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What is JavaScript Engine?

The JavaScript engine is a program or interpreter used to execute JavaScript code in a browser. It converts JavaScript code into lower-level or machine code for execution. Notable examples include V8 (Google), SpiderMonkey (Mozilla), and JavaScriptCore (Apple). Different engines have varying optimization techniques and performance characteristics.

When you’re surfing the internet, many of the interactive elements you interact with are powered by JavaScript. But how does your browser make sense of this JavaScript code? That’s where a JavaScript engine comes in.

It’s like your browser’s translator, converting JavaScript code into something your computer can understand and execute.

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Note: The page is part of the browser vocabulary to help you understand technical terms.

How Does a JavaScript Engine Work?

A JavaScript engine doesn’t just read the JavaScript code line by line. Instead, it performs a couple of key steps to ensure your computer can execute the code as efficiently as possible.

  1. Parsing and Compilation: First, the JavaScript engine takes the raw JavaScript code and parses it into a more computer-friendly format, known as bytecode. Some JavaScript engines, such as V8 (used in Google Chrome), go a step further and convert this bytecode into machine code, which can be executed even more directly by your computer’s processor.
  2. Execution: After the code has been compiled, the JavaScript engine executes it, meaning it starts carrying out the instructions that the code contains.
  3. Optimization: While the code is being executed, the JavaScript engine is also looking for ways to make it run faster. It identifies frequently used code (“hot” code) and optimizes it for better performance.

An Easy-to-Understand Example

Imagine you’re a tourist in a foreign country where you don’t speak the language (JavaScript). You want to order food from a local restaurant, but the menu is written in the local language. Luckily, you have a trusty translator (JavaScript engine) by your side.

First, your translator reads the menu (parses the code) and translates it into a language you understand (compiles it into bytecode or machine code). Next, they relay the translated menu items to you (execution), and you start ordering and enjoying the local cuisine.

While you’re eating, you find that you really like a particular dish and order it frequently. Seeing this, your translator takes note and learns to translate that dish’s name more quickly and efficiently (optimization).

Bottom Line

Just as a translator helps you navigate a foreign language, a JavaScript engine helps your browser understand and execute JavaScript code.

By parsing and compiling the code, executing it, and constantly optimizing for better performance, the JavaScript engine ensures your web browsing experience is smooth, efficient, and interactive.

The understanding of how this works can provide valuable insight into how the interactive elements of the websites you visit every day come to life.

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Kushal Azza

Kushal is a Bachelor of Engineering, a Certified Google IT Support Professional, and a Digital-Tech Geek. He has over a decade of experience solving tech problems, troubleshooting, and creating digital solutions. Follow him on Twitter and LinkedIn.

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