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Introduction to JavaScript - GeeksforGeeks
Introduction to JavaScript
Last Updated : 12 May, 2025
JavaScript is a versatile, dynamically typed programming language used for interactive web applications, supporting both client-side and server-side development, and integrating seamlessly with HTML, CSS, and a rich standard library.
- JavaScript is a single-threaded language that executes one task at a time.
- It is an Interpreted language which means it executes the code line by line.
- The data type of the variable is decided at run-time in JavaScript that’s why it is called dynamically typed.
"Hello, World!" Program in Browser Console
A "Hello, World!" program is the simplest way to get started with any programming language. Here’s how you can write one using JavaScript.
HTML
<html>
<head></head>
<body>
<h1>Check the console for the message!</h1>
<script>
// This is our first JavaScript program
console.log("Hello, World!");
</script>
</body>
</html>
In this example
- The<script> tag is used to include JavaScript code inside an HTML document.
- console.log() prints messages to the browser's developer console. Open the browser console to see the "Hello, World!" message.
To learn more about it follow the article - JavaScript Hello World
"Hello World" Program in Server Console
We can also print the "Hello World" program directly into the console terminal without embedded it into HTML. Create an index.js file and add the code to it.
JavaScript
/*
This is a multi-line comment.
It can span several lines.
*/
console.log("Hello, World!"); // Prints Hello, World! to the console
In this example
- console.log(): The console.log() method is used to print messages to the browser's developer console. Open the console (usually with F12 or Ctrl + Shift + J) to see the message "Hello, World!" displayed.
- Comments in the Code:
- Multi-line Comment: The /* */ syntax is used to write a comment spanning multiple lines.
- Single-line Comment: The // syntax is used for short, inline comments, like the one explaining the console.log function.
To learn more about comments follow the article - Comments in JavaScript
Key Features of JavaScript
Here are some key features of JavaScript that make it a powerful language for web development:
- Client-Side Scripting:JavaScript runs on the user's browser, so has a faster response time without needing to communicate with the server.
- Versatile: JavaScript can be used for a wide range of tasks, from simple calculations to complex server-side applications.
- Event-Driven: JavaScript can respond to user actions (clicks, keystrokes) in real-time.
- Asynchronous: JavaScript can handle tasks like fetching data from servers without freezing the user interface.
- Rich Ecosystem: There are numerous libraries and frameworks built on JavaScript, such as React, Angular, and Vue.js, which make development faster and more efficient.
Client Side and Server Side nature of JavaScript
JavaScript's flexibility extends to both the client-side and server-side, allowing developers to create complete web applications. Here’s how it functions in each environment:
Client-Side:
- Involves controlling the browser and its DOM (Document Object Model).
- Handles user events like clicks and form inputs.
- Common libraries include AngularJS, ReactJS, and VueJS.
Server-Side:
- Involves interacting with databases, manipulating files, and generating responses.
- Node.js and frameworks like Express.js are widely used for server-side JavaScript, enabling full-stack development.
Programming Paradigms in JavaScript
JavaScript supports both imperative and declarative programming styles:
- Imperative Programming: Focuses on how to perform tasks by controlling the flow of computation. This includes approaches like procedural and object-oriented programming, often using constructs like async/await to handle asynchronous actions.
- Declarative Programming: Focuses on what should be done rather than how it’s done. It emphasizes describing the desired result, such as with arrow functions, without detailing the steps to achieve it.
Applications of JavaScript
JavaScript is used in a wide range of applications, from enhancing websites to building complex applications. Here are some examples:
- Web Development: JavaScript adds interactivity and dynamic behavior to static websites, with popular frameworks like AngularJS enhancing development.
- Web Applications: JavaScript powers robust web applications, leveraging APIs, React, and Electron to create dynamic user experiences like Google Maps.
- Server Applications: Node.js brings JavaScript to the server side, enabling powerful server applications and full-stack development.
- Game Development: JavaScript, combined with HTML5 and libraries like Ease JS, enables the creation of interactive games for the web.
- Smartwatches: Pebble JS allows JavaScript to run on smartwatches, supporting apps that require internet connectivity.
Limitations of JavaScript
Despite its power, JavaScript has some limitations to consider:
- Security Risks : JavaScript can be used for attacks like Cross-Site Scripting (XSS), where malicious scripts are injected into a website to steal data by exploiting elements like <img>, <object>, or <script> tags.
- Performance : JavaScript is slower than traditional languages for complex tasks, but for simple tasks in a browser, performance is usually not a major issue.
- Complexity : To write advanced JavaScript, programmers need to understand core programming concepts, objects, and both client- and server-side scripting, which can be challenging.
- Weak Error Handling and Type Checking : JavaScript is weakly typed, meaning variables don’t require explicit types. This can lead to issues as type checking is not strictly enforced.
Why JavaScript is known as a lightweight programming language ?
JavaScript is considered a lightweight language due to its low CPU usage, minimalist syntax, and ease of implementation. With no explicit data types and a syntax similar to C++ and Java, it’s easy to learn and runs efficiently in browsers. Unlike heavier languages like Dart or Java, JavaScript, especially with Node.js, performs faster and uses fewer resources. While it has fewer built-in libraries, this makes it more flexible, though external libraries are often needed for advanced functionality. JavaScript’s efficiency and simplicity make it a top choice for web development.
Is JavaScript Compiled or Interpreted or both ?
JavaScript is both compiled and interpreted. The V8 engine improves performance by first interpreting code and then compiling frequently used functions for speed. This makes JavaScript efficient for modern web apps. It’s mainly used for web development but also works in other environments. You can learn it through tutorials and examples.
Just-In-Time (JIT) compilation is a technique used by JavaScript engines (like V8) to improve performance. Here’s how it works
- Interpretation: Initially, the code is interpreted line-by-line by the engine.
- Hot Code Detection: The engine identifies frequently executed code, such as often-called functions.
- Compilation: The “hot” code is compiled into optimized machine code for faster execution.
- Execution: The compiled machine code is then executed directly, improving performance compared to repeated interpretation.
- JIT compilation balances between interpretation (for quick startup) and compilation (for faster execution).
JavaScript Versions
Let’s take a look at the different versions of ECMAScript, their release years, and the key features they introduced
Note: Older versions of browsers do not support ES6.
To learn more about it follow the article - JavaScript Versions
To start your coding journey in JavaScript you can follow JavaScript Tutorial
Introduction of JavaScript First program - Hello Geeks
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