Common memory leak patterns in Node.js - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a timer that repeats every second.

Node.js

setInterval(() => { console.log('Tick'); }, [1]);

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

A10

B100

C1000

D5000

Attempts:

3 left

💡 Hint

Common Mistakes

Using 100 instead of 1000 causes the timer to tick too fast.

Using 5000 makes the timer tick every 5 seconds, not 1.

The setInterval function takes the delay in milliseconds. 1000 ms means 1 second.

2fill in blank

medium

Complete the code to remove the event listener properly.

Node.js

emitter.on('data', handler);
// Later remove listener
emitter.[1]('data', handler);

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

AremoveListener

BaddListener

Cemit

Donce

Attempts:

3 left

💡 Hint

Common Mistakes

Using addListener again adds more listeners, causing leaks.

Using emit triggers events but does not remove listeners.

To avoid memory leaks, remove event listeners with removeListener when no longer needed.

3fill in blank

hard

Fix the error in the code to avoid a memory leak caused by global variables.

Node.js

global.cache = {};
function addToCache(key, value) {
  [1].cache[key] = value;
}

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

Awindow

Bcache

Cthis

Dglobal

Attempts:

3 left

💡 Hint

Common Mistakes

Using window causes errors because it is a browser object.

Using this may not refer to the global object here.

In Node.js, global is the global object. Using it explicitly avoids confusion and leaks.

4fill in blank

hard

Fill both blanks to create a WeakMap and store an object without preventing garbage collection.

Node.js

const cache = new [1]();
const obj = {};
cache.[2](obj, 'data');

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

AWeakMap

BMap

Cset

Dadd

Attempts:

3 left

💡 Hint

Common Mistakes

Using Map keeps strong references, causing leaks.

Using add is not a method of Map or WeakMap.

A WeakMap allows objects to be garbage collected if no other references exist. Use set to add entries.

5fill in blank

hard

Fill all three blanks to create a closure that does not cause a memory leak by holding unnecessary references.

Node.js

function createCounter() {
  let count = 0;
  return function() {
    count [1] 1;
    return [2];
  };
}
const counter = createCounter();
counter(); // [3]

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

A+=

Bcount

C1

D++

Attempts:

3 left

💡 Hint

Common Mistakes

Using ++ alone without return causes confusion.

Returning 1 always does not reflect the count.

Use count += 1 to increment, return count, and calling counter() returns 1.

Practice

1. Which of the following is a common cause of memory leaks in Node.js applications?

easy

A. Keeping references to objects that are no longer needed

B. Using asynchronous functions

C. Writing synchronous code

D. Using the latest Node.js version

Solution

1. Step 1: Understand what causes memory leaks

   Memory leaks happen when your program holds onto data it no longer needs, preventing the system from freeing memory.

2. Step 2: Identify the correct cause among options

   Keeping references to unused objects prevents garbage collection, causing leaks. Other options do not inherently cause leaks.

3. Final Answer:

   Keeping references to objects that are no longer needed -> Option A

4. Quick Check:

   Memory leaks = unused references [OK]

Hint: Memory leaks happen when unused data is still referenced [OK]

Common Mistakes:

• Thinking async code always causes leaks
• Confusing synchronous code with leaks
• Assuming Node.js version causes leaks

2. Which syntax correctly removes an event listener in Node.js to help prevent memory leaks?

easy

A. emitter.deleteListener('event', callback);

B. emitter.off('event');

C. emitter.remove('event', callback);

D. emitter.removeListener('event', callback);

Solution

1. Step 1: Recall Node.js event listener removal syntax

   The correct method to remove a specific listener is removeListener(event, callback).

2. Step 2: Check each option's validity

   emitter.removeListener('event', callback); uses the correct method. emitter.off('event'); removes all listeners but omits the specific callback. Options A and C are not valid methods.

3. Final Answer:

   emitter.removeListener('event', callback); -> Option D

4. Quick Check:

   Correct method = removeListener(event, callback) [OK]

Hint: Use removeListener with event and callback to remove listeners [OK]

Common Mistakes:

• Using emitter.off without callback
• Using non-existent methods like remove or deleteListener
• Omitting the callback function

3. Consider this code snippet:

const cache = {};
function addToCache(key, value) {
  cache[key] = value;
}
addToCache('user1', {name: 'Alice'});
addToCache('user2', {name: 'Bob'});
console.log(Object.keys(cache).length);

What will be the output and what memory issue might this cause if keys are never removed?

medium

A. 0; No memory leak because cache is empty

B. 2; Memory leak due to unbounded cache growth

C. Error; Syntax error in code

D. Undefined; cache is not defined

Solution

1. Step 1: Analyze the code output

   The cache object stores two keys: 'user1' and 'user2'. Object.keys(cache).length returns 2.

2. Step 2: Identify memory issue

   Since keys are never removed, cache grows indefinitely, causing a memory leak.

3. Final Answer:

   2; Memory leak due to unbounded cache growth -> Option B

4. Quick Check:

   Cache size = 2, leak if keys never removed [OK]

Hint: Uncleared caches cause leaks; count keys to check size [OK]

Common Mistakes:

• Thinking cache is empty initially
• Assuming syntax error without checking code
• Confusing undefined with empty object

4. You have this code causing a memory leak:

const listeners = [];
function addListener(emitter, callback) {
  emitter.on('data', callback);
  listeners.push({emitter, callback});
}
// Later you forget to remove listeners

What is the best fix to prevent the leak?

medium

A. Remove listeners with emitter.removeListener and clear the listeners array

B. Do nothing; Node.js cleans up automatically

C. Replace emitter.on with emitter.once

D. Use global variables instead of arrays

Solution

1. Step 1: Identify cause of leak

   Listeners are added and stored in an array but never removed, causing memory to stay used.

2. Step 2: Apply fix to remove listeners and clear references

   Use emitter.removeListener with stored callbacks and clear the listeners array to free memory.

3. Final Answer:

   Remove listeners with emitter.removeListener and clear the listeners array -> Option A

4. Quick Check:

   Remove listeners + clear refs = fix leak [OK]

Hint: Always remove event listeners and clear stored references [OK]

Common Mistakes:

• Assuming Node.js auto-cleans listeners
• Using emitter.once without removing listeners
• Using global variables increases leaks

5. You have a Node.js app caching user sessions in a global object. Over time, memory usage grows unexpectedly. Which combined approach best prevents this memory leak?

hard

A. Use global variables for caching without cleanup

B. Store all sessions indefinitely to avoid losing data

C. Implement session expiration and remove expired sessions from cache regularly

D. Avoid using any caching to prevent leaks

Solution

1. Step 1: Understand the problem with indefinite caching

   Storing sessions forever causes the cache to grow without limit, leading to memory leaks.

2. Step 2: Apply session expiration and cleanup

   Removing expired sessions regularly frees memory and prevents leaks while keeping useful data.

3. Final Answer:

   Implement session expiration and remove expired sessions from cache regularly -> Option C

4. Quick Check:

   Expire and clean cache to prevent leaks [OK]

Hint: Expire and clear cached data regularly to avoid leaks [OK]

Common Mistakes:

• Keeping all data forever causes leaks
• Avoiding caching is not practical
• Using globals without cleanup worsens leaks