Why is JavaScript not Interpreted?

Hey there! Imagine you're building a LEGO set. You have the instructions and all the pieces, but you need to follow the steps one by one to create your awesome LEGO castle. JavaScript, a programming language used to make websites interactive and fun, works a bit like that too. But instead of you putting the LEGO pieces together, your computer does the job of building the castle by reading instructions called code.

In this blog, we'll explore how JavaScript works, focusing on two key concepts: being interpreted and Just-in-Time (JIT) compilation. We'll break it down into simple steps, just like how you follow LEGO instructions, so even a 10-year-old can understand it!

What is JavaScript?

First things first, let's understand what JavaScript is. JavaScript is a language that allows developers to create interactive effects inside web browsers. Whenever you click a button, see an animation, or fill out a form on a website, JavaScript is likely behind it making things happen.

JavaScript is Everywhere

Here are a few examples of where you might encounter JavaScript in your daily life:

  1. Online Games: Many online games are built using JavaScript.

  2. Interactive Websites: Buttons, forms, slideshows, and more.

  3. Apps: Some mobile and web apps use JavaScript to function.

Interpreted Language: What Does It Mean?

JavaScript is often called an "interpreted" language. But what does that mean?

The Interpreter: Your Computer's Helper

Think of the interpreter like a translator who helps you understand instructions in a language you don't know. When you write JavaScript code, the interpreter translates it into actions that your computer can understand and execute right away.

Step-by-Step Execution

When you write JavaScript code, it looks something like this:

console.log("Hello, World!");

Here's what happens:

  1. You Write the Code: You type out the code in a text editor.

  2. Interpreter Reads the Code: The interpreter reads your code line by line.

  3. Actions are Performed: For each line, the interpreter performs the actions specified. In this case, it prints "Hello, World!" on the screen.

Why Interpreted?

The main advantage of being interpreted is that it allows for quick and easy execution of code. You don't have to wait for the entire program to be compiled before running it. It just runs immediately!

Just-in-Time (JIT) Compilation: Making JavaScript Faster

While being an interpreted language is great for quick execution, it can sometimes be a bit slow. That's where Just-in-Time (JIT) compilation comes in to save the day!

What is JIT Compilation?

JIT compilation is like a superhero for JavaScript. It combines the best of both worlds: the speed of compiled languages and the flexibility of interpreted languages.

How JIT Works

Here's a simple way to understand JIT compilation:

  1. Interpreting Code: At first, JavaScript runs using the interpreter, reading and executing code line by line.

  2. Finding Hotspots: The JIT compiler looks for parts of the code that are run frequently, called "hotspots."

  3. Compiling Hotspots: The JIT compiler translates these hotspots into faster machine code.

  4. Executing Faster Code: The next time those parts of the code are run, the computer uses the faster machine code, making the program run quicker.

An Example with LEGO

Imagine you're building the same LEGO castle over and over. The first few times, you follow the instructions step by step (interpreted). But then you realize that you keep building the same parts, like the walls and towers, repeatedly. So, you decide to memorize those steps (JIT compilation). Now, when you build the castle again, you can assemble those parts much faster because you know them by heart.

Why JIT is Important

JIT compilation makes JavaScript much faster and more efficient. This is especially important for complex web applications like games and interactive websites where speed is crucial.

Putting It All Together: Interpreted + JIT Compilation

JavaScript uses both interpretation and JIT compilation to give you the best of both worlds: quick start-up times and fast execution. This combination is what makes JavaScript so powerful and widely used.

Real-World Example: An Interactive Web Page

Let's look at a simple example to see how JavaScript works in the real world.

Example: Interactive Button

We'll create a button on a web page that changes color when clicked. Here's the HTML and JavaScript code:

<!DOCTYPE html>
    <title>Interactive Button</title>
        #myButton {
            padding: 10px 20px;
            background-color: blue;
            color: white;
            border: none;
            cursor: pointer;
    <button id="myButton">Click Me!</button>

        // JavaScript code to change the button color
        document.getElementById('myButton').addEventListener('click', function() {
            this.style.backgroundColor = 'green';

How It Works

  1. HTML: Defines the structure of the web page with a button.

  2. CSS: Styles the button to look nice.

  3. JavaScript: Adds an event listener to the button. When the button is clicked, the background color changes to green.

Step-by-Step Execution

  1. Loading the Page: When you open the web page, the browser reads the HTML and CSS first.

  2. Executing JavaScript: The browser then reads and executes the JavaScript code.

  3. Interpreted Execution: The JavaScript interpreter runs the code line by line.

  4. JIT Compilation: If you click the button multiple times, the JIT compiler may optimize the code to change the color faster.


JavaScript is a powerful and versatile language that powers much of the web. Understanding how it works, including being interpreted and Just-in-Time (JIT) compiled, can help you appreciate the magic behind the scenes. By breaking down these concepts into simple steps, we hope you now have a better grasp of how JavaScript makes your favorite websites interactive and fun.

Remember, just like building a LEGO set, learning JavaScript step by step can be both fun and rewarding. Happy coding!