JavaScript: Where it is used and how it works

Introduction

The JavaScript programming language is an interpreted language. It is often referred to as a scripting language—a set of instructions executed when web pages load. Although JavaScript, or JS for short, is primarily used in web development, its applications are broad, ranging from server-side applications to mobile platforms. According to GitHub, JavaScript has remained the most popular programming language since 2014 and will only lose the top spot to Python in 2024.

What does JavaScript mean?

JavaScript was created in 1995, simultaneously with Java. Initially, the language was called LiveScript, which translates as “living script.” The name conveyed the essence of the product: it was a language for predicting user behavior patterns and system responses. LiveScript was later renamed JavaScript for marketing reasons. This was to emphasize that Java and JavaScript work well together as frontend and backend languages. Aside from the name, the two languages ​​have little in common. Moreover, with the evolution of JavaScript, writing the backend in Java is no longer necessary.

Where is JavaScript used?

JavaScript is one of the most popular programming languages. Calculating which language is most in demand is difficult. There are a number of criteria: the number of job postings, the number of discussions on specialized forums, new language features, and so on. The most well-known ranking, TIOBE, uses a complex algorithm based on all of these factors. According to its estimates, JavaScript is among the top ten most popular languages. This is partly due to its versatility. Let’s look at the niches where JavaScript is used.

Web development (frontend)

JavaScript is the primary language for creating dynamic and interactive web pages. It allows you to control the behavior and appearance of pages in real time. Here are some popular JavaScript features:

• Working with the DOM (Document Object Model). JavaScript allows you to manipulate web page elements, such as text, images, forms, and styles. For example, you can change the text on a button or hide/show elements on the page without reloading.
• Event handling. JavaScript is used to respond to user actions such as clicks, mouse hovers, form input, page scrolling, and more.
• API interaction. Technologies such as AJAX (Asynchronous JavaScript and XML) and the Fetch API allow you to send requests to the server without reloading the page, updating only part of the content. This is used for dynamic data, such as in sliders and search filters.

Backend development

With the development of Node.js, JavaScript has expanded beyond the browser and begun working with server-side solutions. Node.js is a C++ programming platform that can execute JavaScript code. This allows developers to use a single language for the entire application stack—both front-end and back-end.

The symbiosis of Node.js and JavaScript provides developers with the following benefits:

• High performance. Node.js uses an event-driven model, allowing it to process large numbers of requests asynchronously.
• Interacting with databases and external services. Server-side JavaScript can work with databases such as MongoDB or MySQL, as well as exchange data with external APIs.
• Scalability. Node.js allows for easy scaling of applications, making it suitable for high-load solutions or software that requires real-time data updates.

Full-cycle development

Because JavaScript combines client-side and server-side development capabilities, it can be used to create fully functional web applications. These applications can integrate modern solutions, such as page refresh without a full reload, as well as search and form submission. Interacting with the server without reloading the page allows for a smooth and responsive user interface, which is essential for web applications like Gmail, Twitter, or Facebook, where content is updated dynamically.

Mobile applications

JavaScript is actively used to create cross-platform mobile apps that work across multiple operating systems. Unlike native solutions built only for Android or iOS, cross-platform apps reduce development time and resources while maintaining quality and performance.

To optimize mobile development, frameworks like React Native and Ionic are often used:

• React Native provides access to native operating system components, such as the camera, geolocation, or file system, and supports hot reloading, speeding up the development process.
• Ionic integrates with Angular and React, providing tools for front-end development, and supports access to native device features through Capacitor or Cordova plugins.

Embedded systems and IoT solutions

JavaScript is actively used in developing Internet of Things (IoT) solutions. Platforms such as Johnny-Five and Espruino allow programmers to interact with microcontrollers, sensors, and actuators using familiar JavaScript syntax.

JavaScript can be used to create a smart home ecosystem. For example, Node.js can be used to set up a server that will collect data from temperature and humidity sensors, analyze it, and send notifications to the owner via messenger.

In industrial environments, JavaScript is used to create IoT solutions that integrate multiple devices into a single system. For example, the Espruino platform can be used to program microcontrollers that process data from pressure, speed, or temperature sensors, transmit it to a central server, and receive commands to optimize workflows.

Basic JavaScript Features

Let’s look at some of the features of the JavaScript programming language from a development perspective.

Working with objects and arrays

Since JavaScript is an object-oriented language, everything, including functions, can be an object. Therefore, JavaScript utilizes the core principles of OOP: abstraction, encapsulation, inheritance, and polymorphism. An array is a data type—a collection of identical elements with one or more indices—that allows you to store multiple values ​​for a single variable.

Asynchronous programming

Asynchrony is one of JavaScript’s strengths, allowing for more responsive applications. Key approaches:

• Callbacks. Simple callback functions are called after an asynchronous operation completes.
• Promises. Objects that represent the result of an asynchronous operation.
• Async/await. Syntactic sugar overpromises, making code more readable.

Modules

JavaScript supports a modular architecture that allows you to split code into smaller chunks. Modules are imported and exported using ES6 syntax:

// module.js
export function greet(name) {
  return `Hello, ${name}!`;
}

// main.js
import { greet } from './module.js';
console.log(greet('Alice'));

This code demonstrates code splitting into modules:

• Exporting the greet function from one module (module.js).
• Import and use this function in another module (main.js).
• Outputting the greeting “Hello, Alice!” to the console.

How does JavaScript work?

JavaScript is a programming language without strong typing that allows both synchronous and asynchronous execution of functions. This determines the principles of its operation. Here are some of its features.

Variables and scopes

JavaScript provides several ways to declare variables, each with a different scope—the part of the program that can access the variable. Let’s look at the main methods:

• let. Block-scoped. Suitable for working with mutable values, but does not allow variable reassignment.
• var. Functional scope allows for variables to be updated and redefined outside of a block. The Hexlet team does not recommend using it, as it can lead to confusing code.
• const. Used for immutable variable values ​​that cannot be updated or redefined.

Error handling mechanism

JavaScript supports error handling using try-catch blocks, which detect exceptions, handle them, and prevent application crashes. Promises and async/await are also used for error management. Here’s an example of using try-catch blocks.

try {
  // Code that may throw an error
  let result = 10 / 0;
  if (!isFinite(result)) {
    throw new Error("Division by zero!");
  }
  console.log(result);
} catch (error) {
  // Intercepting and handling errors
  console.error("Error:", error.message);
} finally {
  // Code that will be executed in any case
  console.log("The operation is complete..");
}

The code inside the try block is executed. If an error occurs, execution transfers to the catch block. The finally block can contain code to free resources or terminate the process.

This is how errors are managed using promises.

function fetchData(url) {
  return new Promise((resolve, reject) => {
    const isValidUrl = url.startsWith("http");
    if (isValidUrl) {
      resolve("Data received successfully!");
    } else {
      reject(new Error("Incorrect URL"));
    }
  });
}

fetchData("http://example.com")
  .then((data) => {
    console.log(data);
  })
  .catch((error) => {
    console.error("Error:", error.message);
  });

resolve is called for a successful promise fulfillment, and reject is called for an error. The catch method intercepts and handles the error if one occurs.

This is how errors are handled using async/await.

async function getData(url) {
  try {
    if (!url.startsWith("http")) {
      throw new Error("Incorrect URL");
    }
    // Simulating a successful request
    const response = await Promise.resolve("Data received successfully!");
    console.log(response);
  } catch (error) {
    // Error handling
    console.error("Error:", error.message);
  } finally {
    console.log("The operation is complete..");
  }
}

getData("http://example.com"); // Successful execution
getData("invalid-url");       //Error 

await waits for the promise to be fulfilled. If an error occurs, execution moves to the catch block.

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Popular JavaScript frameworks and libraries

JavaScript uses hundreds of libraries and frameworks that help developers solve common problems. Here are a few of them.

Frontend

• React is a user interface library developed by Facebook. It is suitable for single-page applications (SPAs).
• Angular is a framework for building complex web applications from Google. It supports two-way data binding, simplifying synchronization between the model and view.
• Vue is a framework for building user interfaces with easy customization.

Backend

• Express is a minimalist and flexible framework for Node.js used to build web applications and APIs.
• NestJ is a framework that provides tools for scalable server-side applications.
• Fastify is a modern, high-performance Node.js framework for developing fast and lightweight APIs. It features built-in support for data validation, logging, and plugins, making it easy to use.

Mobile applications

• React Native is a framework from Facebook that allows you to develop mobile applications using JavaScript and React.
• Ionic is a framework for building hybrid applications using HTML, CSS, and JavaScript.

Conclusion

JavaScript is a versatile programming language that continues to evolve and find new applications. It’s ideal for building web applications, servers, mobile apps, and IoT software, meaning JavaScript developers are in demand across a wide range of fields. Because JavaScript can be used to create numerous solutions, and active projects require support, the demand for JavaScript will remain high. If you want to master this in-demand profession, you can start with the “JavaScript Fundamentals” course. This free course allows you to learn the basics of the language, its operating principles, and put your new knowledge into practice.

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