Web Workers for parallel processing in Deno - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a new Web Worker in Deno.

Deno

const worker = new Worker(new URL('worker.js', import.meta.url), { type: '[1]' });

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

Aworker

Bmodule

Cclassic

Dscript

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'classic' or 'script' which are not supported in Deno workers.

Omitting the type option.

In Deno, Web Workers are created with type 'module' to support ES modules.

2fill in blank

medium

Complete the code to send a message from the main thread to the worker.

Deno

worker.[1]('Hello from main thread');

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

ApostMessage

BsendMessage

Cemit

Ddispatch

Attempts:

3 left

💡 Hint

Common Mistakes

Using non-existent methods like 'sendMessage' or 'emit'.

Trying to call 'dispatch' which is not a worker method.

The postMessage method sends data to the worker thread.

3fill in blank

hard

Fix the error in the worker code to receive messages correctly.

Deno

self.[1] = (event) => {
  console.log('Message from main:', event.data);
};

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

Aonreceive

BpostMessage

Csend

Donmessage

Attempts:

3 left

💡 Hint

Common Mistakes

Using postMessage instead of onmessage to receive messages.

Using incorrect event handler names like 'onreceive'.

The worker listens for messages using the onmessage event handler.

4fill in blank

hard

Fill both blanks to send a message back from the worker to the main thread.

Deno

self.[1] = (event) => {
  self.[2]('Received: ' + event.data);
};

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

Aonmessage

BpostMessage

CsendMessage

Donreceive

Attempts:

3 left

💡 Hint

Common Mistakes

Using sendMessage which does not exist.

Mixing up event handler and sending method names.

The worker uses onmessage to receive and postMessage to send messages.

5fill in blank

hard

Fill all three blanks to properly terminate the worker after receiving a message.

Deno

worker.[1] = (event) => {
  console.log('Worker says:', event.data);
  worker.[2]();
  worker.[3] = null;
};

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

Aonmessage

Bterminate

Conerror

Donclose

Attempts:

3 left

💡 Hint

Common Mistakes

Using onerror or onclose instead of onmessage.

Not calling terminate() to stop the worker.

The main thread listens with onmessage, calls terminate() to stop the worker, and clears the onmessage handler.

Practice

1. What is the main purpose of using Web Workers in Deno?

easy

A. To create a new HTTP server

B. To run tasks in parallel without blocking the main thread

C. To manage file system permissions

D. To handle database connections

Solution

1. Step 1: Understand Web Workers role

   Web Workers allow running scripts in background threads separate from the main thread.

2. Step 2: Identify the benefit

   This prevents the main thread from freezing during heavy tasks, enabling parallel processing.

3. Final Answer:

   To run tasks in parallel without blocking the main thread -> Option B

4. Quick Check:

   Web Workers = Parallel tasks without blocking [OK]

Hint: Web Workers run code in background threads [OK]

Common Mistakes:

• Thinking Web Workers handle HTTP or database tasks directly
• Confusing Web Workers with main thread code
• Assuming Web Workers manage permissions

2. Which of the following is the correct way to send a message from the main thread to a Web Worker in Deno?

easy

A. worker.postMessage('Hello')

B. worker.emit('Hello')

C. worker.send('Hello')

D. worker.message('Hello')

Solution

1. Step 1: Recall message sending method

   In Web Workers, the method to send data to the worker is postMessage.

2. Step 2: Match correct syntax

   Among the options, only worker.postMessage('Hello') is valid and recognized.

3. Final Answer:

   worker.postMessage('Hello') -> Option A

4. Quick Check:

   Send message = postMessage [OK]

Hint: Use postMessage to send data to workers [OK]

Common Mistakes:

• Using send() instead of postMessage()
• Confusing emit() with event emitters
• Trying to call message() which doesn't exist

3. Consider this Deno Web Worker code snippet:

const worker = new Worker(new URL('./worker.ts', import.meta.url).href, { type: 'module' });
worker.onmessage = (e) => console.log('Result:', e.data);
worker.postMessage(5);

And the worker.ts contains:

self.onmessage = (e) => {
  const num = e.data;
  self.postMessage(num * 2);
};

What will be printed in the console?

medium

A. Result: 10

B. Result: 5

C. Result: undefined

D. No output, error occurs

Solution

1. Step 1: Understand message flow

   The main thread sends 5 to the worker. The worker receives it as num.

2. Step 2: Calculate worker response

   The worker multiplies 5 by 2 and sends back 10. The main thread logs 'Result: 10'.

3. Final Answer:

   Result: 10 -> Option A

4. Quick Check:

   5 * 2 = 10 output [OK]

Hint: Worker doubles input and sends back result [OK]

Common Mistakes:

• Assuming worker returns original number
• Confusing onmessage event usage
• Expecting error due to URL usage

4. What is wrong with this Deno Web Worker code snippet?

const worker = new Worker('./worker.ts', { type: 'module' });
worker.postMessage('start');
worker.onmessage = (e) => console.log(e.data);
worker.terminate();

medium

A. Worker path './worker.ts' is invalid in Deno

B. postMessage is called before onmessage, causing error

C. Worker is terminated immediately, so no message is received

D. Missing event listener for error events

Solution

1. Step 1: Analyze termination timing

   The worker is terminated right after sending the message, likely before it can respond.

2. Step 2: Effect on message reception

   Because of immediate termination, the onmessage handler never receives data.

3. Final Answer:

   Worker is terminated immediately, so no message is received -> Option C

4. Quick Check:

   Terminate too soon = no response [OK]

Hint: Terminate workers only after receiving messages [OK]

Common Mistakes:

• Thinking postMessage order causes error
• Ignoring worker path resolution rules
• Forgetting to handle errors but not critical here

5. You want to process a large array of numbers in parallel using Deno Web Workers. Which approach correctly splits the work and collects results efficiently?

hard

A. Process the array in the main thread without workers for simplicity

B. Create one worker and send the entire array repeatedly for each number

C. Use a single worker and process numbers sequentially inside it

D. Create multiple workers, send each a slice of the array, collect partial results, then combine them

Solution

1. Step 1: Understand parallel processing strategy

   Splitting the array into parts and assigning each to a separate worker allows true parallelism.

2. Step 2: Collect and combine results

   Each worker returns partial results; the main thread combines them for the final output.

3. Final Answer:

   Create multiple workers, send each a slice of the array, collect partial results, then combine them -> Option D

4. Quick Check:

   Split work + combine results = efficient parallelism [OK]

Hint: Split data among workers, then merge results [OK]

Common Mistakes:

• Sending full array multiple times wastes resources
• Using one worker loses parallel benefit
• Ignoring main thread processing slows app