JavaScript has several data types, including string, number, boolean, object, undefined, null, and symbol. These data types can be used to store and manipulate different kinds of data.

JavaScript is a versatile programming language used to create dynamic and interactive content on web pages. It can manipulate the DOM, handle events, validate forms, and much more.

JavaScript has two main types of data:

1. Primitive:

   • Number: Represents numeric values.
   • String: Represents text.
   • Boolean: Represents true or false.
   • Undefined: Represents a variable that hasn't been assigned a value.
   • Null: Represents an intentional absence of value.
   • Symbol: Represents unique and immutable values (ES6).
   • BigInt: Represents integers with arbitrary precision (ES2020).

2. Object:

   • Includes objects, arrays, functions, and other complex data types. Objects can hold multiple values in key-value pairs.

A closure is a function that has access to its own scope, the scope of the outer function, and the global scope. Closures are created whenever a function is defined inside another function.

var is function-scoped, while let and const are block-scoped. const is used for variables that should not be reassigned, whereas let is used for variables that can be reassigned.

The this keyword refers to the object from which the function was called. Its value depends on the context

The DOM (Document Object Model) is an API that allows JavaScript to interact with and manipulate the structure, style, and content of a web page. It represents the document as a tree of nodes, where each node corresponds to a part of the document.

You can select an element by its ID using the document.getElementById() method.

Example:

Code in JavaScript:

var element = document.getElementById('myElement');

Event delegation is a technique that allows you to handle events at a higher level in the DOM, rather than adding event listeners to individual child elements. It takes advantage of event bubbling to handle events more efficiently.

== is the loose equality operator that compares two values for equality, performing type conversion if necessary. === is the strict equality operator that compares both the value and type, without performing type conversion.

Closures are functions that have access to their own scope, the scope of the outer function, and the global scope. They are useful for creating private variables and functions, and for maintaining state between function calls.

Objects in JavaScript can be created using object literals, constructors, or the Object.create() method. Example using object literals:

Code in JavaScript:

var person = {

  firstName: "John",

  lastName: "Doe",

  age: 30

};

A callback function is a function passed as an argument to another function, to be executed after the completion of that function. Callbacks are often used for asynchronous operations.

Errors in JavaScript can be handled using the try...catch statement. The try block contains the code that may throw an error, and the catch block contains the code that handles the error.

The this keyword refers to the object from which the function was called. Its value depends on the context in which it is used, such as in a method, constructor, or globally.

undefined indicates that a variable has been declared but not yet assigned a value. null is an assignment value that represents no value or an empty object.

A promise in JavaScript can be created using the Promise constructor, which takes a function with resolve and reject parameters.

Example:

Code in JavaScript:

var promise = new Promise(function(resolve, reject) {

  // asynchronous operation

  if (success) {

    resolve('Success!');

  } else {

    reject('Error!');

  }

});

The map() function creates a new array by calling a provided function on every element in the calling array. It does not mutate the original array.

Arrow functions are a concise way to write functions in JavaScript. They have a shorter syntax and do not have their own this, arguments, or super bindings. Example:

JavaScript:

const add = (a, b) => a + b;

Event bubbling can be prevented by calling event.stopPropagation() on the event object within the event handler.

The fetch API is a modern interface that allows you to make network requests similar to XMLHttpRequest. It returns a promise that resolves to the response of the request.

Example:

JavaScript:

fetch('https://api.example.com/data')

  .then(response => response.json())

  .then(data => console.log(data));

Both forEach and map iterate over an array. However, forEach does not return a new array, while map returns a new array with the results of the callback function applied to each element.

Hoisting is a JavaScript mechanism where variables and function declarations are moved to the top of their containing scope during the compilation phase. Only the declarations are hoisted, not the initializations.

Classes in JavaScript can be created using the class keyword.

Example:

JavaScript:

class Person {

  constructor(name, age) {

    this.name = name;

    this.age = age;

  }

  greet() {

    console.log(`Hello, my name is ${this.name}`);

  }

}

apply and call are used to invoke functions with a specific this value, but apply takes arguments as an array, while call takes arguments individually. bind returns a new function with a specific this value, but does not invoke the function immediately.

The typeof operator returns a string indicating the type of the operand.

Example:

JavaScript:

typeof 42; // "number"

typeof 'hello'; // "string"

Modules in JavaScript allow you to organize and reuse code across different files. They can be created using the export and import keywords.

Example:

JavaScript:

// module.js

export const greeting = 'Hello';

// main.js

import { greeting } from './module.js';

console.log(greeting); // Hello

Events can be created using the Event constructor and dispatched using element.dispatchEvent(). Example:

JavaScript:

var event = new Event('customEvent');

element.dispatchEvent(event);

The reduce function applies a function against an accumulator and each element in the array to reduce it to a single value. Example:

JavaScript:

const sum = [1, 2, 3, 4].reduce((acc, val) => acc + val, 0); // 10

Asynchronous operations in JavaScript can be handled using callbacks, promises, or async/await. async/await allows writing asynchronous code in a synchronous style.

The spread operator (...) allows you to spread out elements of an array or object.

Example:

JavaScript:

const arr = [1, 2, 3];

const newArr = [...arr, 4, 5]; // [1, 2, 3, 4, 5]

localStorage allows you to store data in the browser that persists even after the page is closed. Example:

JavaScript:

localStorage.setItem('key', 'value');

const value = localStorage.getItem('key');

 Promises are objects that represent the eventual completion (or failure) of an asynchronous operation. They have three states: pending, fulfilled, and rejected.

An object can be cloned using Object.assign() or the spread operator.

Example using the spread operator:

JavaScript:

const obj = { a: 1, b: 2 };

const clone = { ...obj };

NaN stands for "Not-a-Number" and is a value representing a computational error. You can check if a value is NaN using isNaN() or Number.isNaN().

Exceptions can be handled using try...catch. You can also use finally to execute code regardless of whether an exception was thrown.

Example:

JavaScript:

try {

  // code that may throw an error

} catch (error) {

  console.error('Error:', error);

} finally {

  console.log('This will run regardless');

}