JavaScript is a high-level, interpreted programming language primarily used for web development. It enables interactive web pages and is an essential part of web technologies, along with HTML and CSS. JavaScript can manipulate HTML content dynamically, handle events, and make asynchronous requests. It supports object-oriented, functional, and imperative programming styles. Modern JavaScript engines optimize performance, making it efficient. It is widely used in front-end as well as back-end development (Node.js).

JavaScript provides several data types that categorize different types of values. These include Primitive types such as String, Number, Boolean, Undefined, Null, BigInt, and Symbol. Additionally, there are Non-primitive types, mainly Objects, which include arrays and functions. Objects allow structured data storage and manipulation. Understanding these types is crucial for memory management and optimizing performance. Proper type handling prevents runtime errors and improves code reliability.

The `var` keyword allows function-scoped variables and can be redeclared or updated. However, `let` introduces block-scoped variables, preventing redeclaration within the same block and enhancing security. `const` also has block scope but ensures the variable cannot be reassigned, making it ideal for constants. Unlike `var`, both `let` and `const` are not hoisted to the top in a usable state, avoiding issues of undefined behavior. Choosing between these ensures better scoping control and maintainable code. Modern JavaScript favors `let` and `const` over `var`.

A closure is a function that retains access to its parent scope, even after the parent function has closed. This happens because functions in JavaScript form a lexical scope. Closures allow data encapsulation and are frequently used in event handlers, callbacks, and module patterns. For example, a function inside another function can use variables from its outer function even after execution. Closures help in creating private variables, reducing global scope pollution. They are an essential concept for mastering JavaScript programming.

The `==` operator checks for equality but performs type conversion if necessary, meaning `5 == "5"` evaluates to true. However, `===` checks both value and type, ensuring stricter comparisons. Using `===` prevents unintended type coercion issues and avoids logical errors. It is recommended to use `===` in most cases for more predictable behavior. Similarly, `!=` allows type conversion, while `!==` does not. Understanding these operators prevents common bugs in JavaScript applications.

A Promise in JavaScript represents a value that might be available now, later, or never. It is used for handling asynchronous operations and avoids callback hell. A Promise has three states: Pending, Resolved (Fulfilled), and Rejected. The `.then()` method is used for resolved values, and `.catch()` handles errors. Promises improve code readability and maintainability. They are extensively used in API calls and asynchronous programming.

`async/await` is a modern approach to handle asynchronous operations in JavaScript. `async` functions always return a Promise, while `await` pauses execution until the Promise resolves. This makes asynchronous code easier to read and maintain compared to Promises and callbacks. `try...catch` can be used for handling errors in `async/await`. It helps in writing cleaner and more readable asynchronous JavaScript code.

The Event Loop is a crucial part of JavaScript's concurrency model, allowing non-blocking I/O operations. JavaScript is single-threaded but can handle asynchronous tasks using an Event Loop. It continuously checks the Call Stack and Task Queue, executing functions in order. The Event Loop ensures smooth execution of asynchronous tasks without blocking the main thread. Understanding the Event Loop helps in writing efficient JavaScript code.

Hoisting is JavaScript's default behavior of moving function and variable declarations to the top of their scope before execution. `var` variables are hoisted but initialized as `undefined`, while `let` and `const` are hoisted but not initialized. Function declarations are fully hoisted, allowing them to be called before their definition. Understanding hoisting helps avoid unexpected errors in JavaScript code.

A function declaration defines a named function that can be hoisted, meaning it can be called before its definition. Example: `function greet() {}`. A function expression assigns a function to a variable and is not hoisted. Example: `const greet = function() {}`. Arrow functions are also function expressions. Understanding these differences helps in writing predictable JavaScript code.

JavaScript objects are collections of key-value pairs, where keys are strings (or Symbols) and values can be any type. Objects allow storing structured data and are fundamental in JavaScript. Example: `const person = {name: 'John', age: 30};`. Objects can have methods and can be manipulated dynamically. Understanding objects is essential for JavaScript programming.

Destructuring is a feature that allows extracting values from arrays or properties from objects into separate variables. Example: `const [a, b] = [1, 2];` or `const {name, age} = person;`. It makes code more readable and concise. Destructuring is widely used in modern JavaScript development.

`map()`, `filter()`, and `reduce()` are array methods used for transformation. `map()` creates a new array by applying a function to each element. `filter()` returns a new array containing elements that pass a condition. `reduce()` executes a function on each element, reducing the array to a single value. These methods are essential for functional programming in JavaScript.

`forEach()` executes a provided function once for each array element but does not return anything. `map()` applies a function to each element and returns a new array. `map()` is preferred when transformation is needed, while `forEach()` is used for side effects like logging. Choosing the correct method optimizes code efficiency and readability.

`localStorage` stores data with no expiration time, while `sessionStorage` keeps data only for the session. `cookies` store small amounts of data and can be sent with HTTP requests. `localStorage` and `sessionStorage` provide better performance and security, but `cookies` are useful for server-side operations.

Debouncing is a technique used to delay execution of a function until after a specified time has elapsed since the last time it was called. It is commonly used in scenarios like search inputs to prevent excessive API calls. Example: using `setTimeout()` inside an event listener to optimize performance.

A generator function (`function*`) returns an iterator that can be paused and resumed. The `yield` keyword allows execution to pause and resume later. Example: `function* gen() { yield 1; yield 2; }`. Generators are useful for handling async tasks and infinite sequences efficiently.

Synchronous JavaScript executes code line by line, blocking execution until each operation completes. Asynchronous JavaScript allows operations like API requests and timers to run in the background without blocking. Techniques like callbacks, Promises, and `async/await` handle asynchronous tasks efficiently.

Memoization is an optimization technique that caches function results to avoid redundant calculations. It improves performance in recursive functions like Fibonacci calculations. Example: storing previous results in an object and returning cached values if available.

A shallow copy copies only the reference of nested objects, meaning changes in the copy affect the original. A deep copy duplicates all nested objects, ensuring complete separation. Methods like `Object.assign()` create shallow copies, while `JSON.parse(JSON.stringify())` can create deep copies, though with some limitations.

• A closure is a function that remembers the variables from its outer scope even after the outer function has finished executing.
• Closures allow for data encapsulation and private variables.
• Example:
• function outer() { let count = 0; return function inner() { count++; console.log(count); }; }
• Calling let counter = outer(); counter(); will remember count even after outer execution.
• Closures are widely used in callbacks and event handlers.

• Arrow functions are a concise syntax for defining functions in JavaScript.
• They do not have their own this and inherit it from the surrounding scope.
• Example: const add = (a, b) => a + b;
• Arrow functions are shorter and improve readability.
• They are commonly used in callbacks and functional programming.
• However, they cannot be used as constructors.

• Hoisting is JavaScript's default behavior of moving declarations to the top of the code.
• It applies to both function and variable declarations.
• Example: console.log(x); var x = 5; will not throw an error but print undefined.
• Functions declared with function keyword can be called before their declaration.
• However, variables declared with let and const are hoisted but not initialized, leading to a ReferenceError.
• Understanding hoisting helps in writing better structured and predictable code.

• undefined means a variable has been declared but has not been assigned a value.
• null is an intentional absence of any object value.
• Example: let a; console.log(a); // undefined
• Example: let b = null; console.log(b); // null
• typeof undefined returns "undefined" while typeof null returns "object".
• It is good practice to assign null when intentionally clearing an object reference.

• A higher-order function is a function that takes another function as an argument or returns a function.
• They help in functional programming and abstraction.
• Example:
• function multiplier(factor) { return number => number * factor; }
• Calling const double = multiplier(2); console.log(double(5)); // 10
• Common higher-order functions include map, filter, and reduce.