Chrome DevTools for Node.js in Node.js

Introduction

Chrome DevTools helps you find and fix problems in your Node.js code by letting you see what your program is doing step-by-step.

When your Node.js program is not working as expected and you want to find the bug.

When you want to check how variables change while your program runs.

When you want to understand the flow of your code better by pausing and stepping through it.

When you want to measure how long parts of your code take to run.

When you want to inspect errors with more detail than just console messages.

Syntax

Node.js

node --inspect-brk your_script.js

Use --inspect-brk to start Node.js and pause on the first line for debugging.

After running this, open chrome://inspect in Chrome to connect the debugger.

Examples

Starts app.js with debugging enabled and pauses before the first line.

Node.js

node --inspect-brk app.js

Starts app.js with debugging enabled but does not pause at start.

Node.js

node --inspect app.js

Starts debugging on port 9229 (default port) for remote debugging.

Node.js

node --inspect=9229 app.js

Sample Program

This simple Node.js program adds two numbers and prints the result. You can run it with node --inspect-brk filename.js and use Chrome DevTools to step through each line, watch variables, and understand how the code runs.

Node.js

console.log('Start');

function add(a, b) {
  const result = a + b;
  return result;
}

const sum = add(5, 7);
console.log('Sum is:', sum);

Important Notes

Make sure you have Google Chrome installed to use Chrome DevTools.

Use chrome://inspect in Chrome to find and open your Node.js debugging session.

Remember to remove debugging flags when running your app in production.

Summary

Chrome DevTools lets you pause and inspect your Node.js code while it runs.

You start debugging by running Node.js with --inspect or --inspect-brk flags.

Use Chrome's chrome://inspect page to connect and debug your Node.js app visually.

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