Heap snapshot for memory leaks in Node.js - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to import the module used to take heap snapshots in Node.js.

Node.js

const heapdump = require('[1]');

Drag options to blanks, or click blank then click option'

Aheapdump

Bhttp

Cfs

Dexpress

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'fs' or 'http' instead of 'heapdump'.

Forgetting to install the 'heapdump' module before requiring it.

The heapdump module is used to take heap snapshots in Node.js for memory leak analysis.

2fill in blank

medium

Complete the code to trigger a heap snapshot save with a filename.

Node.js

heapdump.writeSnapshot('[1]', (err, filename) => {
  if (err) console.error(err);
  else console.log('Snapshot saved to', filename);
});

Drag options to blanks, or click blank then click option'

Aleak.heapsnapshot

Bsnapshot.heapsnapshot

Cmemory.heapsnapshot

Dheap.heapsnapshot

Attempts:

3 left

💡 Hint

Common Mistakes

Using filenames without the '.heapsnapshot' extension.

Using unrelated names like 'leak.heapsnapshot' without context.

The filename for the heap snapshot is usually given with a .heapsnapshot extension, such as 'memory.heapsnapshot'.

3fill in blank

hard

Fix the error in the code to correctly listen for a signal to trigger a heap snapshot.

Node.js

process.on('[1]', () => {
  heapdump.writeSnapshot();
});

Drag options to blanks, or click blank then click option'

ASIGTERM

BSIGUSR2

CSIGKILL

DSIGINT

Attempts:

3 left

💡 Hint

Common Mistakes

Using SIGINT or SIGTERM which terminate the process.

Using SIGKILL which cannot be caught.

SIGUSR2 is commonly used to trigger heap snapshots in Node.js without stopping the process.

4fill in blank

hard

Fill both blanks to create a heap snapshot filename with a timestamp.

Node.js

const filename = `heap-[1]-[2].heapsnapshot`;

Drag options to blanks, or click blank then click option'

ADate.now()

Bnew Date()

Cprocess.pid

DMath.random()

Attempts:

3 left

💡 Hint

Common Mistakes

Using new Date() directly in filename without formatting.

Using Math.random() which is not descriptive.

Using Date.now() gives a timestamp, and process.pid adds the process ID to the filename for uniqueness.

5fill in blank

hard

Fill all three blanks to write a heap snapshot and log the filename with error handling.

Node.js

heapdump.writeSnapshot([1], ([2], [3]) => {
  if ([2]) console.error([2]);
  else console.log('Snapshot saved:', [3]);
});

Drag options to blanks, or click blank then click option'

A'snapshot.heapsnapshot'

Berr

Cfilename

D'memory.heapsnapshot'

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing up error and filename parameters in the callback.

Using incorrect filename strings.

The first blank is the filename string, the second is the error parameter, and the third is the filename parameter in the callback.

Practice

1. What is the main purpose of taking a heap snapshot in Node.js?

easy

A. To monitor network requests

B. To speed up the execution of code

C. To compile the Node.js application

D. To see what objects are currently in memory

Solution

1. Step 1: Understand heap snapshot concept

   A heap snapshot captures the objects currently stored in memory at a specific time.

2. Step 2: Identify the purpose of heap snapshots

   They help detect memory leaks by showing which objects remain in memory longer than expected.

3. Final Answer:

   To see what objects are currently in memory -> Option D

4. Quick Check:

   Heap snapshot = current memory objects [OK]

Hint: Heap snapshots show memory objects at a moment [OK]

Common Mistakes:

• Thinking heap snapshots speed up code
• Confusing heap snapshots with network monitoring
• Assuming heap snapshots compile code

2. Which Node.js module provides the getHeapSnapshot() method to create heap snapshots?

easy

A. fs

B. v8

C. http

D. os

Solution

1. Step 1: Recall Node.js modules for memory tools

   The v8 module provides access to V8 engine features including heap snapshots.

2. Step 2: Match method to module

   The getHeapSnapshot() method is part of the v8 module, not fs, http, or os.

3. Final Answer:

   v8 -> Option B

4. Quick Check:

   Heap snapshot method = v8 module [OK]

Hint: Heap snapshot method is in v8 module [OK]

Common Mistakes:

• Choosing fs for file operations only
• Confusing http with memory tools
• Selecting os which handles system info

3. Consider this Node.js code snippet:

import { writeFileSync } from 'fs';
import { getHeapSnapshot } from 'v8';

const snapshotStream = getHeapSnapshot();
const chunks = [];
snapshotStream.on('data', chunk => chunks.push(chunk));
snapshotStream.on('end', () => {
  writeFileSync('heap.heapsnapshot', Buffer.concat(chunks));
  console.log('Snapshot saved');
});

What will this code do when run?

medium

A. Create an empty file named 'heap.heapsnapshot' only

B. Throw an error because getHeapSnapshot is not a function

C. Save a heap snapshot file and print 'Snapshot saved'

D. Print 'Snapshot saved' without creating any file

Solution

1. Step 1: Analyze getHeapSnapshot usage

   The getHeapSnapshot() returns a readable stream of the heap snapshot data.

2. Step 2: Understand stream data handling

   The code collects data chunks from the stream, concatenates them, and writes to 'heap.heapsnapshot'. Then it logs confirmation.

3. Final Answer:

   Save a heap snapshot file and print 'Snapshot saved' -> Option C

4. Quick Check:

   Heap snapshot stream saved to file [OK]

Hint: getHeapSnapshot returns stream; collect and save it [OK]

Common Mistakes:

• Assuming getHeapSnapshot returns a buffer directly
• Expecting no file creation
• Thinking it throws an error

4. You wrote this code to save a heap snapshot but it never finishes and the file is empty:

import { writeFileSync } from 'fs';
import { getHeapSnapshot } from 'v8';

const snapshotStream = getHeapSnapshot();
snapshotStream.on('data', chunk => {
  writeFileSync('heap.heapsnapshot', chunk);
});
console.log('Snapshot saved');

What is the main problem?

medium

A. writeFileSync overwrites file on each data event, so file ends empty

B. getHeapSnapshot does not return a stream

C. Missing 'end' event handler to finalize file writing

D. writeFileSync cannot write buffers

Solution

1. Step 1: Examine file writing inside 'data' event

   Calling writeFileSync inside 'data' overwrites the file each time a chunk arrives.

2. Step 2: Understand effect on file content

   Because of overwriting, only the last chunk remains, or file may appear empty if last chunk is empty.

3. Final Answer:

   writeFileSync overwrites file on each data event, so file ends empty -> Option A

4. Quick Check:

   File overwritten repeatedly in data event [OK]

Hint: Write all chunks after stream ends, not during data event [OK]

Common Mistakes:

• Assuming getHeapSnapshot is not a stream
• Ignoring need for 'end' event
• Thinking writeFileSync can't write buffers

5. You want to detect a memory leak in your Node.js app by comparing heap snapshots over time. Which approach is best?

hard

A. Take multiple heap snapshots at intervals and compare retained objects

B. Only take one snapshot at app start and analyze it

C. Use console.log to print all objects in memory continuously

D. Restart the app frequently to clear memory

Solution

1. Step 1: Understand memory leak detection

   Memory leaks show as objects that remain in memory longer than expected, growing over time.

2. Step 2: Use heap snapshots over time

   Taking snapshots at intervals lets you compare and find objects that accumulate, indicating leaks.

3. Step 3: Evaluate other options

   One snapshot can't show growth; console.log is impractical; restarting hides leaks instead of detecting.

4. Final Answer:

   Take multiple heap snapshots at intervals and compare retained objects -> Option A

5. Quick Check:

   Compare snapshots over time to find leaks [OK]

Hint: Compare snapshots over time to spot leaks [OK]

Common Mistakes:

• Relying on a single snapshot
• Using console.log for memory objects
• Restarting app to avoid leaks instead of fixing