Passing data to workers in Node.js

Introduction

Workers let your program do many things at once. Passing data to workers helps them know what to do.

When you want to run heavy tasks without stopping the main program.

When you need to send information to a worker to start its job.

When you want to split work into smaller parts and share data with each part.

When you want to keep your app fast by doing work in the background.

Syntax

Node.js

const { Worker } = require('worker_threads');

const worker = new Worker('./worker.js', { workerData: yourData });

workerData is the key to send data to the worker.

The data can be any value that can be cloned, like numbers, strings, objects.

Examples

Sends an object with a name to the worker.

Node.js

const { Worker } = require('worker_threads');

const worker = new Worker('./worker.js', { workerData: { name: 'Alice' } });

Sends a number to the worker.

Node.js

const { Worker } = require('worker_threads');

const worker = new Worker('./worker.js', { workerData: 42 });

Sends an array of strings to the worker.

Node.js

const { Worker } = require('worker_threads');

const worker = new Worker('./worker.js', { workerData: ['apple', 'banana'] });

Sample Program

This example shows how to send data to a worker. The main file sends a task and a limit number. The worker adds numbers from 1 to the limit and sends back the result.

Node.js

/* main.js */
const { Worker } = require('worker_threads');

const worker = new Worker('./worker.js', { workerData: { task: 'count', limit: 5 } });

worker.on('message', (msg) => {
  console.log('Message from worker:', msg);
});

worker.on('error', (err) => {
  console.error('Worker error:', err);
});

worker.on('exit', (code) => {
  console.log('Worker stopped with exit code', code);
});

/* worker.js */
const { parentPort, workerData } = require('worker_threads');

if (workerData.task === 'count') {
  let count = 0;
  for (let i = 1; i <= workerData.limit; i++) {
    count += i;
  }
  parentPort.postMessage(`Sum from 1 to ${workerData.limit} is ${count}`);
} else {
  parentPort.postMessage('Unknown task');
}

Important Notes

Data sent with workerData is read-only inside the worker.

Use parentPort.postMessage() to send messages back to the main thread.

Workers run in separate threads, so data is cloned, not shared directly.

Summary

Workers get data through the workerData option when created.

This data tells the worker what to do or what information to use.

Workers send results back using messages with parentPort.postMessage().

Practice

1. In Node.js, how do you pass initial data to a worker thread when creating it?

easy

A. By using the workerData option in the Worker constructor

B. By sending a message after the worker starts

C. By setting a global variable inside the worker file

D. By importing a module inside the worker

Solution

1. Step 1: Understand worker creation

   When creating a worker thread, you can pass data directly using the workerData option in the Worker constructor.

2. Step 2: Recognize data passing method

   This method allows the worker to access the data immediately via the imported workerData from 'worker_threads'.

3. Final Answer:

   By using the workerData option in the Worker constructor -> Option A

4. Quick Check:

   Initial data to worker = workerData option [OK]

Hint: Pass data at creation with workerData option [OK]

Common Mistakes:

• Trying to set global variables inside the worker
• Sending data only after worker starts
• Using imports to pass data

2. Which of the following is the correct syntax to create a worker and pass data to it?

easy

A. new Worker('worker.js', { workerData: { value: 10 } })

B. new Worker('worker.js', { message: { value: 10 } })

C. new Worker('worker.js', { initialData: { value: 10 } })

D. new Worker('worker.js', { data: { value: 10 } })

Solution

1. Step 1: Recall Worker constructor options

   The correct option to pass data when creating a worker is workerData.

2. Step 2: Match syntax with correct option

   Only new Worker('worker.js', { workerData: { value: 10 } }) uses workerData correctly; others use invalid keys.

3. Final Answer:

   new Worker('worker.js', { workerData: { value: 10 } }) -> Option A

4. Quick Check:

   Correct option key = workerData [OK]

Hint: Use workerData option exactly in Worker constructor [OK]

Common Mistakes:

• Using incorrect option names like data or initialData
• Confusing message passing with initial data passing
• Missing curly braces around data

3. Given this worker code snippet:

const { parentPort, workerData } = require('worker_threads');
parentPort.postMessage(workerData.num * 2);

And main thread code:

const { Worker } = require('worker_threads');
const worker = new Worker('./worker.js', { workerData: { num: 5 } });
worker.on('message', (result) => console.log(result));

What will be printed to the console?

medium

A. 5

B. 10

C. undefined

D. Error

Solution

1. Step 1: Understand workerData usage

   The worker receives workerData with { num: 5 } and multiplies num by 2.

2. Step 2: Trace message sent back

   The worker sends 5 * 2 = 10 back via parentPort.postMessage, which the main thread logs.

3. Final Answer:

   10 -> Option B

4. Quick Check:

   workerData.num * 2 = 10 [OK]

Hint: Multiply workerData.num by 2 and print [OK]

Common Mistakes:

• Confusing workerData with message event data
• Expecting original number instead of doubled
• Not listening to 'message' event

4. What is wrong with this code snippet for passing data to a worker?

const { Worker } = require('worker_threads');
const worker = new Worker('./worker.js', { workerData: 123 });
worker.on('message', (msg) => console.log(msg));

Assuming worker.js expects workerData to be an object with a value property.

medium

A. Worker file path is incorrect

B. Missing event listener for 'error'

C. postMessage is missing in main thread

D. workerData should be an object, not a number

Solution

1. Step 1: Check workerData type

   The worker expects workerData as an object with a value property, but a number 123 is passed.

2. Step 2: Identify mismatch impact

   This mismatch can cause errors or undefined behavior inside the worker when accessing workerData.value.

3. Final Answer:

   workerData should be an object, not a number -> Option D

4. Quick Check:

   workerData type must match expected structure [OK]

Hint: Pass workerData as expected object type [OK]

Common Mistakes:

• Passing primitive instead of object
• Ignoring workerData structure requirements
• Not handling errors from wrong data

5. You want to create a worker that receives an array of numbers as workerData.numbers, doubles each number, and sends back the new array. Which main thread code correctly passes data and listens for the result?

hard

A. const worker = new Worker('./worker.js', { workerData: [1,2,3] });
worker.on('message', (result) => console.log(result));

B. const worker = new Worker('./worker.js', { data: [1,2,3] });
worker.on('message', (result) => console.log(result));

C. const worker = new Worker('./worker.js', { workerData: { numbers: [1,2,3] } });
worker.on('message', (result) => console.log(result));

D. const worker = new Worker('./worker.js');
worker.postMessage([1,2,3]);
worker.on('message', (result) => console.log(result));

Solution

1. Step 1: Verify the correct workerData structure

   The worker accesses the array as workerData.numbers, so only code passing { workerData: { numbers: [1,2,3] } } works correctly.

2. Step 2: Confirm result handling

   The correct code includes worker.on('message', (result) => console.log(result)) to log the doubled array sent back by the worker.

3. Final Answer:

   const worker = new Worker('./worker.js', { workerData: { numbers: [1,2,3] } }); worker.on('message', (result) => console.log(result)); -> Option C

4. Quick Check:

   Pass array inside object via workerData and listen for message [OK]

Hint: Wrap array in object for workerData and listen to message [OK]

Common Mistakes:

• Passing array directly without object wrapper
• Using wrong option name like data
• Trying to send data after worker creation