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Archit Sharma
Archit Sharma

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What actually is JavaScript?

To build great software we should have deep enough understanding of the internals. There are so many developers who don't have any idea of what is going on β€œunder the covers”.
So in this article ill try my best to explain the internal working of JavaScript in simplest way.

Before we begin, here is a fact for you:

Did you know JavaScript supports a modern touchscreen-based UI for the SpaceX Dragon spacecraft?

Yes JavaScript was actually used to build UI inside SpaceX spacecraft.

JavaScript Runtime Environment

JavaScript works on a environment called JavaScript Runtime Environment. To use JavaScript you basically install this environment and than you can simply use JavaScript.
So in order to use JavaScript you install a Browser or NodeJS both of which are JavaScript Runtime Environment.

Depending on the context, the runtime environment can take on different forms; for example, the runtime environment in a browser is very different from that of Node.js. Because these differences are primarily at the implementation level, the majority of the following concepts remain applicable.

JS Engine

JavaScript Engine is a part of JavaScript Runtime Environment and each web browser has its own version of the JS engine.
One thing to note here is that JavaScript can be used to write only logic like for ex: for loops, while loops, if-else etc.
Example, Chrome uses the V8 JS engine which has been developed by the Chromium Project. Firefox uses SpiderMonkey which was first written by Brendan Eich at Netscape and is now maintained by the folks at Mozilla. Apple's Safari uses Webkit.

The purpose of the JavaScript engine is to translate source code that developers write into machine code that allows a computer to perform specific tasks.
JS Engine is made up of the heap and the call stack

The heap, also called the β€˜memory heap’, is a section of unstructured memory that is used for the allocation of objects and variables.

The call stack is a data structure that tracks where we are in the programme and operates on a last-in, first-out basis. Each entry in the stack is referred to as a stack frame. This means that the engine is focused on the frame at the top of the stack, and it will not move on to the next function unless the function above it is removed from the stack.

As the JS engine steps into a function, it is pushed onto the stack. When a function returns a value or gets sent to the Web APIs, it is popped off the stack. If a function doesn’t explicitly return a value then the engine will return undefined and also pop the function off the stack. This is what is meant by the term β€œJavaScript runs synchronously”; it is single-threaded, so can only do one thing at a time.

JavaScript Runtime Environment also provide APIs which are not a part of JS Engine. We have different APIs for different runtime environment. So features like console.log and DOM are not provided by JavaScript but by the APIs of browsers runtime environment and same for setTimeout() this method is not provide by JS but NodeJS runtime environmet.
So JavaScript can be used to write only logic with JS Engine but with help of APIs we have a lot more features.

Electron is also a runtime environment used for creating Desktop Applications

This is the reason you never install JavaScript remotely like Java or Python in your Desktop because it comes pre installed with Browser or NodeJS or other JS Runtime Environment.

Thanks for reading this article, follow for more

Top comments (2)

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codewithtee profile image
Tabassum Khanum

Good article thanks for sharing.

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iarchitsharma profile image
Archit Sharma Author

Glad to know you like it.

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