When you run Javascript code it's processed by a javascript engine. At a high level, the engine performs two key tasks:

-    **Manages Execution(Call Stack)**: it determines what piece of code to run and when
-    **Manages Memory(Memory Heap):** It allocates memory for variables and objects, and cleans up memory that is no longer needed.

## What is the Call Stack?

The **Call Stack** is a data structure where the JavaScript engine keeps track of where your code is during execution. Every time a function is called, it gets **pushed** onto the stack. Once a function finishes executing, it’s **popped** off the stack. When the stack is empty, it indicates that the engine has finished executing the script. You can think of the call stack like a stack of plates where you can only remove the plate on top (first in,last out). Newer function calls are placed on top, while older ones stay lower until they’re done. If functions keep calling each other endlessly (like a recursive function without an exit condition), a **"Stack Overflow"** error occurs because the stack grows too large for the engine to manage.

## How the Call Stack Works? (Example) 

```js
function secondFunction() {
  console.log('End of the stack');
}

function firstFunction() {
  secondFunction();
}

firstFunction();

```

## Execution step by step:

1.  firstFunction() is called → pushed onto the stack.
2.  Inside firstFunction, secondFunction() is called → pushed onto the stack.
3.  Inside secondFunction, console.log('End of the stack') is called → pushed onto the stack.
4.  console.log('End of the stack') executes and returns → popped off the stack.
5.  secondFunction() finishes → popped off the stack.
6.  sayHi() finishes → popped off the stack.

## What is the Memory Heap?

The **Memory Heap** is a large region of memory where objects, functions, and variables are stored. Whenever you create objects, arrays, or even functions, they are allocated in the memory heap (primitive constants are stored in the call stack or in the global execution context -ex: window in the browser-).

## Example of Memory Allocation:

```js

const person = {
  name: 'Marko',
  gender: 'male'
};
```

When this code runs:

- The object  **{name: 'Marko', gender: 'male'}** is stored in the heap.
- The variable **person** holds a reference (a pointer) to that location in memory.

## Example of Call Stack and Memory Heap usage:


```js

function createProfile(name, gender) {
  return {
    name: name,
    gender: gender
  };
}

const userProfile = createProfile('Marko', 'male');

console.log(userProfile);

```

## What happens:

- **createProfile('Marko', 'male')** is called → pushed onto the **call stack**.
- a new object is created in the **memory heap** (the function is still on the stack)
- **createProfile** returns a reference to the object -> poped of the **call stack**
- reference is stored in the **userProfile** variable
- **console.log(userProfile)**  logs the object, accessing it via the reference.


## How the Call Stack and Memory Heap Interact

- The **call stack** manages where you are in the code and what function is currently running (keeps track of function calls)
- The **memory** heap manages where your data lives in memory (stores actual data during function calls)


Call Stack and Memory Heap

How Javascript manages execution and memory, focusing on the call stack and memory heap.

Javascript

JavaScript Internals

## What is a single-threaded language?

A **shingle-threaded** language is one that can execute only **one task at a time**. It executes the tasks one after the other, and each task has to finish before the next task starts. There is only one call stack, no parallel execution like in multhithreaded languages such as Java and C++

## How works the Call Stack in a single-threaded language?

The **Javascript call stack** works on a **"last in, first out"**. Only one function runs at a time. When a function finishes, it's removed of the stack. If something takes too long to finish, it blocks the stack, which means nothing else can run until it's done. For more information about how the call stack works, you can see my [post](http://localhost:3000/blog/article/call-stack-and-memory-heap) about the call stack.

## How can we prevent long running tasks from blocking our code?

There are several approaches to **prevent long-running tasks from blocking our code**. Some of these include using **asynchronous programming** techniques, such as Web API's callbacks, promises, async/await or **web workers**.

Web workers don't block our code because they run on a **separate thread** in the browser.

Asynchronous programming doesn't block our code because it uses the **event loop**,

## How does the event loop work?

1. The event loop starts by executing the code that is on the call stack. When it encounters asynchronous operations, it sends them to the Web Api, which is responsible to place them in the right queue. once the stack is empty (all synchronous tasks are completed), it checks the queues (callback queue, microtask queue )  to decide what will execute next. 
2.  Microtask queue has higher priority than the callback queue. The event loop will process all microtasks such as (promises resolutions, .then . catch) before moving to the callback queue.
4.  After microtask queue is emptied, the event loop will start procesing the calback queue (callbacks from web Api such as setTimeout, setInterval, event listeners)
5.  Will repeat this cycle over and over again

## Example:

```js
console.log('1: Start');

setTimeout(() => {
  console.log('2: setTimeout callback');
}, 0);

Promise.resolve().then(() => {
  console.log('3: Promise resolved');
});

console.log('4: End');

```

## The output will be:

1. Start
2. End
3. Promise resolved
4. setTimeout callback



The Single-Threaded Nature of Javascript

Understand how Javascript, being single-threaded, handles asynchronous operations using the call stack and event loop

Callstack

Call Stack and Memory Heap

The Single-Threaded Nature of Javascript