Web Workers for parallel processing in Deno - Performance & Optimization

Performance: Web Workers for parallel processing

HIGH IMPACT

Improves interaction responsiveness by offloading heavy tasks from the main thread to background threads.

Running CPU-intensive tasks without blocking the UI

Deno

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

Runs heavy computation in a separate thread, keeping the main thread free for UI updates.

📈 Performance GainMain thread remains responsive; input latency (INP) improves significantly.

Running CPU-intensive tasks without blocking the UI

Deno

const result = heavyComputation(); // runs on main thread
console.log(result);

Blocks the main thread causing UI freezes and slow input response.

📉 Performance CostBlocks rendering and input for the duration of the task, increasing INP significantly.

Performance Comparison

Pattern	DOM Operations	Reflows	Paint Cost	Verdict
Heavy computation on main thread	Minimal	Blocks reflows during execution	Blocks paint updates	[X] Bad
Heavy computation in Web Worker	Minimal	No reflows blocked	No paint blocking	[OK] Good

Rendering Pipeline

Web Workers run scripts in background threads separate from the main UI thread, preventing blocking during heavy computations.

→JavaScript Execution

→Input Handling

→Rendering

⚠️ BottleneckMain thread JavaScript execution blocking input and rendering

Core Web Vital Affected

INP

Improves interaction responsiveness by offloading heavy tasks from the main thread to background threads.

Optimization Tips

1Offload heavy computations to Web Workers to keep the main thread responsive.

2Avoid running CPU-intensive tasks directly on the main thread to reduce input latency.

3Use message passing to communicate between main thread and workers without blocking.

Performance Quiz - 3 Questions

What is the main performance benefit of using Web Workers?

A They run scripts in parallel without blocking the main thread

B They reduce the size of JavaScript bundles

C They improve CSS selector matching speed

D They decrease image loading times

DevTools: Performance

How to check: Record a performance profile while interacting with the page. Look for long tasks on the main thread and check if heavy computations appear there.

What to look for: Long tasks blocking main thread indicate poor use of Web Workers; absence means good parallel processing.

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