When you run code in Javascript it operates within an **Execution Context**. The execution Context is responsible for keeping track of what is going on: figuring out **which variables and functions are accesible** at a given point in time, determining the **value of this**, applying **hoisting rules** and setting up the **scope chain**.

## Types of Execution Context
 
1. Global Execution Context:
     - Is Created when Javascript engine starts executing the code
     - It sets up the global this (In the browser refers to the window object)
     -  There's only one global context
 
2. Function Execution Context:
     -  Created every time a function is invoked
     -  Each functuion has its own execution context (separate from the global context)
     -  A new execution context is created for each call, even if it's the same function  

## Phases of Execution Context:

1. **Creation Phase**:
    - Creation of the **Variable Environment**
    - Creation of The **Lexical Environment**
    - Establishing the **Scope Chain**
    - Determination of **this Binding**
    - **Hoisting**

2. **Execution Phase**
   - This phase runs the code line by line: Variables are assignet to their actual values, function are executed (if they are called)
    


## Lexical environment and Scope Chain:

Using the **lexical environment** javascript knows where a piece of code was **declared** (this is known as lexical scoping).
Each execution context's lexical environment gets **linked to its outer environment**, forming the **scope chain**. 
Javascript uses the scope chain to determine the variable accessibility (it starts from the current context and works its way to the global context)


         
 ```js
const globalVariable = "Global Variable";

function a() {
  return globalVariable; // 'a' accesses variable from the global lexical environment
}

function b() {
  connst bVariable = "Inside b function variable";

  function c() {
    return bVariable+ " and " + globalVariable; // 'c' accesses variable from 'b's lexical environment
  }

  return c();
}

console.log(a()); // Outputs: Global Variable
console.log(b()); // Outputs: "Inside b function variable and Global Variable"

```

**Explanation**:  a accesses a variable declared in the **global lexical environment** , c accesses a variable declared in the **lexical environment of b and global lexical environment**. The scope chain links c's environment to b's then to global

**Types Of Scopes:**

1. Global Scope:
A variable declared **outside all functions or blocks** is in the **global scope** and accessible anywhere in your code.
```js
const globalVariable = "I'm in global scope";

function showGlobal() {
  console.log(globalVariable); // Accessible here
}

showGlobal(); // Output: I'm in global scope
```
2. Function Scope: 
A variable declared **inside a function** using var, let, or const is only accessible ** within that function**.
```js
function functionScopeExample() {
  const a = "Function Scope";
  console.log(a); // Works here
}

functionScopeExample();

console.log(a); // Error: a is not defined

```

3. Block scope: 
 Variables declared inside a block (e.g., an if, for, or {} block) using let or const are only accessible within that block. Variables declared with var aren't block scoped they are function scoped.
 
 ```js
{
  let blockLet = "I'm a block-scoped let variable";
  const blockConst = "I'm a block-scoped const variable";
  var blockVar = "I'm declared with var inside a block";
  console.log(blockLet);   // Accessible here
  console.log(blockConst); // Accessible here
}

console.log(blockLet);   //  Error: blockLet is not defined
console.log(blockConst); //  Error: blockConst is not defined
console.log(blockVar); //  Still accessible here — var is not block-scoped



```
 
## Variable environment:

   During the creation phase of an execution context, the variable enviroment  is responsable for creating memory space   for all the variables and functions declared in that scope applying hoisting. Hoisting allows certain declarations to be accesed before their line of code is actually reached.  

 Example:
 
 ```js

function one(){
var variable;
}

function two(){
var variable = true;
one(); // new Execution Context
}

var variable = false

two();

// Execution Contexts:
// one --> 'variable' is undefined in the 'one' functions local scope
//two() -->'variable' is true in the two functions local scope
//global() --> 'variable' is false in the global scope

// Each execution context has its own **Variable Environment**, the same variable can exist in all three without conflict.
```

## Hoisting

Hoisting is the behaviour of "moving" the variables or function declarations to the top of their respective environments during the creation phase. Variables declared with **var** are **partially** **hoisted**, **functions are fully hoisted**, variables declared with **let and const**  are **hoisted but not initialized** (They reimain in a Temporal Dead Zone and accessing them before declaration causes a RefrenceError) . 

Example2:
```js
console.log(partialyHoisted);
console.log(fullyHoisted);

var partialyHoisted = "partialyHoisted";

function fullyHoisted(){
   console.log("Fully Hoisted");
}

// Logs: 
// undefined
// "Fully hoisted"
```

## This binding

**What is  _this_  ?**

**This** is the object that the function is a property of

Example:

```js

function example() {
  console.log(this); // Global object
}



const user = {
  name: "Marko",
  example() {
    console.log(this);
  }
};

user.example(); 

// {
//   name: "Marko",
//   example: ƒ example()
//  }

```

JavaScript Execution Context, Scope, Hoisting, this keyword

How Javascript creates and manages execution context

Javascript

JavaScript Execution Context, Scope, Hoisting, and this

JavaScript's object-oriented behavior is built on the **prototype chain**. When an object is created via a constructor function, all instances inherit shared methods and properties from the constructor’s **.prototype**.

```js
function Animal(name) {
  this.name = name;
}

Animal.prototype.speak = function () {
  console.log(`${this.name} makes a noise.`);
};
  
const dog = new Animal("Rex");
dog.speak(); // Rex makes a noise.

```

Behind the scenes:
- dog. __ proto __ points to Animal.prototype
- That’s how dog can access speak() even though it’s not directly on the dog object

When you use the new keyword, JavaScript performs these steps:

1. Creates a new empty object
2. Sets its internal [[Prototype]] (i.e., __proto__) to the constructor's .prototype
3. Calls the constructor function with this bound to the new object
4. Returns the new object

```js
const rex = new Animal("Rex");

//equivalent to:

const rex = {};
rex.__proto__ = Animal.prototype;
Animal.call(rex, "Rex");
```

The class syntax introduced in ES6 provides a cleaner, more familiar structure for OOP, but it's just syntactic sugar—the underlying prototype-based inheritance model remains unchanged.

```js
class Animal {
  constructor(name) {
    this.name = name;
  }

  speak() {
    console.log(`${this.name} makes a noise.`);
  }
}
```

This modern syntax feels more like traditional OOP, but under the hood:

- It still creates a constructor function
- Methods are still placed on the prototype
- Inheritance still uses the prototype chain


OOP in Javascript: Classes, prototypes and constructors

JavaScript’s object-oriented model. How class syntax simplifies, but doesn't replace, prototype-based inheritance. What happens under the hood with prototype, __proto__, and new.

OOP in JavaScript

## 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

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