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tanishparashar
tanishparashar

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How JS works behind the scenes?

How JavaScript Works

JavaScript operates in a single-threaded environment, meaning it executes one task at a time. It handles the execution of code in Execution Contexts (EC), which can be either Global or Functional.

Global Execution Context (GEC)

  • Created when the code file is loaded.

  • The GEC is responsible for setting up the global object (in browsers, it's window) and the (this) keyword.

  • All variables and functions declared in the global scope are stored in the Global EC.

Execution Context Phases

  • Memory Creation Phase:
    During the memory phase, JavaScript allocates memory for variables and functions.
    Variables are assigned the value undefined initially, while functions are stored in memory with their full definition.

  • Execution Phase:
    After the memory allocation phase, JavaScript starts executing the code line by line.
    Variables are assigned actual values and functions are executed.

Functional Execution Context (FEC)

  • Whenever a function is invoked, JavaScript creates a new Execution Context specifically for that function.

  • Execution context for each function has two phases:

  • Memory Phase: Memory is allocated to the local variables, and functions inside the function.

  • Execution Phase: The function body is executed, and variables get their actual values.

  • After execution, the function's execution context is destroyed, freeing up memory.

Return Statements
Once a function reaches a return statement, the function immediately exits, and its execution context is removed from the Call Stack. Any code written after a return statement won't execute.

The Call Stack
JavaScript uses the Call Stack to manage execution contexts.

  • LIFO (Last In, First Out) concept is used:

  • When a function is called, its execution context is placed on top of the stack.

  • Once the function completes (or reaches return), its execution context is popped off the stack.

JavaScript is Single-Threaded

Single-threaded means JavaScript can only perform one operation at a time but still it can perform asynchronous operations isn't that interesting in my next post we will discuss about this amazing functionality of JavaScript.

Top comments (21)

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georgewl profile image
George WL • Edited

This was great, only minor quibble, I think it could make the keywords such as this stand out if wrapped in codeblocks (``)

So refreshing to see articles of high quality instead of all the listicles that other folks are spamming

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tanishparashar profile image
tanishparashar

Thankyou @georgewl.
Positive Criticism is much-needed as i am just a fresher and i wanna keep growing ✨️.

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vale58ntina profile image
Valentina

Great description of how JavaScript works. I'm curious how JavaScript manages asynchronous activities while being single-threaded. Could you explain how the Event Loop and Web APIs work together with the Call Stack in your upcoming post? thanks

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tanishparashar profile image
tanishparashar • Edited

That's really motivating for me, i will make to sure explain it through code snippets and diagrams 😊.
Thankyou @vale58ntina

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ritesh_pawar09 profile image
Ritesh Pawar

Really informative!!

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weitend profile image
Vladislav Voronin

but so little

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tanishparashar profile image
tanishparashar

I never really thought anyone would read this so i didn't put much efforts, I will go all out next time!!

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jennavisions profile image
Jenna • Edited

@tanishparashar ,

If you plan to write more about the topic you may want to create a series :-)

Happy writing ✍️

Best practices for writing on DEV:

dev.to/sloan/series/22620

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tanishparashar profile image
tanishparashar

dev.to/tanishparashar/javascript-a...

The next part of this post is here!!
Do check it out

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nishanthank profile image
Nishanthan K

Great and well groomed explanation!

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tanishparashar profile image
tanishparashar

Thankyou So much

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suhani_acharya profile image
Suhani Acharya

So informative, keep going ✊🏻

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cookiemonsterdev profile image
Mykhailo Toporkov 🇺🇦

I would also recommend to read You-Dont-Know-JS series.

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tanishparashar profile image
tanishparashar

Sure!!

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henry_johnson profile image
Codelab

Hi there. as a mern stack developer, i launched my first web. Feel free to submit your feedback if it needs improving
codelabs.uz/

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tanishparashar profile image
tanishparashar

which component library did you use?

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kamlesh_gupta_539c974fb0d profile image
Kamlesh Gupta

That's exciting

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sunilkhadka001 profile image
Sunil khadka

I think including event loop can add great depth to this concept

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tanishparashar profile image
tanishparashar
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be_rajeevkumar profile image
Rajeev Kumar

You write so well, but still, it does not look complete. Together, we can write much more about this topic. So, Let's collaborate!

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sheraz4194 profile image
Sheraz Manzoor

Too much short, ho javascript works behind the scenes is a field in itself, it requires a long content.. Anyways good try. Keep it up boy

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