Understanding the Event Loop, Callbacks, Promises, and Async/Await in JavaScript

Introduction

In the early days of the internet, websites often consisted of static data in an HTML page. But now that web applications have become more interactive and dynamic, it has become increasingly necessary to do intensive operations like make external network requests to retrieve API data. To handle these operations in JavaScript, a developer must use asynchronous programming techniques.

Since JavaScript is a single-threaded programming language with a synchronous execution model that processes one operation after another, it can only process one statement at a time. However, an action like requesting data from an API can take an indeterminate amount of time, depending on the size of data being requested, the speed of the network connection, and other factors. If API calls were performed in a synchronous manner, the browser would not be able to handle any user input, like scrolling or clicking a button, until that operation completes. This is known as blocking.

In order to prevent blocking behavior, the browser environment has many Web APIs that JavaScript can access that are asynchronous, meaning they can run in parallel with other operations instead of sequentially. This is useful because it allows the user to continue using the browser normally while the asynchronous operations are being processed.

As a JavaScript developer, you need to know how to work with asynchronous Web APIs and handle the response or error of those operations. In this article, you will learn about the event loop, the original way of dealing with asynchronous behavior through callbacks, the updated ECMAScript 2015 addition of promises, and the modern practice of using async/await.

Note:  This article is focused on client-side JavaScript in the browser environment. The same concepts are generally true in the Node.js  environment, however Node.js uses its own C++ APIs  as opposed to the browser’s Web APIs . For more information on asynchronous programming in Node.js, check out How To Write Asynchronous Code in Node.js .

The Event Loop

This section will explain how JavaScript handles asynchronous code with the event loop. It will first run through a demonstration of the event loop at work, and will then explain the two elements of the event loop: the stack and the queue.

JavaScript code that does not use any asynchronous Web APIs will execute in a synchronous manner—one at a time, sequentially. This is demonstrated by this example code that calls three functions that each print a number to the console:

// Define three example functions
function first() {
  console.log(1)
}

function second() {
  console.log(2)
}

function third() {
  console.log(3)
}

Copy

In this code, you define three functions that print numbers with console.log().

Next, write calls to the functions:

// Execute the functions
first()
second()
third()

Copy

The output will be based on the order the functions were called—first(), second(), then third():

Output
1
2
3

When an asynchronous Web API is used, the rules become more complicated. A built-in API that you can test this with is setTimeout, which sets a timer and performs an action after a specified amount of time. setTimeout needs to be asynchronous, otherwise the entire browser would remain frozen during the waiting, which would result in a poor user experience.

Add setTimeout to the second function to simulate an asynchronous request:

// Define three example functions, but one of them contains asynchronous code
function first() {
  console.log(1)
}

function second() {
  setTimeout(() => {
    console.log(2)
  }, 0)
}

function third() {
  console.log(3)
}

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Understanding JavaScript: Promises, Async & Await!!

Analogy

We all know the importance of promises in our life. We even have a special day dedicated to it :) But how well do we know the importance of promises in JavaScript? Well if you don’t know it yet, it’s a great time to know it because they are becoming more and more popular. So what are promises? Let’s try to understand it through an analogy.

Suppose you are a top class rapper and you haven’t released an album for a while and fans are asking for it day and night. So what you do is that you “promise” them that whenever it will be out, all of them would be notified. To get this done you give your fans a list. They can fill in their email addresses, so that when the album becomes available, all the subscribers instantly receive it. And even if something goes wrong, say a pandemic, so that you can’t release the album, they will still be notified.

Now everyone is happy: You, because the people don’t crowd you anymore, and fans, because they won’t miss any news on the album.

This is a real-life analogy for things we often have in programming:

1. A “producing code” that does something and may take time. That’s a “rapper”.
2. A “consuming code” that wants the result of the “producing code” once it’s ready. Many functions may need that result. These are the “fans”.
3. A promise is a special JavaScript object that links the “producing code” and the “consuming code” together. In terms of our analogy: this is the “subscription list”. The “producing code” takes whatever time it needs to produce the promised result, and the “promise” makes that result available to all of the subscribed code when it’s ready.

JavaScript promises are much more complex than a simple subscription list: they have additional features and limitations. But it’s fine to begin with.

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JavaScript Loops Tutorial

JavaScript loops made simple.

📺 The video in this post was made by Programming with Mosh
The origin of the article: https://www.youtube.com/watch?v=s9wW2PpJsmQ&list=PLTjRvDozrdlxEIuOBZkMAK5uiqp8rHUax&index=8
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What is Callback-Function in JavaScript? How it is Replaced By Promises?

Let us understand term _Callback _by an real-world example: Suppose, you are calling to your Girlfriend (If you have) and she is busy in another call then she send message to you : “I am busy right now, Call back you later.!!”. After completing her work, she calls you back and this is what call back in JavaScript as well.

In JavaScript, When a function is executing (Girlfriend is talking with someone) then after function execution is completed another function is started for execution this is call back function.

Now you are thinking that its depend upon when, where you are calling function and all function call is “Call-back function”.

Here, _printWorld() _function is executed after _printHello() _complete its execution but this is not call-back function example because _printHello() _is not Asynchronous function. Suppose, _printHello() _prints after 1 Second then _printWorld() _executes first.

What if we want “Hello World” output when Asynchronous function is there. We can pass function as argument and calls it after _printHello() _complete its execution. Here below code snippet of how _function pass as argument _:

Callback function can be defined as a function passed by argument and executes when one function completes its execution.

Suppose, If you have API (Application Programming Interface) to get Students Roll numbers and select one of Roll number — getting that roll number’s data and print that data. We don’t have API to get students data so we are using _setTimeout() _Async function and getting roll number after 2s, We are also selecting one of roll number manually after 2s and print Roll number’s data after 2s. This can be done by call back function.

The program became complex and complex if we have too many things to do like Getting Students data, Selecting one of them student, get student’s roll number and get result by roll number then it become very complex. If you have any Error in this then debugging is also tedious task, this things is called “Callback Hell”, which is shape like “Pyramid Of Doom”.

To overcome with this problem, Promises is introduced in JavaScript. Promises has three states : Pending, Resolved, Reject. Promises is created by Constructor : new Promise(). It has one executor function which has two arguments (resolve, reject).

Promise object has three methods: then(), catch() & finally().

If Promise is successfully executed then its data is transferred through resolve function and if it has error then passed through reject function.

We have implemented same task which is done using call back function in Promises and its easily understandable However it is complicated compare to callback function but when you use promises for sometimes then it’s easy to implement.

In _getRollNumber(), _resolve method’s data is caught by then() functions arguments and reject method’s data is caught by catch() function. Here In Promises, Every task has different promises because of that it is easy to debug and readable compare to call back function. You can see that there is no shape like “Pyramid of Doom” in Promises. This is how Callback function is replaced by Promises.

Thank you for reading!

This article was originally published on Medium.com

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JavaScript Fetch API Explained | Callbacks, Promises, Async Await

Learn about Callbacks, Promises, and Async Await as the JavaScript Fetch API is explained in this tutorial. You will also learn about thenables and how async / await replaces them in our JS code. The first 30 minutes covers the concepts. The last 30 minutes gives examples of retrieving data from different APIs with Fetch.

Quick Concepts outline:
Fetch API with Async / Await
(0:00) Intro
(0:29) What is a callback function?
(1:15) What is the problem with callbacks?
(3:00) JavaScript Promises have 3 states
(5:28) A promise may not return a value where you expect it to: You need to wait for a promise to resolve
(6:58) Using thenables with a promise
(20:15) An easy mistake to make with promises
(24:00) Creating an async function
(25:00) Applying await inside the function
(33:45) Example 1: Retrieving user data
(40:00) Example 2: Retrieving dad jokes
(47:00) Example 3: Posting data
(49:40) Example 4: Retrieving data with URL parameters
(54:55) Abstract it all into single responsibility functions

Subscribe: https://www.youtube.com/c/DaveGrayTeachesCode/featured 

#async  #await  #fetch #javascript 

Javascript : Async Await in Javascript | Javascript Interview questions.

Explained in detail about

• Async
• Await
• Fetching country data using Async & Await
• How Async & Await better than promise

Dont miss to watch the video & ask your questions or doubts in comments

https://youtu.be/pJbsd2vAqdI

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