How To Do

What It Is and How to Use It

When writing JavaScript purposes, you could have encountered asynchronous features, such because the fetch perform within the browser or the readFile perform in Nodejs.

You could have gotten sudden outcomes in the event you used both of those features such as you usually would. This is as a result of they’re asynchronous features. This article guides what which means and how to use asynchronous features like a professional.

Introduction to Synchronous Function

JavaScript is a single-threaded language that may solely do one factor at a time. This signifies that if the processor encounters a perform that takes a very long time, JavaScript waits till the entire perform has been executed earlier than transferring on to different components of this system.

Most features are executed wholly by the processor. This signifies that throughout the execution of the stated features, no matter how lengthy it takes, the processor might be fully busy. These are known as synchronous features. An instance synchronous perform has been outlined under:

perform add(a, b) {
    for (let i = 0; i < 1000000; i ++) {
        // Do nothing
    return a + b;

// Calling the perform will take a very long time
sum = add(10, 5);

// However, the processor can't transfer to the 

This perform performs a big loop whose execution takes a very long time earlier than returning the sum of its two arguments.

After defining the perform, we known as it and saved its outcome within the sum variable. Next, we logged the worth of the sum variable. Even although executing the add perform takes some time, the processor can’t transfer on to log the sum till the execution is full.

The overwhelming majority of features you’ll encounter will behave in predictable methods just like the one above. However, some features are asynchronous and don’t behave like common features.

Introduction to Asynchronous Function

Asynchronous features do nearly all of their work exterior the processor. This signifies that although the perform would possibly take some time to full execution, the processor might be unoccupied and free to do extra work.

Here’s an instance of such a perform:


To improve effectivity, JavaScript allows the processor to transfer on to different duties that require the CPU even earlier than the asynchronous perform’s execution is full.

Since the processor moved on earlier than the asynchronous perform’s execution was full, its outcome is not going to be instantly obtainable. It might be pending. If the processor tried to execute different components of this system that relied on the pending outcome, we might get errors.

Therefore, the processor ought to solely execute components of this system that don’t rely upon the pending outcome. To do that, trendy JavaScript makes use of guarantees.

What is a Promise in JavaScript?

In JavaScript, a promise is a short lived worth that an asynchronous perform returns. Promises are the spine of recent asynchronous programming in JavaScript.

After a promise is created, both of two issues occurs. It both resolves when the asynchronous perform’s return worth is efficiently produced or rejects within the occasion of an error. These are occasions inside a promise’s lifecycle. Therefore, we will connect occasion handlers to the promise to be known as when it resolves or rejects.

All the code that requires the ultimate worth of an asynchronous perform will be hooked up to the promise’s occasion handler for when it resolves. All the code that handles the error of a rejected promise can even be hooked up to its corresponding occasion handler.

Here’s an instance the place we learn information from a file in Nodejs.

const fs = require('fs/guarantees');

fileReadPromise = fs.readFile('./good day.txt', 'utf-8');

fileReadPromise.then((information) => console.log(information));

fileReadPromise.catch((error) => console.log(error));

In the primary line, we import the fs/guarantees module.

In the second line, we name the readFile perform, passing within the title and encoding for the file whose contents we wish to learn. This perform is asynchronous; subsequently, it returns a promise. We retailer the promise within the fileReadPromise variable.

In the third line, we hooked up an occasion listener for when the promise resolves. We did this by calling the then technique on the promise object. As an argument to our name to the then technique, we handed within the perform to run if and when the promise resolves.

In the fourth line, we hooked up a listener for when the promise rejects. This is finished by calling the catch technique and passing within the error occasion handler as an argument.

A screenshot of an async javascript program.

An different strategy is to use the async and await key phrases. We will cowl this strategy subsequent.

Async and Await Explained

Async and Await key phrases can be utilized to write asynchronous Javascript in a manner that appears higher syntactically. In this part, I’ll clarify how to use the key phrases and what impact they’ve in your code.

The await key phrase is used to pause the execution of a perform whereas ready for an asynchronous perform to full. Here’s an instance:

const fs = require('fs/guarantees');

perform readData() {
	const information = await fs.readFile('./good day.txt', 'utf-8');

    // This line is not going to be executed till the info turns into obtainable


We used the await key phrase whereas making a name to readFile. This instructed the processor to wait till the file has been learn earlier than the subsequent line (the console.log) will be executed. This helps make sure that code that will depend on an asynchronous perform’s outcome is not going to be executed till the outcome turns into obtainable.

If you tried to run the above code, you’ll encounter an error. This is as a result of await can solely be used inside an asynchronous perform. To declare a perform as asynchronous, you utilize the async key phrase earlier than the perform declaration like so:

const fs = require('fs/guarantees');

async perform readData() {
	const information = await fs.readFile('./good day.txt', 'utf-8');

    // This line is not going to be executed till the info turns into obtainable

// Calling the perform so it runs

// Code at this level will run whereas ready for the readData perform to full
console.log('Waiting for the info to full')

Running this code snippet, you will notice that JavaScript executes the outer console.log whereas ready for the info learn from the textual content file to turn out to be obtainable. Once it’s obtainable, the console.log inside readData is executed.

A screen shot of an async python script on a computer.

Error dealing with whereas utilizing the async and await key phrases is often carried out utilizing try/catch blocks. It can be essential to understand how to loop with asynchronous code.

Async and await can be found in trendy JavaScript. Traditionally, asynchronous code was written by using callbacks.

Introduction to Callbacks

A callback is a perform that might be known as as soon as the result’s obtainable. All code that requires the return worth might be put contained in the callback. Everything else exterior the callback doesn’t rely upon the outcome and is, subsequently, free to be executed.

Here is an instance that reads a file in Nodejs.

const fs = require("fs");

fs.readFile("./hello.txt", "utf-8", (err, information) => {

	// In this callback, we put all code that requires 
	if (err) console.log(err);
	else console.log(information);

// In this half right here we will carry out all of the duties that don't require the outcome
console.log("Hello from the program")

In the primary line, we imported the fs module. Next, we known as the readFile perform of the fs module. The readFile perform will learn textual content from a file we specify. The first argument is which file that’s, and the second specifies the file format.

The readFile perform reads the textual content from recordsdata asynchronously. To do that, it takes in a perform as an argument. This perform argument is a callback perform and might be known as as soon as the info has been learn.

The first argument handed when the callback perform known as is an error that can have a price if an error arises whereas the perform is operating. If no error is encountered, it will likely be undefined.

The second argument handed to the callback is the info learn from the file. The code inside this perform will entry the info from the file. Code exterior this perform doesn’t require information from the file; subsequently will be executed whereas ready for information from the file.

Running the above code would produce the next outcome:

A screen shot of an async python script on a computer.

Key JavaScript Features

There are some key options and traits that affect how async JavaScript works. They are effectively defined within the video under:

YouTube video

I’ve briefly outlined the 2 essential options under.

#1. Single-threaded

Unlike different languages that enable the programmer to use a number of threads, JavaScript solely permits you to use one thread. A thread is a sequence of directions that logically rely upon each other. Multiple threads enable this system to execute a unique thread when blocking operations are encountered.

However, a number of threads add complexity and make understanding the packages that use them more durable. This makes it extra seemingly that bugs might be launched within the code, and it could be arduous to debug the code. JavaScript was made single-threaded for simplicity. As a single-threaded language, it depends on being event-driven to deal with blocking operations effectively.

#2. Event-driven

JavaScript can be event-driven. This signifies that some occasions occur throughout a JavaScript program’s lifecycle. As a programmer, you possibly can connect features to these occasions, and at any time when the occasion occurs, the hooked up perform might be known as and executed.

Some occasions might outcome from a blocking operation’s outcome being obtainable. In this case, the related perform is then known as with the outcome.

Things to Consider When Writing Asynchronous JavaScript

In this final part, I’ll point out some issues to think about when writing asynchronous JavaScript. This will embrace browser help, finest practices, and significance.

Browser Support

This is a desk displaying the help of guarantees in numerous browsers.

A player's stats displayed in a game using Javascript.

This is a desk displaying the help of async key phrases in numerous browsers.

A screen shot displaying multiple numbers generated by a JavaScript async function.

Best Practices

  • Always go for async/await, because it helps you write cleaner code that’s straightforward to take into consideration.
  • Handle errors in attempt/catch blocks.
  • Use the async key phrase solely when it’s essential to watch for the results of a perform.

Importance of Asynchronous Code

Asynchronous code allows you to write extra environment friendly packages that solely use one thread. This is essential as JavaScript is used to construct web sites that make a lot of asynchronous operations, reminiscent of community requests and studying or writing recordsdata to disk. This effectivity has enabled runtimes like NodeJS to develop in reputation as the popular runtime for software servers.

Final Words

This has been a prolonged article, however in it, we have been in a position to cowl how asynchronous features differ from common synchronous features. We additionally lined how to use asynchronous code utilizing simply guarantees, async/await key phrases, and callbacks.

In addition, we lined key options of JavaScript.In the final part, we wrapped up by overlaying browser help and finest practices.

Next, try Node.js’ steadily requested interview questions.

Vikash Gorayan

Vikash Gorayan is a technology enthusiast with a passion for gadgets and product management. He is constantly seeking new opportunities to learn and grow in his field.

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