JavaScript Core Concepts

• JavaScript is a high-level, versatile programming language essential for web development
• It enables interactive and dynamic content on websites

Core Concepts

• Variables: Used to store data values
  • Declared using var, let, or const
  • let and const are block-scoped, while var is function-scoped
• Data Types: Include primitive types (String, Number, Boolean, Null, Undefined, Symbol) and object types (Object, Array, Function)
• Operators: Perform operations on variables and values
  • Arithmetic operators (+, -, *, /)
  • Comparison operators (==, ===, !=, !==, >, <, >=, <=)
  • Logical operators (&&, ||, !)
• Control Flow: Manages the order in which code is executed
  • Conditional statements (if, else if, else)
  • Switch statements (switch, case, break, default)
  • Loops (for, while, do...while)
• Functions: Reusable blocks of code
  • Defined using the function keyword
  • Can accept arguments (parameters) and return values
  • Can be defined as function declarations or function expressions
  • Arrow functions (=>) provide a concise syntax
• Objects: Collections of key-value pairs
  • Created using object literals ({}) or the new keyword with a constructor function
  • Accessed using dot notation (.) or bracket notation ([])
• Arrays: Ordered lists of values
  • Created using array literals ([]) or the new Array() constructor
  • Accessed using index numbers (starting from 0)
  • Have various methods for manipulation (push, pop, shift, unshift, splice, slice, concat, join, etc.)

Document Object Model (DOM)

• The DOM is a programming interface for HTML and XML documents
• It represents the structure of a document as a tree-like structure
• JavaScript can manipulate the DOM to dynamically update content, structure, and styles of a web page
• Common DOM methods include:
  • document.getElementById(): Returns the element with a specified ID
  • document.getElementsByClassName(): Returns a collection of elements with a specified class name
  • document.getElementsByTagName(): Returns a collection of elements with a specified tag name
  • document.querySelector(): Returns the first element that matches a specified CSS selector
  • document.querySelectorAll(): Returns a collection of elements that match a specified CSS selector
  • element.innerHTML: Gets or sets the HTML content of an element
  • element.textContent: Gets or sets the text content of an element
  • element.setAttribute(): Sets the value of an attribute on an element
  • element.getAttribute(): Gets the value of an attribute on an element
  • element.appendChild(): Adds a new child node to an element
  • element.removeChild(): Removes a child node from an element

Events

• Events are actions or occurrences that happen in the browser, such as a user clicking a button or a page loading
• JavaScript can respond to these events by executing specific code
• Event listeners are used to wait for specific events and trigger a function when the event occurs
• Common event types include:
  • click: Occurs when an element is clicked
  • mouseover: Occurs when the mouse pointer moves over an element
  • mouseout: Occurs when the mouse pointer moves out of an element
  • keydown: Occurs when a key is pressed down
  • keyup: Occurs when a key is released
  • submit: Occurs when a form is submitted
  • load: Occurs when a page or an element has finished loading
• Event listeners can be added using:
  • Inline event handlers (e.g., <button onclick="myFunction()">)
  • DOM properties (e.g., element.onclick = myFunction;)
  • addEventListener() method (e.g., element.addEventListener('click', myFunction);)

Asynchronous JavaScript

• JavaScript is single-threaded, meaning it executes code line by line in a sequential manner
• Asynchronous operations allow JavaScript to perform tasks without blocking the execution of other code
• Callbacks: Functions passed as arguments to other functions, to be executed after the completion of an asynchronous operation
• Promises: Represent the eventual completion (or failure) of an asynchronous operation and its resulting value
  • A promise can be in one of three states: pending, fulfilled, or rejected
  • Promises provide a more structured way to handle asynchronous code compared to callbacks
  • then() method: Used to handle the fulfilled state of a promise
  • catch() method: Used to handle the rejected state of a promise
  • finally() method: Used to execute code regardless of whether the promise is fulfilled or rejected
• Async/Await: Syntactic sugar built on top of promises that makes asynchronous code look and behave more like synchronous code
  • async keyword: Used to define an asynchronous function
  • await keyword: Used to pause the execution of an asynchronous function until a promise is resolved

Error Handling

• Errors can occur during the execution of JavaScript code
• Proper error handling is essential to prevent unexpected behavior and provide a better user experience
• try...catch statement: Used to handle exceptions (errors)
  • Code that might throw an error is placed inside the try block
  • If an error occurs, the code in the catch block is executed
  • The finally block can be used to execute code regardless of whether an error occurred or not
• throw statement: Used to explicitly throw an error
• Error object: Contains information about the error, such as the error message and the line number where the error occurred

Scope

• Scope refers to the accessibility of variables in different parts of the code
• Global scope: Variables declared outside of any function or block have global scope and can be accessed from anywhere in the code
• Function scope: Variables declared inside a function have function scope and can only be accessed within that function
• Block scope: Variables declared with let or const inside a block (e.g., inside an if statement or a loop) have block scope and can only be accessed within that block
• Lexical scope: The ability of a function to access variables from its surrounding (parent) scope
• Closures: A function that has access to the variables in its surrounding scope, even after the outer function has finished executing

Modules

• Modules allow you to organize code into reusable and maintainable units
• ES Modules: The standard module system in JavaScript
  • import statement: Used to import modules
  • export statement: Used to export modules
  • Named exports: Export individual variables, functions, or classes using their names
  • Default exports: Export a single value as the default export of a module
• CommonJS: An older module system commonly used in Node.js
  • require() function: Used to import modules
  • module.exports object: Used to export modules

Modern JavaScript (ECMAScript)

• ECMAScript (ES) is the standard that JavaScript is based on
• New versions of ECMAScript are released regularly, introducing new features and improvements to the language
• ES6 (ECMAScript 2015) introduced many significant features, including:
  • let and const keywords
  • Arrow functions
  • Classes
  • Modules
  • Template literals
  • Destructuring assignment
  • Spread syntax
  • Default parameters
• Other notable ES features include:
  • Async/await (ES2017)
  • Object rest/spread properties (ES2018)
  • Optional chaining (ES2020)
  • Nullish coalescing operator (ES2020)

Common Libraries and Frameworks

• React: A JavaScript library for building user interfaces
  • Uses a component-based architecture
  • Uses a virtual DOM for efficient updates
• Angular: A comprehensive framework for building complex web applications
  • Uses TypeScript
  • Provides features like dependency injection, routing, and data binding
• Vue.js: A progressive framework for building user interfaces
  • Easy to learn and use
  • Flexible and versatile
• Node.js: A JavaScript runtime environment that allows you to run JavaScript code on the server-side
  • Used for building scalable and high-performance applications
• jQuery A library designed to simplify HTML DOM tree traversal and manipulation, as well as event handling, CSS animation, and Ajax.

Explore JavaScript's key concepts: variables, data types, and operators. Understand control flow, conditional statements, and loops. Learn to define and use functions to write reusable code blocks.