JavaScript, Asynchronous Programming, and Promises

In this tutorial, you’ll learn what promises are in JavaScript, which states JavaScript promises can be in, and how to handle asynchronous errors in JS promises.

If you’re already familiar with this topic, you might be interested in learning how to handle promise chains in JavaScript.

Introduction

Until now, you have only worked with regular values. You’ve created a variable or constant, saved something there, and it was immediately available for use. For example, you could have printed it to the console.

But what if the value does not appear immediately, but rather take time to appear? We often get data from a database or an external server. These operations take time and there are two ways to work with them:

• We can try to block the execution of the program until we receive the data.
• We can continue the execution and deal with the data later when it appears.

This is not to say that one method is definitely better than the other. Both suit different needs as we need different behavior in different situations.

If the data you are waiting for is critical to moving forward, then you need to block the execution and you can’t get around it. And if you can postpone the processing, then, of course, it’s not worth wasting time, because you can do something else.

What Is a JavaScript Promise Exactly?

A promise is a special type of object that helps you work with asynchronous operations.

Many functions will return a promise to you in situations where the value cannot be retrieved immediately.

const userCount = getUserCount();3
console.log(userCount); // Promise {<pending>}

In this case, getUserCount is the function that returns a Promise. If we try to immediately display the value of the userCount variable, we get something like Promise {<pending>}.

This will happen because there is no data yet and we need to wait for it.

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Asynchronous Programming in JavaScript

When a program is executed by the processor it runs straight, with only one thing happening at a time. There are many programs that interact with the things outside other than the processor like a network request, data read from the hard disk. In this era in which computers have multiple processor cores available there’s no sense sitting there waiting for something when you could let the other task come along on another processor core and let you know when it’s done.

This lets us get other work done in the meantime, which is the basis of asynchronous programming. It is up to the programming environment we are using (web browsers/node in our case) to provide us with APIs that allow us to run such tasks asynchronously.

There are two main types of asynchronous code style one can come across in JavaScript code, old-style callbacks, and newer promise-style code. Let us review each of these in turn.

Callbacks:

A callback is a function that is to be executed after another function has finished executing — hence the name ‘call back’.

It is a function passed into another function as an argument to be executed later.

The function that takes a callback function as an argument is known as a High-Order function.

Callbacks are a way to make sure a certain code doesn’t execute until the other code has already finished execution.

Ideally, the function that performs an async action takes an extra argument, a callback function. When the action finishes, the callback function is called with the result.

Let us see an example:

fs.readFile('/input.json', function (error, data) {
  if(error) {
    //handle error logic
    console.log(error);
    return;
  }
//no error process data
  console.log(data);
})

In the above code snippet, we are using the readFile method in fs class (available as node API). This method reads a file asynchronously. It takes a callback function as an argument (here second argument) which is being called when the reading of the file is completed or there is an error in the process.

The problem with the callback arises only when we have a callback having another callback function as an argument creating a nested callback structure known as “Callback Hell”. As the number of nesting increases, it becomes difficult to manage and understand. It looks something like this.

firstFunction(args, function() {
  secondFunction(args, function() {
    thirdFunction(args, function() {
      // And so on…
    });
  });
});

Promises:

Promises are a first-class representation of a value that may be made available in the future.

It allows us to associate handlers with an asynchronous action’s eventual success value or failure reason.

This lets asynchronous methods return values like synchronous methods: instead of immediately returning the final value, the asynchronous method returns a promise to supply the value at some point in the future.

#promises #asynchronous #javascript #programming

A Beginner’s Guide To Promises In JavaScript

Welcome to my post about the JavaScript Promises. I hope it can be helpful since when we start programming it is difficult to enter the context.

Promises are one of the largest tools in the world of JavaScript that helps us to manage future situations in the flow of execution of a program.

The promises originated in the realm of functional programming, generally to handle asynchronous programming. In short, they allow us to define how data that will only be available in the future will be treated, specifying what will be done with that data later.

The promises were introduced in the standard in ES6, the truth is that they have been used for a long time since several libraries had implemented them to solve their needs in a more elegant way.

The Promise object represents the eventual completion or failure of an asynchronous operation and its resulting value.

#programming #coding #javascript-promise #promises #javascript

JavaScript Asynchronous Operation — Read File Directory with Callback or Promises

Asynchronous Operation — Why do we care?

JavaScript is a synchronous, blocking, single threaded language. Synchronous single threaded means JavaScript waits for the task to be completed before being able to move on to the next task. However, Asynchronous JavaScript makes it so much faster if each task takes a really long time! Instead of waiting for the task beforehand to move on, there can be independent threads to start the tasks simultaneously.

http://www.phpmind.com/blog/2017/05/synchronous-and-asynchronous/

In the computer programming world, synchronous and asynchronous may still be considered “fast” in human time. But let’s imagine a scenario if a request was sent to read an entire database of files and there are files in there are REALLY huge therefore take a long time to read. Instead of waiting, what if we can start reading the other files in the directory at the same time. This is Asynchronous JavaScript.

Asynchronous Operation — Code Styles

In JavaScript, functions are first class citizens. This means functions can be treated like values of other types. Functions can be passed to other functions as arguments, returned from other functions as values, and stored in variables.

Async operations are put into an event queue which runs after the main thread has finished processing so that they do not stop subsequent JavaScript from running.

There are 2 main asynchronous concepts that JavaScript uses which leverage functions first class citizenship:

1. Async Callbacks
2. Promises

For other examples, to help you further solidify your understanding of these 2 concepts.

Introducing asynchronous JavaScript

In this article we briefly recap the problems associated with synchronous JavaScript, and take a first look at some of…

developer.mozilla.org

Callbacks, Promises, and Async

Synchronous operations in JavaScript entails having each step of an operation waits for the previous step to execute…

scotch.io

Let’s consider an AJAX (Asynchronous JavaScript and XML) request to the server.

http://www.phpmind.com/blog/2017/05/synchronous-and-asynchronous/

The user clicks on the browser which results in sending an HTTP request to the server to read an entire directory, which could take some time. Instead of having the browser stop running any other functions and wait for the response, another function is set up to wait for this response and react when the response is received.

Let’s see how we can write these Asynchronous Operations.

#programming #javascript #asynchronous #promises #coding

JavaScript Promise: Methods Comparison

Introduction

Promises in JavaScript are used to handle asynchronous operations by keeping track of whether a certain event has happened. If that certain event has taken place, it determines what happens next. Promises return a value which is either a resolved value or a reason why it’s rejected. They can handle multiple asynchronous operations easily and they provide better error handling than callbacks and events.

Callback: A callback is a function that is passed into another function as an argument to be executed later.

Events: Events provide a dynamic interface to a WebPage and are connected to elements in the Document Object Model(DOM), for example: onclick(), onmouseover() etc.

A Promise has four states

Pending: Before the event has happened, the promise is in the pending state.

Settled: Once the event has happened it is then in the settled state.

Fulfilled: Action related to the promise has succeeded.

Rejected: Action related to the promise has failed.

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Object-oriented Programming in JavaScript: Made Super Simple. DO NOT MISS!!!

Object-oriented programming in JavaScript: learn all about objects, prototypes, prototypical inheritance, this and more.
TABLE OF CONTENT:

00:00: What is OOP?
01:46: Four Pillars of OOP
08:50: Setting Up the Development Environment 11:07: Objects
11:53: Object Literals
14:58: Factories
17:50: Constructors
23:27: Constructor Property
25:53: Functions are Objects
31:09: Value vs Reference Types
37:00: Adding or Removing Properties
40:54: Enumerating Properties
43:45: Abstraction
47:48: Private Properties and Methods
51:55: Getters and Setters
57:32: Exercise
59:42: Solution

📺 The video in this post was made by Programming with Mosh
The origin of the article: https://www.youtube.com/watch?v=PFmuCDHHpwk&list=PLTjRvDozrdlxEIuOBZkMAK5uiqp8rHUax&index=3
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