Interview JS Part 2 - JavaScript Engines
Tags: javascript
Published: 2020-11-26 05:14:00

**Disclaimer:-** This article is a mixture of 
1. [Understanding core concepts of nodejs chrome v8 engine](https://www.freecodecamp.org/news/understanding-the-core-of-nodejs-the-powerful-chrome-v8-engine-79e7eb8af964/)
2. [How JS Works](https://blog.sessionstack.com/how-javascript-works-inside-the-v8-engine-5-tips-on-how-to-write-optimized-code-ac089e62b12e)

A JavaScript engine is a program or an interpreter that executes JavaScript code. A JavaScript engine can be implemented as a standard interpreter, or just-in-time compiler that compiles JavaScript to bytecode in some form.

This is a list of popular projects that are implementing a JavaScript engine:

* V8 — open-source, developed by Google, written in C++
* Rhino — managed by the Mozilla Foundation, open-source, developed entirely in Java
* SpiderMonkey — the first JavaScript engine, which back in the days powered Netscape Navigator, and today powers Firefox
* JavaScriptCore — open-source, marketed as Nitro and developed by Apple for Safari
* KJS — KDE’s engine originally developed by Harri Porten for the KDE project’s Konqueror web browser
* Chakra (JScript9) — Internet Explorer
* Chakra (JavaScript) — Microsoft Edge
* Nashorn, open-source as part of OpenJDK, written by Oracle Java Languages and Tool Group
* JerryScript — is a lightweight engine for the Internet of Things.


We will be discussing mostly Chrome V8 Engine.

**The Chrome V8 engine** :

* The V8 engine is written in C++ and used in Chrome and Nodejs.
* It implements ECMAScript as specified in ECMA-262.
* The V8 engine can run standalone we can embed it with our own C++ program.

**V8 can run standalone and at the same time, we can add our own function implementation in C++ to add new features to JavaScript.**

![](https://cdn-media-1.freecodecamp.org/images/1*PA3QZ_7EWgoDGNyJID_7MA.png)

So for example: `print('hello world')`is not a valid statement in Node.js. It will give an error if we compile it. But we can add our own implementation of the print function in C++ on top of the V8 which is open source at Github, thus making the print function work natively. This allows the JavaScript to understand more than what the ECMAScript standard specifies the JavaScript should understand.

This is a powerful feature since C++ has more features as a programming language as compared to JavaScript, as it is much closer to hardware like dealing with files and folders on the hard drive.

Allowing us to write code in C++ and making it available to JavaScript makes it so we can add more features to JavaScript.

Node.js in itself is a C++ implementation of a V8 engine allowing server-side programming and networking applications.

**How to write optimized JavaScript**

1. Order of object properties: always instantiate your object properties in the same order so that hidden clauses, and subsequently optimized code, can be shared.

2. Dynamic properties: adding properties to an object after instantiation will force a hidden class change and slow down any methods that were optimized for the previously hidden class. Instead, assign all of an object’s properties in its constructor.

3. Methods: code that executes the same method repeatedly will run faster than code that executes many different methods only once (due to inline caching).

4. Arrays: avoid sparse arrays where keys are not incremental numbers. Sparse arrays that don’t have every element inside them are a hash table. Elements in such arrays are more expensive to access. Also, try to avoid pre-allocating large arrays. It’s better to grow as you go. Finally, don’t delete elements in arrays. It makes the keys sparse.

5. Tagged values: V8 represents objects and numbers with 32 bits. It uses a bit to know if it is an object (flag = 1) or an integer (flag = 0) called SMI (SMall Integer) because of its 31 bits. Then, if a numeric value is bigger than 31 bits, V8 will box the number, turning it into a double and creating a new object to put the number inside. Try to use 31 bit signed numbers whenever possible to avoid the expensive boxing operation into a JS object.