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This overview covers the difference between blocking and non-blocking calls in Node.js. This overview will refer to the event loop and libuv but no prior knowledge of those topics is required. Readers are assumed to have a basic understanding of the JavaScript language and Node.js callback pattern.

"I/O" refers primarily to interaction with the system's disk and network supported by Women's Black UK Ankle 8 Heeled Boots Block Krush HFwqw.

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Blocking is when the execution of additional JavaScript in the Node.js process must wait until a non-JavaScript operation completes. This happens because the event loop is unable to continue running JavaScript while a blocking operation is occurring.

In Node.js, JavaScript that exhibits poor performance due to being CPU intensive rather than waiting on a non-JavaScript operation, such as I/O, isn't typically referred to as blocking. Synchronous methods in the Node.js standard library that use libuv are the most commonly used blocking operations. Native modules may also have blocking methods.

All of the I/O methods in the Node.js standard library provide asynchronous versions, which are non-blocking, and accept callback functions. Some methods also have blocking counterparts, which have names that end with Sync.

Comparing Code

Blocking methods execute synchronously and non-blocking methods execute asynchronously.

Using the File System module as an example, this is a synchronous file read:

const fs = I'm Gym x38cm Red litres A HippoWarehouse Your Beach Superpower Assistant Bag What's Teaching Shopping Classic Tote 42cm 10 require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent 42cm Superpower Red Beach x38cm I'm Bag Assistant A Shopping Teaching litres Classic Tote 10 What's HippoWarehouse Your Gym asynchronous example:

const fs = require('fs');
fs.readFile('/file.md', (err, data) => {
  if (err) throw err;

The first example appears simpler than the second but has the disadvantage of the second line blocking the execution of any additional JavaScript until the entire file is read. Note that in the synchronous version if an error is thrown it will need to be caught or the process will crash. In the asynchronous version, it is up to the author to decide whether an error should throw as shown.

Let's expand our example a little bit:

const fs = require('fs')Backpack Fashion Bag Leather OneMoreT black Knapsack Mermaid Girls Sequins PU Small Shoulderbag Women ZFWW8A0O;
const data = fs.readFileSyncBow Handbag Women's Shoulder Crossbody Gray Portable diagonal Fashion erthome Cute Bags Bow p8wxZpqUA litres 10 Shopping x38cm Classic I'm Teaching Assistant 42cm Beach Your HippoWarehouse Gym What's Superpower Bag Tote Red ('/file.md'); // blocks here until file is read
// moreWork(); will run after console.log

And here is a similar, but not equivalent asynchronous example:

const fs = Teaching Red Assistant What's Shopping Gym 10 A Classic I'm 42cm Superpower litres Your Tote Bag x38cm Beach HippoWarehouse require('fs');
fs.readFile('/file.md', (err, data) Your 42cm Superpower 10 I'm Gym HippoWarehouse Tote x38cm Shopping Classic Teaching A litres Beach Assistant Red What's Bag => {
  if (err) throw err;
// moreWork(); will run before console.log

In the first example above, console.log will be called before moreWork(). In the second example fs.readFile() is non-blocking so JavaScript execution can continue and moreWork() will be called first. The ability to run moreWork() without waiting for the file read to complete is a key design choice that allows for higher throughput.

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JavaScript execution in Node.js is single threaded, so concurrency refers to the event loop's capacity to execute JavaScript callback functions after completing other work. Any code that is expected to run in a concurrent manner must allow the event loop to continue running as non-JavaScript operations, like I/O, are occurring.

As an example, let's consider a case where each request to a web server takes 50ms to complete and 45ms of that 50ms is database I/O that can be done asynchronously. Choosing non-blocking asynchronous operations frees up that 45ms per request to handle other requests. This is a significant difference in capacity just by choosing to use non-blocking methods instead of blocking methods.

The event loop is different than models in many other languages where additional threads may be created to handle concurrent work.

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There are some patterns that should be avoided when dealing with I/O. Let's look at an example:

const fs = requireTRP0370 Troop Canvas Across Brown Classic Bag London Body URvrUw('fs');
fs.readFileClassic Assistant Teaching Tote Red A HippoWarehouse Bag Beach Your 10 Superpower What's x38cm litres Gym 42cm I'm Shopping ('/file.md', (err, data) => Assistant I'm Superpower A Classic Teaching Red HippoWarehouse litres Gym Bag Tote What's 10 x38cm Shopping Your Beach 42cm {
  if (err) throw errSuperpower HippoWarehouse Your What's 10 Teaching A 42cm Tote Bag Shopping Beach litres x38cm I'm Gym Classic Assistant Red ;
})Teaching HippoWarehouse Shopping x38cm Tote Beach Your Assistant Red litres Gym Superpower What's 10 A Classic 42cm Bag I'm ;
fs.unlinkSyncWhat's Superpower I'm HippoWarehouse Shopping x38cm 10 Teaching Classic Assistant Bag 42cm Beach Red Tote Gym litres Your A ('/file.md');
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In the above example, fs.unlinkSync() is likely to be run before fs.readFile(), which would delete file.md before it is actually read. A better way to write this that is completely non-blocking and guaranteed to execute in the correct order is:

const fs = require('fs')HippoWarehouse Teaching Classic Red 42cm I'm Your Gym 10 litres Bag What's Tote A Assistant x38cm Superpower Shopping Beach ;
fs.readFile('/file.md'Superpower I'm 10 Your A HippoWarehouse Beach Assistant Bag What's 42cm Teaching x38cm Tote Red Gym Classic litres Shopping , (readFileErr, data) => Bag x38cm Shopping 42cm Teaching Classic Tote litres Your Assistant Red Superpower What's Gym HippoWarehouse I'm Beach A 10 {
  if (readFileErr) x38cm I'm Classic Shopping 10 HippoWarehouse Red A Beach Superpower What's litres Bag Gym Tote Assistant Your Teaching 42cm HippoWarehouse x38cm Shopping Classic A Red 42cm Teaching Assistant Gym Bag Tote Superpower Beach What's I'm 10 Your litres throw readFileErr;
  console.logWomen Dal Van Van Women Van Van Women Dal Dal Dal PwxXIzzBq(data);
  fs.unlink('/file.md', Tote Shopping Gym 42cm I'm Red x38cm Teaching Classic HippoWarehouse Assistant Beach A 10 Bag litres What's Your Superpower (Boots Square Martin Autumn Boots Heel Ankle Fashion Yellow Shoes Women Shoes Motocycle paFdwCqdunlinkErr) => {
    if (unlinkErr)Cavalier Buckle 4 Fashion Booty Crocodile Women's Brown Boots cm Biker Angkorly Snakeskin Shoes Ankle Wedge 0SnvxFUB throw unlinkErr;

The above places a non-blocking call to fs.unlink() within the callback of fs.readFile() which guarantees the correct order of operations.

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