🔹 JAVASCRIPT – REVISION SHEET

1️⃣ What is JavaScript?
> JavaScript is a high-level, dynamic, untyped, and interpreted programming language primarily used for enhancing interactivity in web pages.

2️⃣ Key Features of JavaScript
• Interpreted Language: Runs directly in the browser without the need for compilation.
• Event-driven: Responds to user actions such as clicks and keyboard input.
• Prototype-based: Supports object-oriented programming through prototypes instead of classes.

3️⃣ Data Types
• Primitive Types:
– String: Represents text (e.g., "Hello World")
– Number: Represents numeric values (e.g., 42, 3.14)
– Boolean: Represents true or false values (true, false)
– Undefined: A variable that has been declared but not assigned a value.
– Null: Represents an intentional absence of any object value.
– Symbol: A unique and immutable primitive value (ES6).

• Reference Types:
– Object: Collections of key-value pairs.
– Array: An ordered list of values.
– Function: A callable object.

4️⃣ Variables
• Declaration Keywords:
– var: Function-scoped or globally scoped.
– let: Block-scoped and can be reassigned.
– const: Block-scoped and cannot be reassigned.

5️⃣ Control Structures
• Conditional Statements:

  if (condition) {
      // code to execute if condition is true
  } else {
      // code to execute if condition is false
  }
  

• Switch Statement:

  switch (expression) {
      case value1:
          // code block
          break;
      case value2:
          // code block
          break;
      default:
          // default code block
  }
  

• Loops:
– For Loop:

    for (let i = 0; i < array.length; i++) {
        // code to execute
    }
    

• While Loop:

    while (condition) {
        // code to execute
    }
    

6️⃣ Functions
• Function Declaration:

  function functionName(parameters) {
      // code to execute
      return value;
  }
  

• Arrow Functions (ES6):

  const functionName = (parameters) => {
      // code to execute
      return value;
  };
  

7️⃣ Objects and Arrays
• Creating Objects:

  const person = {
      name: "John",
      age: 30,
      greet: function() {
          console.log("Hello!");
      }
  };
  

• Accessing Object Properties:

  console.log(person.name); // Dot notation
  console.log(person['age']); // Bracket notation
  

• Arrays:

  const fruits = ["apple", "banana", "cherry"];
  console.log(fruits[0]); // Accessing first element
  

8️⃣ ES6 Features
• Template Literals:

const greeting = Hello, ${name}!;

• Destructuring Assignment:

const { name, age } = person;

• Spread Operator:

const newArray = [...oldArray, newItem];

9️⃣ Asynchronous JavaScript
• Callbacks:

function fetchData(callback) {
    // Simulate async operation
    setTimeout(() => {
        callback("Data received");
    }, 1000);
}

• Promises:

const promise = new Promise((resolve, reject) => {
    // Async operation
    if (success) {
        resolve("Success!");
    } else {
        reject("Error!");
    }
});

• Async/Await:

async function fetchData() {
    try {
        const result = await promise;
        console.log(result);
    } catch (error) {
        console.error(error);
    }
}

🔟 Best Practices
• Use let and const instead of var for better scope control.
• Keep your functions small and focused on a single task.
• Use meaningful variable and function names.
• Comment your code for clarity.
• Avoid global variables to prevent conflicts.

1️⃣1️⃣ Common JavaScript Libraries/Frameworks
• jQuery: Simplifies DOM manipulation and event handling.
• React: A library for building user interfaces.
• Node.js: A runtime environment for executing JavaScript server-side.