Reading files synchronously in Node.js - Performance & Optimization

Performance: Reading files synchronously

HIGH IMPACT

This affects the main thread blocking time, delaying other operations and slowing overall responsiveness.

Reading a file in a Node.js application

Node.js

const fs = require('fs');
fs.readFile('file.txt', 'utf8', (err, data) => {
  if (err) throw err;
  console.log(data);
});

Reads file asynchronously, allowing other code and events to run without waiting.

📈 Performance GainNon-blocking I/O improves responsiveness and reduces input delay.

Reading a file in a Node.js application

Node.js

const fs = require('fs');
const data = fs.readFileSync('file.txt', 'utf8');
console.log(data);

Blocks the main thread until the file is fully read, preventing other code from running.

📉 Performance CostBlocks main thread for file read duration, causing input lag and delayed event handling.

Performance Comparison

Pattern	DOM Operations	Reflows	Paint Cost	Verdict
Synchronous file read	N/A	Blocks event loop, no DOM changes during read	Delays paint and input handling	[X] Bad
Asynchronous file read	N/A	Event loop remains free	Paint and input handled promptly	[OK] Good

Rendering Pipeline

Synchronous file reading blocks the Node.js event loop, delaying all subsequent JavaScript execution and UI updates in environments like Electron or server-side rendering.

→JavaScript Execution

→Event Loop

⚠️ BottleneckMain thread blocking during synchronous I/O

Core Web Vital Affected

INP

This affects the main thread blocking time, delaying other operations and slowing overall responsiveness.

Optimization Tips

1Avoid synchronous file reading to prevent blocking the main thread.

2Use asynchronous file reading to keep the event loop responsive.

3Blocking the main thread delays input handling and slows user experience.

Performance Quiz - 3 Questions

What is the main performance drawback of reading files synchronously in Node.js?

A It uses more memory than asynchronous reading.

B It blocks the main thread, causing input lag.

C It increases network latency.

D It causes CSS reflows in the browser.

DevTools: Performance

How to check: Record a performance profile while running the code; look for long tasks blocking the main thread during file read.

What to look for: Long blocking periods in the main thread timeline indicate synchronous file reading causing input delay.