Modern JavaScript From Beginner to Advanced Lesson 1 of 30

JavaScript Engine Explained - Working, Compilation, Interpreter vs Compiler

Hello friends, welcome to shrash studio learning. In this article we will understand:

What is JavaScript Runtime
What is JavaScript Engine
How JavaScript code runs
What is V8 Engine
What is Call Stack & Memory Heap
Important Interview Questions

This article is written in simple Indian spoken English so students can learn quickly and easily.

What is JavaScript Runtime Environment?

JavaScript cannot run alone. It needs an environment to execute the code.

That environment is called JavaScript Runtime Environment.

Runtime gives extra features like:

Feature	Purpose
console.log()	Print output
setTimeout()	Run code later
fetch()	API calls
DOM APIs	Handle HTML elements

Simple Remember:

Runtime = JavaScript Engine + Extra APIs

What is JavaScript Engine?

JavaScript Engine is a program that reads and executes JavaScript code.

It converts JavaScript code into machine understandable instructions.

Engine	Used In
V8	Chrome & Node.js
SpiderMonkey	Firefox
JavaScriptCore	Safari

Interview Tip:
Most interviewers ask: "Which engine is used in Node.js?" Answer: V8 Engine

How JavaScript Code Runs

Step	What Happens
1	Engine reads code
2	Code is parsed
3	Code gets compiled
4	Execution starts

Simple JavaScript Example


const num = 5;

function square(n) {
   return n * n;
}

console.log(square(num));

// Output: 25

Engine reads the code, stores variables in memory, executes the function, and prints the output.

What is Call Stack?

Call Stack keeps track of function execution.

Whenever a function runs, it enters the stack. After execution, it gets removed.

Important:

Call Stack follows LIFO rule.

Last In → First Out

What is Memory Heap?

Memory Heap stores:

Objects
Arrays
Functions
Variables

JavaScript automatically manages memory using Garbage Collector.

What is Garbage Collector?

Garbage Collector removes unused memory automatically.

This helps improve application performance.

Common Mistake:
Many beginners think JavaScript removes all memory problems automatically. But memory leaks can still happen if objects are not cleared properly.

Difference Between Runtime and Engine

JavaScript Engine	Runtime Environment
Executes code	Provides complete environment
Example: V8	Example: Browser, Node.js
Focuses on execution	Provides APIs and tools

Most Asked Interview Questions

Quick Revision Table

Concept	Simple Meaning
Runtime	Environment to run JS
Engine	Executes JavaScript
V8	Google's JS Engine
Call Stack	Handles function calls
Memory Heap	Stores data
Garbage Collector	Removes unused memory

Conclusion

JavaScript Runtime and JavaScript Engine are very important concepts for every developer.

Engine executes JavaScript code, while Runtime provides APIs and complete environment support.

Once you understand these basics, advanced topics like Event Loop, Async/Await, Closures, and Promises become much easier.

Back to course overview