Web Workers for parallel processing in Deno

Introduction

Web Workers let your program do many things at once without slowing down the main part. This helps keep your app smooth and fast.

When you need to do heavy calculations without freezing the user interface.

When you want to load or process data in the background while the user keeps interacting.

When you want to split tasks into smaller parts to finish faster.

When you want to keep animations or buttons responsive during complex work.

Syntax

Deno

const worker = new Worker(new URL('./worker.ts', import.meta.url).href, { type: 'module' });

worker.postMessage(data);

worker.onmessage = (event) => {
  console.log('Result from worker:', event.data);
};

worker.onerror = (event) => {
  console.error('Worker error:', event.message);
};

Use new Worker() with a module script URL in Deno.

Communicate with the worker using postMessage and listen with onmessage.

Examples

Start a worker and send it a number to process.

Deno

const worker = new Worker(new URL('./worker.ts', import.meta.url).href, { type: 'module' });
worker.postMessage({ number: 10 });
worker.onmessage = (e) => console.log(e.data);

Listen for messages from the worker and print them.

Deno

worker.onmessage = (event) => {
  console.log('Worker says:', event.data);
};

Handle errors that happen inside the worker.

Deno

worker.onerror = (event) => {
  console.error('Error in worker:', event.message);
};

Sample Program

This example creates a worker that calculates the square of a number. The main script sends the number 5, and the worker sends back 25. The main script then prints the result and stops the worker.

Deno

// main.ts
const worker = new Worker(new URL('./worker.ts', import.meta.url).href, { type: 'module' });

worker.postMessage(5);

worker.onmessage = (event) => {
  console.log(`Square of 5 is ${event.data}`);
  worker.terminate();
};

worker.onerror = (event) => {
  console.error('Worker error:', event.message);
};

// worker.ts
self.onmessage = (event) => {
  const num = event.data;
  const result = num * num;
  self.postMessage(result);
};

Important Notes

Workers run in a separate thread, so they cannot access the main thread's variables directly.

Always terminate workers when done to free resources.

Use message passing to send data back and forth between main and worker.

Summary

Web Workers help run tasks in parallel without freezing the app.

Use postMessage and onmessage to communicate.

Remember to handle errors and terminate workers when finished.

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