The Main and Tricky Concepts in JavaScript

1.scopes

2.Hoisting

3.Functions

4.Arrow Functions

5.Promises

6.Fetching API

7.Async/Await

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1. Scopes 🌎

Scope determines the accessibility of variables in JavaScript.

Types of Scope

• Global Scope 🌍: Variables declared outside any function are accessible everywhere.
• Local Scope 🔒: Variables declared inside a function are only accessible within that function.
• Block Scope 🧱: Introduced with let and const, variables declared inside {} cannot be accessed outside.
• Function Scope 🎯: Variables declared using var inside a function are only accessible within that function.

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2. Hoisting 🚀

Hoisting is JavaScript’s behavior of moving variable and function declarations to the top of their scope before execution.

Example

console.log(a); // undefined (not error)
var a = 10;

foo(); // ✅ Works! Function declarations are hoisted
function foo() {
    console.log("Function Hoisted!");
}

// Function Expressions are **not** hoisted
bar(); // ❌ Error: bar is not a function
var bar = function () {
    console.log("Function Expression");
};

Note: Only function declarations are fully hoisted, but variables declared with var are hoisted with undefined value.

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3. Functions 🎭

Function Declaration vs Function Expression

Feature	Function Declaration	Function Expression
Hoisted?	✅ Yes	❌ No
Can be called before declaration?	✅ Yes	❌ No
Syntax	function func() {}	const func = function() {}

Example

// Function Declaration
function greet() {
    return "Hello!";
}
console.log(greet()); // ✅ Works!

// Function Expression
const greetExpression = function() {
    return "Hello from expression!";
};
console.log(greetExpression()); // ✅ Works only after definition

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4. Arrow Functions ➡️

Arrow functions provide a more concise syntax for writing functions.

Syntax

const functionName = (parameters) => expression;

Example

// Traditional Function
function add(a, b) {
    return a + b;
}

// Arrow Function Equivalent
const addArrow = (a, b) => a + b;

console.log(addArrow(3, 4)); // Output: 7

Key Differences

• this is lexically bound (inherits from the surrounding scope).
• Cannot be used as constructors (new keyword).
• No arguments object available.

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5. Promises 🤝

A Promise is an object that represents the eventual completion or failure of an asynchronous operation.

States of a Promise

1. Pending 🟡 - Initial state, before the result is available.
2. Fulfilled ✅ - The operation was successful.
3. Rejected ❌ - The operation failed.

Why use Promises?

✔️ Handle asynchronous operations efficiently.

✔️ Avoid callback hell.

Example

const myPromise = new Promise((resolve, reject) => {
    let success = true;
    setTimeout(() => {
        if (success) resolve("Promise resolved! ✅");
        else reject("Promise rejected! ❌");
    }, 2000);
});

myPromise
    .then(response => console.log(response))  // Runs if resolved
    .catch(error => console.log(error));      // Runs if rejected

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6. Fetching Data from an API 🌍

Fetch is used to request data from a server.

Example

fetch("https://jsonplaceholder.typicode.com/posts/1")
    .then(response => response.json()) // Convert response to JSON
    .then(data => console.log(data))   // Log data
    .catch(error => console.log("Error:", error));

It returns a Promise, which we handle using .then() and .catch().

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7. Async/Await ⏳

async/await makes handling asynchronous code cleaner and more readable.

Example

async function fetchData() {
    try {
        let response = await fetch("https://jsonplaceholder.typicode.com/posts/1");
        let data = await response.json();
        console.log(data);
    } catch (error) {
        console.log("Error:", error);
    }
}

fetchData();

Why use async/await?

✔️ Avoids callback hell

✔️ Improves readability

✔️ Easier error handling using try...catch

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🚀 Summary

• Scope: Determines where variables are accessible.
• Hoisting: Moves function declarations and var variables to the top before execution.
• Function Declarations vs Expressions: Declarations are hoisted, expressions are not.
• Arrow Functions: Shorter syntax, no this binding.
• Promises: Handle async operations with .then() and .catch().
• Fetch API: Used for making network requests.
• Async/Await: Syntactic sugar over Promises for cleaner async code.

🔹 Master these concepts, and you're on your way to becoming a JavaScript pro! 🚀