NodeJS Interview and Prep: What is blocking vs. non-blocking?

Marika Lam

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Jun 19, 2024

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Blocking

• Blocking is when the execution of additional JavaScript in the NodeJS process must wait until a non-Javscript operation completes. This happens because the event loop is unable to continue running Javascript while a blocking operation is occurring.
• Blocking methods execute synchronously.
• Synchronous execution usually refers to code executing in sequence.
• Using the file system module as an example, this is a synchronous file read.

const fs = require('node:fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

• Blocking refers to operations that block further execution until that operation finishes.
• Below, fetch is a non-blocking operation as it does not stall alert(3) from execution

// Blocking: 1,... 2
alert(1);
var value = localStorage.getItem('foo');
alert(2);

• An example of synchronous, blocking operations is how some web servers like ones in Java or PHP handle I/O or network requests. If your code reads from a file or the database, your code blocks everything after it from executing. In that period, your machine is holding onto memory and processing time for a thread that isn’t doing anything!