SharedArrayBuffer for shared memory in Node.js - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a SharedArrayBuffer of 1024 bytes.

Node.js

const sharedBuffer = new [1](1024);

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

AUint8Array

BSharedArrayBuffer

CArrayBuffer

DBuffer

Attempts:

3 left

💡 Hint

Common Mistakes

Using ArrayBuffer instead of SharedArrayBuffer

Using Buffer which is Node.js specific but not shared memory

The SharedArrayBuffer constructor creates a buffer that can be shared between threads.

2fill in blank

medium

Complete the code to create a typed array view over the shared buffer.

Node.js

const sharedArray = new [1](sharedBuffer);

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AInt16Array

BArray

CFloat32Array

DUint8Array

Attempts:

3 left

💡 Hint

Common Mistakes

Using Array which is not a typed array

Using Float32Array which is for 4-byte floats, not bytes

Uint8Array creates a typed array view over the buffer, allowing byte-level access.

3fill in blank

hard

Fix the error in the code to safely increment a shared counter using Atomics.

Node.js

Atomics.[1](sharedArray, 0, 1);

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Aadd

Bincrement

Cstore

Dload

Attempts:

3 left

💡 Hint

Common Mistakes

Using Atomics.increment which does not exist

Using Atomics.store which replaces the value instead of adding

Atomics.add safely adds a value to the shared array at the given index.

4fill in blank

hard

Fill both blanks to create a shared buffer and a typed array view of 32-bit integers.

Node.js

const buffer = new [1](128);
const intArray = new [2](buffer);

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ASharedArrayBuffer

BArrayBuffer

CInt32Array

DUint8Array

Attempts:

3 left

💡 Hint

Common Mistakes

Using ArrayBuffer instead of SharedArrayBuffer

Using Uint8Array instead of Int32Array for 32-bit integers

Use SharedArrayBuffer to create shared memory and Int32Array to view it as 32-bit integers.

5fill in blank

hard

Fill all three blanks to atomically compare and exchange a value in the shared array.

Node.js

const oldValue = Atomics.[1](sharedArray, [2], [3], 42);

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AcompareExchange

B0

C41

Dexchange

Attempts:

3 left

💡 Hint

Common Mistakes

Using Atomics.exchange which does not compare before swapping

Using wrong index or old value parameters

Atomics.compareExchange compares the value at index 0 with 41, and if equal, replaces it with 42.

Practice

1. What is the main purpose of SharedArrayBuffer in Node.js?

easy

A. To create a memory area that multiple threads can access simultaneously

B. To store large strings efficiently

C. To replace regular arrays with faster versions

D. To handle file system operations asynchronously

Solution

1. Step 1: Understand Shared Memory Concept

   SharedArrayBuffer is designed to create a block of memory that can be shared between multiple threads or workers.

2. Step 2: Compare with Other Options

   Options B, C, and D describe unrelated features: string storage, array speed, and file system operations, which are not the purpose of SharedArrayBuffer.

3. Final Answer:

   To create a memory area that multiple threads can access simultaneously -> Option A

4. Quick Check:

   Shared memory = multiple threads access [OK]

Hint: SharedArrayBuffer is about sharing memory across threads [OK]

Common Mistakes:

• Thinking it stores strings or files
• Confusing with normal arrays
• Assuming it handles async file tasks

2. Which of the following is the correct way to create a SharedArrayBuffer of 1024 bytes in Node.js?

easy

A. const sab = SharedArrayBuffer(1024);

B. const sab = SharedArrayBuffer.new(1024);

C. const sab = new SharedArrayBuffer();

D. const sab = new SharedArrayBuffer(1024);

Solution

1. Step 1: Check the correct constructor usage

   The SharedArrayBuffer must be created with the new keyword and a size in bytes as argument.

2. Step 2: Validate each option

   const sab = new SharedArrayBuffer(1024); uses new SharedArrayBuffer(1024), which is correct. const sab = SharedArrayBuffer(1024); misses new. const sab = new SharedArrayBuffer(); misses size argument. const sab = SharedArrayBuffer.new(1024); uses invalid syntax.

3. Final Answer:

   const sab = new SharedArrayBuffer(1024); -> Option D

4. Quick Check:

   Use new with size in bytes [OK]

Hint: Always use 'new' with SharedArrayBuffer and specify size [OK]

Common Mistakes:

• Omitting 'new' keyword
• Not providing size argument
• Using incorrect constructor syntax

3. Given the code below, what will be the output?

const sab = new SharedArrayBuffer(4);
const int32 = new Int32Array(sab);
int32[0] = 10;
Atomics.add(int32, 0, 5);
console.log(int32[0]);

medium

A. 10

B. 5

C. 15

D. NaN

Solution

1. Step 1: Understand initial value and Atomics.add

   The int32[0] is set to 10. Then Atomics.add adds 5 to this value atomically.

2. Step 2: Calculate the new value

   10 + 5 = 15, so int32[0] becomes 15.

3. Final Answer:

   15 -> Option C

4. Quick Check:

   Atomics.add adds value safely = 15 [OK]

Hint: Atomics.add adds value and returns old value, array updates [OK]

Common Mistakes:

• Expecting Atomics.add to return new value
• Ignoring atomic operation effect
• Confusing initial and updated values

4. What is wrong with the following code snippet?

const sab = new SharedArrayBuffer(8);
const uint8 = new Uint8Array(sab);
uint8[0] = 255;
Atomics.store(uint8, 0, 256);
console.log(uint8[0]);

medium

A. Atomics.store cannot be used with Uint8Array

B. 256 is out of range for Uint8Array element

C. SharedArrayBuffer size is too small

D. Uint8Array cannot be created from SharedArrayBuffer

Solution

1. Step 1: Check Uint8Array element range

   Uint8Array elements can only hold values from 0 to 255. The value 256 is out of this range.

2. Step 2: Understand effect of storing 256

   Storing 256 wraps around to 0 because 256 mod 256 = 0, so uint8[0] becomes 0, not 256.

3. Final Answer:

   256 is out of range for Uint8Array element -> Option B

4. Quick Check:

   Uint8 max value 255, 256 wraps to 0 [OK]

Hint: Uint8Array values must be 0-255; higher values wrap [OK]

Common Mistakes:

• Assuming Atomics.store rejects Uint8Array
• Ignoring value wrapping behavior
• Thinking SharedArrayBuffer size is insufficient

5. You want to safely increment a shared counter in a Node.js worker thread using SharedArrayBuffer. Which code snippet correctly increments the counter without race conditions? // Shared buffer and typed array const sab = new SharedArrayBuffer(4); const counter = new Int32Array(sab); counter[0] = 0; // Increment function function increment() { // Which line correctly increments? }

hard

A. Atomics.add(counter, 0, 1);

B. counter[0] = counter[0] + 1;

C. counter[0] += 1;

D. counter[0]++;

Solution

1. Step 1: Understand race conditions in shared memory

   Directly modifying counter[0] with normal operators can cause race conditions when multiple threads run concurrently.

2. Step 2: Use atomic operations for safe increments

   Atomics.add(counter, 0, 1) safely increments the value at index 0 without conflicts.

3. Final Answer:

   Atomics.add(counter, 0, 1); -> Option A

4. Quick Check:

   Use Atomics for safe shared memory updates [OK]

Hint: Always use Atomics methods to update shared memory safely [OK]

Common Mistakes:

• Using normal arithmetic on shared memory
• Ignoring atomicity causing race conditions
• Assuming ++ or += are thread-safe