9229 by default, so it cannot connect to 9230 without manual configuration.

💡 Hint

Check the default port Chrome DevTools expects for Node.js debugging.

By default, Chrome DevTools connects to port 9229 for Node.js debugging. If you specify a different port like 9230, you must manually tell DevTools to connect to that port. Otherwise, it will fail to connect.

❓ state_output

advanced

Effect of Pausing Execution in Chrome DevTools on Node.js Event Loop

If you pause a running Node.js script in Chrome DevTools, what happens to the Node.js event loop and asynchronous callbacks?

A The event loop continues running, so asynchronous callbacks execute even while paused.

B The event loop is paused, so no asynchronous callbacks or timers run until resumed.

C Only timers pause, but I/O callbacks continue running during the pause.

D The event loop crashes and the Node.js process exits.

💡 Hint

Think about what pausing execution means for the entire JavaScript runtime.

When you pause execution in Chrome DevTools, the entire Node.js event loop is paused. This means no asynchronous callbacks, timers, or I/O events will run until you resume the script.

🧠 Conceptual

expert

Understanding Source Map Support in Node.js Debugging with Chrome DevTools

You are debugging a Node.js application written in TypeScript using Chrome DevTools. You notice that breakpoints in the original TypeScript files are not hit, only in the compiled JavaScript files. What is the most likely cause?

A Source maps are not properly generated or linked, so DevTools cannot map JS back to TS files.

B Chrome DevTools does not support debugging TypeScript files at all.

C You must run Node.js with <code>--inspect-brk</code> to debug TypeScript source maps.

D TypeScript files must be renamed to .js for DevTools to recognize breakpoints.

💡 Hint

Check if the compiled JavaScript includes source map references.

For Chrome DevTools to map breakpoints from JavaScript back to TypeScript, source maps must be generated during compilation and properly linked in the JavaScript files. Without source maps, DevTools only sees the compiled JS.

Practice

1. What is the main purpose of using Chrome DevTools with Node.js?

easy

A. To convert Node.js code to browser JavaScript

B. To write Node.js code faster

C. To deploy Node.js apps to the cloud

D. To pause and inspect Node.js code while it runs

Solution

1. Step 1: Understand Chrome DevTools role

   Chrome DevTools is a tool to debug and inspect running code visually.

2. Step 2: Connect to Node.js debugging

   Using DevTools with Node.js lets you pause and inspect your code during execution.

3. Final Answer:

   To pause and inspect Node.js code while it runs -> Option D

4. Quick Check:

   Debugging = Pause and inspect code [OK]

Hint: DevTools helps you pause and check code live [OK]

Common Mistakes:

• Thinking DevTools is for writing code
• Confusing debugging with deployment
• Assuming DevTools converts code formats

2. Which command correctly starts a Node.js app with debugging enabled for Chrome DevTools?

easy

A. node --inspect app.js

B. node app.js --inspect

C. node app.js --debug

D. node debug app.js

Solution

1. Step 1: Recall correct flag usage

   The debugging flag must come before the script name in the command.

2. Step 2: Check each option

   node --inspect app.js uses 'node --inspect app.js' which is the correct syntax to enable debugging.

3. Final Answer:

   node --inspect app.js -> Option A

4. Quick Check:

   Flag before script = correct syntax [OK]

Hint: Put --inspect before your script name [OK]

Common Mistakes:

• Placing --inspect after the script name
• Using deprecated --debug flag
• Typing 'node debug' which is not a valid command

3. Given this command: node --inspect-brk app.js, what happens when you run it?

medium

A. The app crashes immediately

B. The app runs normally without pausing

C. The app pauses on the first line waiting for debugger

D. The app runs but debugging is disabled

Solution

1. Step 1: Understand --inspect-brk flag

   This flag tells Node.js to start debugging and pause before executing any code.

2. Step 2: Predict behavior on running

   The app will wait for a debugger to connect before running the first line.

3. Final Answer:

   The app pauses on the first line waiting for debugger -> Option C

4. Quick Check:

   --inspect-brk = pause at start [OK]

Hint: --inspect-brk pauses app before first line [OK]

Common Mistakes:

• Thinking app runs immediately
• Confusing --inspect with --inspect-brk
• Assuming app crashes on this flag

4. You run node --inspect app.js but Chrome DevTools does not connect. What is a likely cause?

medium

A. You forgot to open chrome://inspect in Chrome

B. You used --inspect-brk instead of --inspect

C. Your Node.js version is too new

D. You ran the command without node

Solution

1. Step 1: Check connection setup

   To debug, you must open Chrome's chrome://inspect page to connect to Node.js.

2. Step 2: Analyze other options

   Using --inspect-brk is valid and pauses app; Node.js version too new is unlikely cause; running without node would cause error.

3. Final Answer:

   You forgot to open chrome://inspect in Chrome -> Option A

4. Quick Check:

   Open chrome://inspect to connect debugger [OK]

Hint: Always open chrome://inspect to connect debugger [OK]

Common Mistakes:

• Assuming --inspect-brk is required
• Ignoring the need to open Chrome inspect page
• Not running with 'node' command

5. You want to debug a Node.js app that crashes immediately on start. You run node --inspect app.js and connect Chrome DevTools but can't find where it crashes. What should you do to catch the crash at the start?

hard

A. Add debugger; inside app.js after the crash line

B. Run with node --inspect-brk app.js to pause before code runs

C. Run node --inspect app.js with extra logging

D. Use node debug app.js instead

Solution

1. Step 1: Understand crash timing

   If the app crashes immediately, you need to pause before any code runs to inspect it.

2. Step 2: Use correct flag to pause early

   The --inspect-brk flag pauses execution on the first line, letting you catch early crashes.

3. Step 3: Evaluate other options

   Adding debugger; after crash won't help if crash happens before it; extra logging may miss early crash; node debug is outdated.

4. Final Answer:

   Run with node --inspect-brk app.js to pause before code runs -> Option B

5. Quick Check:

   --inspect-brk pauses early to catch crashes [OK]

Hint: Use --inspect-brk to pause before app starts [OK]

Common Mistakes:

• Placing debugger after crash point
• Relying only on logs for early crashes
• Using deprecated debug command