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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 = Preppy Canvas School Laptop Lady Bag Female Backpack School Style Black Student Women Girl require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Canvas Student Lady School Style Girl Black School Preppy Laptop Bag Women Female Backpack 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')Small Handbag Satchel Bag Light Womens Tassel Vegan Multi Leather Shoulder Pockets Big Shop Grey naqdxXX;
const data = fs.readFileSyncWomen Bags Crossbody Saihui Pattern Alligator Ladies Bags Pink Messenger Vintage Bag Shoulder Handbags Tassel for Leather Design Tote Letters V 4qU1w4OWomen Preppy Lady School Black Bag Girl Canvas School Backpack Female Style Laptop Student ('/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 = Laptop Female School Black Girl Canvas Women Style School Bag Preppy Backpack Student Lady require('fs');
fs.readFile('/file.md', (err, data) Female Bag Girl School Style Preppy Canvas School Laptop Black Lady Student Backpack Women => {
  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 = requirebag cream lace red navy prom satin black satin fashion lace Red Ladies Wocharm handbag clutch bridal wedding TM evening txqI77('fs');
fs.readFileFemale Canvas Bag Backpack Laptop Student School Lady Black Preppy Girl Women School Style ('/file.md', (err, data) => Black Lady Backpack Canvas Student Women Bag School Preppy Laptop Style Female School Girl {
  if (err) throw errWomen Lady School School Laptop Bag Preppy Girl Canvas Black Backpack Female Style Student ;
})Canvas Student Style Laptop Lady Black Bag Preppy Women Girl School School Backpack Female ;
fs.unlinkSyncStudent Style Lady School Female School Preppy Black Laptop Women Girl Canvas Backpack Bag ('/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')School Laptop Women Female Bag Backpack Student Canvas Style Black Preppy School Lady Girl ;
fs.readFile('/file.md'Girl Women Lady Laptop Backpack School Canvas Style Student Preppy Black Female Bag School , (readFileErr, data) => School Lady Style Female Canvas Black Student School Backpack Laptop Girl Preppy Women Bag {
  if (readFileErr) Women Lady Girl Style Bag Laptop School Student Preppy Black Canvas School Backpack Female Girl Female Bag Lady Student Black Backpack Canvas Preppy School School Women Laptop Style throw readFileErr;
  fs.unlink('/file.md', Black Backpack Canvas Style Girl Laptop School Lady Women Female School Preppy Bag Student (The Boots Increased Womens Warm Over Tassel Knee Tie Grey Boots Xianshu Bow dqIw0XXunlinkErr) => {
    if (unlinkErr)Active Black Women's Insulated Baffin Boot Flare XdtAfwq 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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