How does Puppeteer affect callback execution in Node.js?

Other Questions Puppeteer
1

I’m trying to figure out why my Node.js script behaves differently when I change the order of operations. Here’s what’s going on:

When I run this code, it opens a browser and reads a text file line by line:

const fs = require('fs');
const readline = require('readline');
const puppeteer = require('puppeteer');

(async () => {
  const fileReader = readline.createInterface({
    input: fs.createReadStream('myfile.txt'),
    output: process.stdout,
    terminal: false,
  });

  const browser = await puppeteer.launch({headless: false});

  fileReader.on('line', (line) => console.log(line));
  fileReader.on('close', () => console.log('Done reading'));
})();

However, if I move the puppeteer.launch() call before creating the readline interface, the callbacks for the stream don’t execute as expected. The browser opens, but no output appears in the console. Can someone explain why this behavior occurs? Is Puppeteer somehow blocking the callbacks?

2

The issue you’re experiencing is related to how Node.js handles the event loop and asynchronous operations. When you launch Puppeteer before setting up the file reader, it’s likely consuming significant resources and potentially blocking the event loop. This can cause delays in processing other asynchronous operations, including file I/O.

To mitigate this, consider using Promise.all() to launch Puppeteer and set up your file reader concurrently. This approach should allow both operations to initialize without one blocking the other:

(async () => {
  const [browser, fileReader] = await Promise.all([
    puppeteer.launch({headless: false}),
    createFileReader('myfile.txt')
  ]);

  // Rest of your code here
})();

function createFileReader(filename) {
  return new Promise((resolve) => {
    const reader = readline.createInterface({
      input: fs.createReadStream(filename),
      output: process.stdout,
      terminal: false,
    });
    resolve(reader);
  });
}

This should help ensure that both operations complete without interfering with each other’s execution.

3

hey soaringeagle, i had a simlar issue. puppeteer’s launch seems to block the event loop, delay your fileReader setup with a setTimeout for next tick. this should let callbacks run properly after the browser starts.

4

I’ve encountered this issue before, and it’s related to how Puppeteer interacts with Node’s event loop. When you launch Puppeteer first, it can monopolize resources, causing delays in other async operations.

One solution I’ve found effective is to use process.nextTick() to defer the file reading operation. This ensures the event loop completes one full cycle before starting to read the file, giving Puppeteer time to initialize without blocking other operations.

Here’s how you could modify your code:

(async () => {
  const browser = await puppeteer.launch({headless: false});

  process.nextTick(() => {
    const fileReader = readline.createInterface({
      input: fs.createReadStream('myfile.txt'),
      output: process.stdout,
      terminal: false,
    });

    fileReader.on('line', (line) => console.log(line));
    fileReader.on('close', () => console.log('Done reading'));
  });
})();

This approach has worked well for me in similar scenarios, allowing both Puppeteer and file operations to run smoothly.