JavaScript Notes
HubYour First JavaScript Program — Hello World & Beyond Complete Guide 2026History of JavaScript — Complete Timeline from 1995 to 2026How JavaScript Runs — Complete Guide to Execution, Call Stack & Event LoopJavaScript Engine Explained — V8, JIT Compilation & How Code Gets FastJavaScript Use Cases — 15 Things You Can Build with JavaScript (2026)JavaScript vs Other Languages — Complete Comparison Guide 2026What is JavaScript? Complete BeginnerWhy Learn JavaScript in 2026? Career, Salary, Scope & Benefits GuideJavaScript Break Statement Tutorial – Complete Guide with ExamplesJavaScript Break and Continue Statements – Complete Guide with ExamplesJavaScript Continue Statement Tutorial – Skip Iterations with ExamplesJavaScript Do While Loop Tutorial – Execute at Least Once with ExamplesJavaScript Do-While Loop Tutorial – Complete Guide with ExamplesJavaScript For Loop Tutorial – Complete Guide with ExamplesJavaScript If Statement Tutorial – Conditional Logic with ExamplesJavaScript If Else Statement Tutorial – Complete Guide with ExamplesJavaScript If Statement Tutorial – Complete Guide with ExamplesJavaScript Nested If Statement Tutorial – Multi-Level Conditions with ExamplesNested Loops in JavaScriptJavaScript Control Flow Practice Programs – 20+ Exercises with SolutionsJavaScript Switch Statement Tutorial – Case Matching with ExamplesSwitch Statement in JavaScriptJavaScript While Loop Tutorial – Complete Guide with ExamplesJavaScript Arrow Functions Tutorial – Syntax, Use Cases & this BindingJavaScript Callback Functions Explained – Synchronous & AsynchronousJavaScript Closures Explained – Lexical Scope, Private State & MemoryJavaScript Function Declaration Tutorial – Hoisting, Syntax & ScopeJavaScript Function Expression Tutorial – Anonymous, Named & AssignedJavaScript Higher Order Functions Tutorial – map, filter, reduce & Custom HOFsJavaScript IIFE Explained – Immediately Invoked Function ExpressionJavaScript Functions Practice Programs – 20+ Exercises with SolutionsJavaScript Recursion Tutorial – Call Stack, Base Case & Recursive PatternsJavaScript Array Basics - Complete BeginnerJavaScript Array Methods - Complete Reference Guide with ExamplesJavaScript Array Programs - 20+ Practice Problems with SolutionsJavaScript Arrays Basics - Complete Guide with Examples & DiagramsJavaScript Array filter() Method - Complete Tutorial with ExamplesJavaScript filter() Method — Complete Guide with Examples 2026JavaScript Array find() Method - Complete Tutorial with ExamplesJavaScript find() & findIndex() — Complete Guide 2026JavaScript Array map() Method - Complete Tutorial with ExamplesJavaScript map() Method — Transform Arrays Complete Guide 2026JavaScript map() vs forEach() - Key Differences with ExamplesJavaScript Array push() and pop() - Complete Tutorial with ExamplesJavaScript Array reduce() Method - Complete Tutorial with ExamplesJavaScript reduce() Method — Complete Deep Dive with Examples 2026JavaScript Array shift() and unshift() - Complete Tutorial with ExamplesJavaScript Array sort() Method - Complete Tutorial with ExamplesJavaScript splice() vs slice() - Complete Tutorial with ExamplesJavaScript Object Destructuring - Complete Guide with ExamplesJavaScript Objects - Complete Guide to Object Basics with ExamplesJavaScript Object Methods - Built-in Methods & Custom Methods Complete GuideJavaScript Object Programs - Practice Problems with SolutionsJavaScript Object Properties - Keys, Values, Descriptors & Dynamic PropertiesJavaScript Rest Operator (...) - Parameters, Destructuring & Complete GuideJavaScript Spread Operator (...) - Complete Guide with ExamplesJavaScript this Keyword - Complete Guide with Binding Rules & ExamplesJavaScript String Basics - Complete Guide to Strings with ExamplesJavaScript String Methods - Complete Reference with Examples & OutputJavaScript String Programs - Practice Problems with SolutionsJavaScript String Replacing - replace(), replaceAll() & Regex Patterns GuideJavaScript String Searching Methods - includes, indexOf, startsWith, match GuideJavaScript Template Literals (ES6) - Complete Guide with Tagged TemplatesJavaScript DOM Changing Content - textContent, innerHTML, setAttribute Complete GuideJavaScript DOM Changing Styles - CSS Manipulation, classList & getComputedStyleJavaScript DOM Creating Elements - createElement, appendChild, DocumentFragment GuideJavaScript DOM Introduction - Complete Guide to Document Object ModelJavaScript DOM Traversal - Navigate Parent, Child & Sibling NodesJavaScript DOM Forms - Form Handling, Validation & FormData Complete GuideJavaScript localStorage - Complete Guide to Web Storage APIlocalStorage vs sessionStorage - JavaScript Web Storage Comparison GuideJavaScript DOM Removing Elements - remove(), removeChild() Complete GuideJavaScript DOM Selecting Elements - querySelector, getElementById Complete GuideJavaScript sessionStorage - Complete Guide to Session-Based Web StorageJavaScript Click Event - addEventListener, Event Handler Complete GuideEvent Bubbling in JavaScript – Complete Guide with Examples (2026)Event Capturing in JavaScript – Complete Guide to Capture Phase (2026)Event Delegation in JavaScript – Complete Guide with Examples (2026)JavaScript Event Object – Complete Guide with Examples (2026)JavaScript Form Events – Complete Guide with Examples (2026)JavaScript Keyboard Events – Complete Guide to keydown, keyup & Key Properties (2026)JavaScript Mouse Events – Complete Guide with Examples (2026)JavaScript Async Await - Complete Guide with Examples & Error HandlingJavaScript Call Stack - Complete Guide with Visual Diagrams & ExamplesJavaScript Callback Queue (Task Queue) - Complete Guide with DiagramsJavaScript Callbacks - Complete Guide with Callback Hell & PatternsJavaScript Event Loop - Complete Guide with Visual Diagrams & ExamplesJavaScript Fetch API - Complete Guide with Examples, Error Handling & Async/AwaitJSON in JavaScript - Complete Guide: Parse, Stringify, Fetch & Error HandlingJavaScript Promise.all, Promise.race, Promise.any, Promise.allSettled — Complete Guide 2026JavaScript Promises - Complete Guide with Chaining, Error Handling & Event LoopSynchronous vs Asynchronous JavaScript — Complete Explanation 2026JavaScript Web APIs - Complete Guide: setTimeout, fetch, DOM Events & MoreClosures in JavaScript — Complete Guide with Examples 2026Hoisting in JavaScript — Complete Guide with Examples 2026Prototype in JavaScript — Complete Guide with Examples 2026Scope in JavaScript — Complete Guide with Examples 2026Browser Object Model (BOM) in JavaScript — Complete GuideCanvas API in JavaScript — Complete Drawing & Animation GuideDrag and Drop API in JavaScript — Complete TutorialGeolocation API in JavaScript — Get User Location TutorialHistory API in JavaScript — SPA Routing & pushState GuideNotification API in JavaScript — Push Notifications TutorialService Workers in JavaScript — Offline, Caching & PWA GuideWeb Storage API in JavaScript — localStorage & sessionStorage GuideWeb Workers in JavaScript — Multithreading & Background Tasks GuideJavaScript Interview — Behavioral Questions and Soft Skills Guide 2026Common JavaScript Interview Mistakes — What to Avoid 2026JavaScript Interview Tips — Strategy and Preparation Guide 2026JavaScript Mock Interview — Practice Questions with Solutions 2026JavaScript System Design — Frontend Architecture Interview Guide 2026JavaScript apply() Method - Complete Guide with ExamplesJavaScript bind() Method - Complete Guide with ExamplesJavaScript call() Method - Complete Guide with ExamplesJavaScript call() vs apply() vs bind() - Complete Comparison GuideJavaScript Currying - Complete Guide with ExamplesJavaScript Debounce - Complete Guide with Timeline DiagramsJavaScript Execution Context - Complete Guide with Stack DiagramsJavaScript Garbage Collection - Memory Lifecycle & Mark-and-Sweep GuideJavaScript Generators - Complete Guide with yield and IterationJavaScript Hoisting - Complete Guide with var, let, const & FunctionsJavaScript Iterators - Complete Guide with Iterator ProtocolJavaScript Lexical Scope - Complete Guide with Scope Chain DiagramsJavaScript Memory Management - Complete Guide with Lifecycle DiagramsJavaScript Modules - Complete Guide with import export ExamplesJavaScript Scope - Complete Guide with var, let, const & Scope ChainJavaScript Symbols - Complete Guide with Well-Known SymbolsJavaScript Throttle - Complete Guide with Timeline DiagramsJavaScript Comments Complete Guide – Single Line, Multi-line & JSDoc with ExamplesJavaScript Data Types Complete Guide – Primitives, Objects & typeof with ExamplesJavaScript Input and Output Complete Guide – console.log, prompt, alert & DOM with ExamplesJavaScript Keywords Complete Guide – Reserved Words List with Examples for BeginnersJavaScript null vs undefined – Complete Guide with Differences, Examples & Interview Q&AJavaScript Operators Complete Guide – Arithmetic, Comparison, Logical & More with ExamplesJavaScript Strict Mode Complete Guide – JavaScript Type Conversion Complete Guide – Explicit & Implicit Coercion with ExamplesJavaScript var vs let vs const – Complete Guide with Scope, Hoisting & Best PracticesJavaScript Variables Complete Guide – Declaration, Naming Rules & Memory Model for BeginnersDeque (Double-Ended Queue) in JavaScript — Complete GuideLinked List in JavaScript — Nodes, Pointers & Operations GuideJavaScript Map — Key-Value Data Structure Complete GuideJavaScript Numbers — IEEE 754, Precision, and Number Methods GuidePriority Queue in JavaScript — Min Heap Implementation GuideQueue Data Structure in JavaScript — FIFO, Enqueue & Dequeue ExplainedJavaScript Set — Unique Values Data Structure GuideSorting Algorithms in JavaScript — Bubble, Merge, Quick Sort GuideStack Data Structure in JavaScript — LIFO, Push, Pop ExplainedJavaScript Typed Arrays — ArrayBuffer, Int32Array, Float64Array GuideJavaScript WeakMap — Weak References, Memory Management GuideJavaScript WeakSet — Complete Guide with Examples 2026Applications of JavaScript — Where JS is Used in 2026 Complete GuideJavaScript Backend Frameworks — Node.js, Express, NestJS Complete Guide 2026JavaScript Frontend Libraries — React, Vue, Angular, Svelte Complete Overview 2026JavaScript Full-Stack Frameworks — Next.js, Remix, Astro Complete Guide 2026JavaScript Career Paths & Jobs — Salary, Roles & Roadmap 2026JavaScript Libraries vs Frameworks — Difference & Examples Complete Guide 2026JavaScript Custom Errors – Extend Error Class, Custom Error Types & instanceofJavaScript Debugging – console Methods, DevTools, Breakpoints & Error Stack TracesJavaScript throw Statement – Throw Errors, Custom Messages & Error TypesJavaScript Try Catch Finally – Error Handling Complete GuideTop 30 JavaScript Closure Interview Questions & Answers 2026Top 30 JavaScript Coding Interview Questions 2026 (With Solutions)Top 25 JavaScript Event Loop Interview Questions & Answers 2026Top 50 JavaScript Interview Questions & Answers 2026Top 30 JavaScript Promise Interview Questions & Answers 202650 Tricky JavaScript Output Questions 2026 (With Answers)Build a JavaScript Calculator - Step by Step Project TutorialBuild a JavaScript Image Carousel - Step by Step Project TutorialBuild a JavaScript Counter Application - Step by Step Project TutorialBuild a JavaScript Digital Clock - Step by Step Project TutorialBuild a Dynamic Resume Builder in JavaScript - Step by Step TutorialBuild a JavaScript Email Validator - Step by Step Project TutorialBuild a JavaScript Employee Database Management App - Step by Step TutorialBuild a JavaScript Expense Tracker - Step by Step Project TutorialBuild a GitHub Profile Search App in JavaScript - Step by Step TutorialBuild a JavaScript Image Slider - Step by Step Project TutorialBuild a JavaScript Multi-Step Progress Bar - Step by Step TutorialBuild JavaScript Nested Chat Comments - Step by Step Project TutorialBuild a JavaScript Notes App - Step by Step Project TutorialBuild a JavaScript OTP Input Field - Step by Step Project TutorialBuild a JavaScript Palindrome Checker - Step by Step Project TutorialBuild a JavaScript Price Range Slider - Step by Step Project TutorialBuild a JavaScript Prime Number Checker - Step by Step Project TutorialBuild a JavaScript QR Code Generator - Step by Step Project TutorialBuild a JavaScript QR Code Scanner - Step by Step Project TutorialBuild a JavaScript Quiz App - Step by Step Project TutorialBuild a JavaScript Random Number Generator - Step by Step TutorialBuild a JavaScript Random Password Generator - Step by Step TutorialJavaScript Image Resize & Compress — Canvas API Project 2026JavaScript Responsive Admin Dashboard — DOM & CSS Project 2026JavaScript Show/Hide Password Toggle — Beginner DOM Project 2026JavaScript Sortable Filterable Table — DOM & Array Methods Project 2026JavaScript Stopwatch Project — setInterval & DOM Tutorial 2026JavaScript Student Grade Calculator — Arrays & Logic Project 2026JavaScript Task Scheduler Project — CRUD & localStorage Tutorial 2026JavaScript Tic-Tac-Toe Game — DOM Manipulation Project Tutorial 2026JavaScript Toast Notification Project — Custom Alerts Tutorial 2026Build a Todo App in JavaScript - Step by Step Project TutorialJavaScript Unicode Character Value Finder — charCodeAt & fromCharCode Project 2026JavaScript Weather App Project — Fetch API & DOM Complete Tutorial 2026JavaScript Cheat Sheet 2026 — Complete Quick ReferenceImportant JavaScript Concepts Every Developer Must Know in 2026JavaScript One Shot Notes 2026 — Complete JS in One PageJavaScript Quick Revision Notes — Interview Prep in 30 Minutes 2026Abstraction in JavaScript OOP – Hide Complexity, Expose SimplicityJavaScript Classes Tutorial – ES6 Class Syntax, Inheritance & MoreJavaScript Constructor Functions – new Keyword, Prototype & OOP PatternsEncapsulation in JavaScript – Private Fields, Getters, Setters & Data ProtectionJavaScript Inheritance – extends, super, and Prototype-Based InheritanceObject-Oriented Programming in JavaScript – OOP Concepts Complete GuideJavaScript OOP Projects – Build Real-World Apps with Classes & OOPPolymorphism in JavaScript – Method Overriding, Duck Typing & Many FormsJavaScript Prototype Chain – How Property Lookup Works in JavaScriptJavaScript Prototypes – Prototype-Based Inheritance, Object.create & prototype PropertyJavaScript Advanced Practice Problems — 15+ Interview-Level Challenges 2026JavaScript Assignments — 10 Project-Based Learning Tasks 2026JavaScript Beginner Practice Problems — 20+ Coding Exercises 2026JavaScript Coding Challenges — 25 Interview-Style Problems 2026JavaScript Intermediate Practice Problems — 20+ Coding Challenges 2026JavaScript MCQs — 50 Multiple Choice Questions with Answers 2026JavaScript Regex Character Classes – [abc], [^abc], Ranges & ShortcutsJavaScript Regex Email Validation – Pattern, Edge Cases & Best PracticesJavaScript Regex Flags – g, i, m, s, u, y Flags Explained with ExamplesJavaScript Regex Groups – Capturing, Non-Capturing & Named GroupsJavaScript Number Validation with Regex — Complete Patterns & Examples 2026JavaScript Password Validation with Regex — Strength Checker & Patterns 2026JavaScript Regex Quantifiers – *, +, ?, {n,m}, Greedy vs Lazy MatchingJavaScript Regex Basics – Regular Expressions Tutorial for BeginnersJavaScript Regex Projects — 10 Practical Mini-Projects with Code 2026JavaScript URL Validation with Regex — Complete Patterns & Examples 2026JavaScript Username Validation with Regex — Patterns & Form Examples 2026JavaScript TutorialJavaScript Lazy Loading — Images, Modules & Components Complete Guide 2026JavaScript Performance Optimization — Speed Up Your Code Complete Guide 2026JavaScript Unit Testing with Jest — Complete Guide for Beginners 2026
Learn JavaScript Event Object properties & methods. Covers event.target, preventDefault, stopPropagation, MouseEvent, KeyboardEvent with examples.
Introduction
Whenever an event occurs in the browser — whether the user clicked, pressed a key, or moved the mouse — the browser automatically creates an Event Object and passes it to the event handler function as the first parameter.
This Event Object is like a treasure box — it contains all the information about the event: what happened, where it happened, which element it happened on, and much more.
js example
button.addEventListener('click', function(event) {
// 'event' yahan Event Object hai
// Browser ne ise automatically pass kiya
console.log(event);
});Output
PointerEvent {isTrusted: true, type: "click", target: button, currentTarget: button, ...}Key Point: You don't need to create the event object manually — the browser creates it automatically and passes it as the first parameter to the handler function. You can name it anything: event, e, or evt.
Key Properties of Event Object
1. event.type
This property tells which event was triggered — "click", "keydown", "submit", etc.
js example
document.addEventListener('click', function(event) {
console.log("Event type:", event.type);
});
document.addEventListener('keydown', function(event) {
console.log("Event type:", event.type);
});Output
Event type: click Event type: keydown
2. event.target
This returns the actual element on which the event occurred. If you clicked inside nested elements, this will be the innermost (deepest) element.
js example
// HTML: <div id="parent"><button id="child">Click Me</button></div>
document.getElementById('parent').addEventListener('click', function(event) {
console.log("Target element:", event.target.id);
console.log("Target tag:", event.target.tagName);
});
// User clicks the button inside divOutput
Target element: child Target tag: BUTTON
3. event.currentTarget
This is the element on which the event listener is attached. It is very important in event delegation.
js example
// HTML: <ul id="menu"><li>Home</li><li>About</li></ul>
document.getElementById('menu').addEventListener('click', function(event) {
console.log("target:", event.target.tagName); // LI (jahan click hua)
console.log("currentTarget:", event.currentTarget.tagName); // UL (jahan listener hai)
});
// User clicks on "Home" liOutput
target: LI currentTarget: UL
4. event.bubbles
A boolean value that indicates whether the event will bubble or not. Most events bubble, but some (like focus, blur) do not.
js example
document.addEventListener('click', function(event) {
console.log("Click bubbles:", event.bubbles);
});
document.querySelector('input').addEventListener('focus', function(event) {
console.log("Focus bubbles:", event.bubbles);
});Output
Click bubbles: true Focus bubbles: false
5. event.clientX / event.clientY
In mouse events, these properties give the cursor position relative to the viewport (visible area).
js example
document.addEventListener('mousemove', function(event) {
console.log(`Mouse position: X=${event.clientX}, Y=${event.clientY}`);
});
// User moves mouse to coordinates (150, 300)Output
Mouse position: X=150, Y=300
6. event.key
In keyboard events, this property indicates which key was pressed. It is the modern replacement for the deprecated keyCode.
js example
document.addEventListener('keydown', function(event) {
console.log("Key pressed:", event.key);
console.log("Key code:", event.code);
console.log("Shift held:", event.shiftKey);
});
// User presses Shift + AOutput
Key pressed: A Key code: KeyA Shift held: true
7. event.timeStamp
When the event was triggered — gives the time in milliseconds since page load.
js example
document.addEventListener('click', function(event) {
console.log("Event occurred at:", event.timeStamp.toFixed(2), "ms after page load");
});Output
Event occurred at: 4523.70 ms after page load
8. event.isTrusted
Indicates whether the event was triggered by a user action (true) or dispatched programmatically (false).
js example
button.addEventListener('click', function(event) {
console.log("Is user-triggered:", event.isTrusted);
});
// User clicks → true
// button.click() → true (browser simulates)
// button.dispatchEvent(new Event('click')) → falseOutput
Is user-triggered: true
Code Examples
Example 1: Accessing All Basic Properties
js example
const btn = document.querySelector('#myBtn');
btn.addEventListener('click', function(event) {
console.log("Type:", event.type);
console.log("Target:", event.target.id);
console.log("CurrentTarget:", event.currentTarget.id);
console.log("Bubbles:", event.bubbles);
console.log("Trusted:", event.isTrusted);
console.log("TimeStamp:", Math.round(event.timeStamp));
});
// User clicks button#myBtnOutput
Type: click Target: myBtn CurrentTarget: myBtn Bubbles: true Trusted: true TimeStamp: 5234
Example 2: Mouse Coordinates (All Types)
js example
document.addEventListener('click', function(event) {
console.log("=== Mouse Coordinates ===");
console.log("clientX/Y (viewport):", event.clientX, event.clientY);
console.log("pageX/Y (page):", event.pageX, event.pageY);
console.log("screenX/Y (screen):", event.screenX, event.screenY);
console.log("offsetX/Y (element):", event.offsetX, event.offsetY);
});
// User clicks at a position on pageOutput
=== Mouse Coordinates === clientX/Y (viewport): 245 180 pageX/Y (page): 245 680 screenX/Y (screen): 545 380 offsetX/Y (element): 45 20
Example 3: Keyboard Event Details
js example
document.addEventListener('keydown', function(event) {
console.log("Key:", event.key);
console.log("Code:", event.code);
console.log("Alt:", event.altKey);
console.log("Ctrl:", event.ctrlKey);
console.log("Shift:", event.shiftKey);
console.log("Meta (Win/Cmd):", event.metaKey);
console.log("Repeat:", event.repeat);
});
// User presses Ctrl + SOutput
Key: s Code: KeyS Alt: false Ctrl: true Shift: false Meta (Win/Cmd): false Repeat: false
Example 4: Form Submit Event
js example
const form = document.querySelector('#loginForm');
form.addEventListener('submit', function(event) {
event.preventDefault(); // Form submit hone se roko
console.log("Event type:", event.type);
console.log("Submitter:", event.submitter.textContent);
console.log("Default prevented:", event.defaultPrevented);
// Handle form data manually
const formData = new FormData(form);
console.log("Username:", formData.get('username'));
});
// User clicks submit buttonOutput
Event type: submit Submitter: Login Default prevented: true Username: rahul123
Example 5: Event Delegation with target
js example
// HTML: <ul id="todo-list">
// <li data-id="1">Buy groceries</li>
// <li data-id="2">Read book</li>
// <li data-id="3">Exercise</li>
// </ul>
const list = document.getElementById('todo-list');
list.addEventListener('click', function(event) {
if (event.target.tagName === 'LI') {
const itemId = event.target.dataset.id;
const itemText = event.target.textContent;
console.log(`Clicked item #${itemId}: "${itemText}"`);
console.log("Listener on:", event.currentTarget.id);
}
});
// User clicks "Read book"Output
Clicked item #2: "Read book" Listener on: todo-list
Example 6: Custom Event with Detail
js example
// Creating a custom event
const customEvent = new CustomEvent('userLogin', {
detail: {
username: 'rahul_dev',
role: 'admin',
loginTime: new Date().toISOString()
},
bubbles: true
});
document.addEventListener('userLogin', function(event) {
console.log("Custom event type:", event.type);
console.log("Username:", event.detail.username);
console.log("Role:", event.detail.role);
console.log("Is trusted:", event.isTrusted);
});
document.dispatchEvent(customEvent);Output
Custom event type: userLogin Username: rahul_dev Role: admin Is trusted: false
MouseEvent vs KeyboardEvent vs FocusEvent
JavaScript has different Event interfaces for different event types. All inherit from Event but bring their own extra properties.
Example
┌──────────────────────────────────────────────────────────────────────┐
│ Event (Base Class) │
│ type, target, currentTarget, bubbles, preventDefault(), ... │
├──────────────┬──────────────────────┬────────────────────────────────┤
│ │ │ │
▼ ▼ ▼ │
┌──────────┐ ┌──────────────────┐ ┌────────────────┐ │
│MouseEvent│ │ KeyboardEvent │ │ FocusEvent │ │
├──────────┤ ├──────────────────┤ ├────────────────┤ │
│clientX │ │ key │ │ relatedTarget │ │
│clientY │ │ code │ │ (previous or │ │
│pageX │ │ altKey │ │ next focused │ │
│pageY │ │ ctrlKey │ │ element) │ │
│screenX │ │ shiftKey │ │ │ │
│screenY │ │ metaKey │ │ Bubbles: NO │ │
│button │ │ repeat │ │ (focus/blur) │ │
│buttons │ │ location │ │ │ │
│offsetX │ │ │ │ Use focusin/ │ │
│offsetY │ │ Bubbles: YES │ │ focusout for │ │
│movementX │ │ │ │ bubbling │ │
│movementY │ │ │ │ │ │
│ │ │ │ │ │ │
│Bubbles:YES│ │ │ │ │ │
└──────────┘ └──────────────────┘ └────────────────┘ │
└──────────────────────────────────────────────────────────────────────┘
Comparison Table
| Feature | MouseEvent | KeyboardEvent | FocusEvent |
|---|---|---|---|
| Trigger | Mouse click, move, hover | Key press/release | Focus gain/loss |
| Events | click, dblclick, mousedown, mousemove | keydown, keyup, keypress(deprecated) | focus, blur, focusin, focusout |
| Coordinates | ✅ clientX, pageX, screenX | ❌ Not available | ❌ Not available |
| Key info | ❌ Not available | ✅ key, code, modifiers | ❌ Not available |
| Bubbles | ✅ Yes | ✅ Yes | ⚠️ focus/blur: No, focusin/focusout: Yes |
| relatedTarget | ✅ (mouseenter/leave) | ❌ | ✅ (previous/next element) |
Example: Checking Event Type
js example
document.addEventListener('click', function(event) {
console.log("Is MouseEvent:", event instanceof MouseEvent);
console.log("Is KeyboardEvent:", event instanceof KeyboardEvent);
});
document.addEventListener('keydown', function(event) {
console.log("Is MouseEvent:", event instanceof MouseEvent);
console.log("Is KeyboardEvent:", event instanceof KeyboardEvent);
});
// User clicks, then presses a keyOutput
Is MouseEvent: true Is KeyboardEvent: false Is MouseEvent: false Is KeyboardEvent: true
event.target vs event.currentTarget
This is the most confusing part of JavaScript events. Let's understand it clearly:
- event.target → Actual element on which the event occurred (the source)
- event.currentTarget → Element on which the event listener is registered
Example
┌─────────────────────────────────────────────────┐
│ <div id="outer"> ← currentTarget │
│ ┌─────────────────────────────────────┐ │
│ │ <div id="inner"> │ │
│ │ ┌─────────────────────────┐ │ │
│ │ │ <button id="btn"> │ │ │
│ │ │ Click Me ← target │ │ │
│ │ │ (user clicked here) │ │ │
│ │ └─────────────────────────┘ │ │
│ │ │ │
│ └─────────────────────────────────────┘ │
│ │
│ Listener is on #outer │
│ User clicked #btn │
│ │
│ event.target = #btn (where click happened) │
│ event.currentTarget = #outer (where listener) │
└─────────────────────────────────────────────────┘
Detailed Example
js example
// HTML:
// <div id="container">
// <div id="wrapper">
// <button id="action-btn">
// <span id="btn-text">Click Me</span>
// </button>
// </div>
// </div>
const container = document.getElementById('container');
container.addEventListener('click', function(event) {
console.log("=== target vs currentTarget ===");
console.log("target (clicked):", event.target.id);
console.log("currentTarget (listener):", event.currentTarget.id);
console.log("Are they same?", event.target === event.currentTarget);
});
// User clicks on the <span> text inside buttonOutput
=== target vs currentTarget === target (clicked): btn-text currentTarget (listener): container Are they same? false
When Are They Same?
js example
const btn = document.getElementById('action-btn');
btn.addEventListener('click', function(event) {
console.log("target:", event.target.id);
console.log("currentTarget:", event.currentTarget.id);
console.log("Same?", event.target === event.currentTarget);
});
// User clicks DIRECTLY on button (not on child span)Output
target: action-btn currentTarget: action-btn Same? true
Rule: Only when the user directly clicks the element that has the listener — that's when both are the same. If a child element is clicked and the event bubbles up, target will be different.
preventDefault() and stopPropagation()
event.preventDefault()
Prevents the default browser behavior. Has no effect on event propagation — the event will continue to bubble.
Common Use Cases:
- Form submission rokna
- Link navigation rokna
- Context menu (right-click) rokna
- Checkbox toggle rokna
js example
// Example 1: Form submit rokna
const form = document.querySelector('form');
form.addEventListener('submit', function(event) {
event.preventDefault();
console.log("Form submitted? No! Default prevented.");
console.log("defaultPrevented:", event.defaultPrevented);
// Do custom validation and AJAX call here
});Output
Form submitted? No! Default prevented. defaultPrevented: true
js example
// Example 2: Link click rokna
const link = document.querySelector('a');
link.addEventListener('click', function(event) {
event.preventDefault();
console.log("Link href:", event.target.href);
console.log("Navigation blocked! Custom handling...");
});Output
Link href: https://example.com/page Navigation blocked! Custom handling...
event.stopPropagation()
Stops the event from bubbling up to parent elements. Other listeners on the current element will still run, but the event won't reach the parent.
js example
// HTML: <div id="parent"><button id="child">Click</button></div>
document.getElementById('parent').addEventListener('click', function(event) {
console.log("Parent clicked!"); // This will NOT run
});
document.getElementById('child').addEventListener('click', function(event) {
event.stopPropagation();
console.log("Child clicked! Propagation stopped.");
console.log("Event will NOT reach parent.");
});
// User clicks child buttonOutput
Child clicked! Propagation stopped. Event will NOT reach parent.
event.stopImmediatePropagation()
This is stricter than stopPropagation() — even other listeners on the same element won't run.
js example
const btn = document.querySelector('#btn');
btn.addEventListener('click', function(event) {
console.log("First listener runs");
event.stopImmediatePropagation();
});
btn.addEventListener('click', function(event) {
console.log("Second listener runs"); // This will NOT run
});
// User clicks buttonOutput
First listener runs
Comparison: preventDefault vs stopPropagation
Example
┌────────────────────────────────────────────────────────────┐
│ preventDefault() vs stopPropagation() │
├────────────────────────────────────────────────────────────┤
│ │
│ preventDefault(): │
│ ✓ Stops DEFAULT browser action (navigate, submit, etc.) │
│ ✗ Does NOT stop event from bubbling to parents │
│ ✗ Does NOT stop other listeners on same element │
│ │
│ stopPropagation(): │
│ ✓ Stops event from reaching parent elements │
│ ✗ Does NOT prevent default browser action │
│ ✗ Other listeners on SAME element still run │
│ │
│ stopImmediatePropagation(): │
│ ✓ Stops bubbling to parents │
│ ✓ Stops other listeners on SAME element │
│ ✗ Does NOT prevent default browser action │
│ │
└────────────────────────────────────────────────────────────┘
js example
// Demonstrating that preventDefault doesn't stop bubbling
// HTML: <div id="outer"><a id="link" href="/page">Go</a></div>
document.getElementById('outer').addEventListener('click', function(event) {
console.log("Parent handler still runs!"); // Runs because bubbling not stopped
});
document.getElementById('link').addEventListener('click', function(event) {
event.preventDefault(); // Only prevents navigation, NOT bubbling
console.log("Link click prevented, but event still bubbles");
});
// User clicks linkOutput
Link click prevented, but event still bubbles Parent handler still runs!
🚫 Common Mistakes
Mistake 1: Forgetting the event parameter
js example
// ❌ WRONG - event variable not defined
button.addEventListener('click', function() {
console.log(event.target); // 'event' is undefined in strict mode
});
// ✅ CORRECT - accept event as parameter
button.addEventListener('click', function(event) {
console.log(event.target);
});Output
// ❌ Uncaught ReferenceError: event is not defined // ✅ <button id="myBtn">Click</button>
Mistake 2: Using event.target instead of event.currentTarget for styling
js example
// ❌ WRONG - target might be a child element
container.addEventListener('click', function(event) {
event.target.classList.add('active'); // Might style wrong element!
});
// ✅ CORRECT - currentTarget is always the listener element
container.addEventListener('click', function(event) {
event.currentTarget.classList.add('active'); // Always styles container
});Output
// ❌ If user clicks a <span> inside container, span gets 'active' class // ✅ Container always gets 'active' class regardless of click position
Mistake 3: Assuming all events bubble
js example
// ❌ WRONG - focus doesn't bubble
parentDiv.addEventListener('focus', function(event) {
console.log("Focus detected!"); // Never fires via bubbling!
});
// ✅ CORRECT - use focusin which bubbles
parentDiv.addEventListener('focusin', function(event) {
console.log("Focus detected via focusin!");
console.log("Focused element:", event.target.tagName);
});Output
// ❌ (nothing happens when child input gets focus) // ✅ Focus detected via focusin! // ✅ Focused element: INPUT
Mistake 4: Using deprecated keyCode instead of key
js example
// ❌ WRONG - keyCode is deprecated
document.addEventListener('keydown', function(event) {
if (event.keyCode === 13) { // Magic number, hard to read
console.log("Enter pressed");
}
});
// ✅ CORRECT - use event.key
document.addEventListener('keydown', function(event) {
if (event.key === 'Enter') { // Clear and readable
console.log("Enter pressed");
}
});Output
Enter pressed Enter pressed
Mistake 5: Stopping propagation unnecessarily (breaking event delegation)
js example
// ❌ WRONG - breaks parent delegation
childButton.addEventListener('click', function(event) {
event.stopPropagation(); // Now parent can't track clicks!
doSomething();
});
// Parent's analytics tracking broke:
document.addEventListener('click', function(event) {
trackClick(event.target); // Never receives child button clicks
});
// ✅ CORRECT - handle locally without stopping propagation
childButton.addEventListener('click', function(event) {
doSomething(); // Handle your logic
// Don't stop propagation unless absolutely necessary
});Output
// ❌ Parent click tracker never fires for child button // ✅ Both child handler AND parent tracker work correctly
Mistake 6: Confusing preventDefault with return false
js example
// ❌ WRONG - return false only works in inline HTML handlers
button.addEventListener('click', function(event) {
return false; // Does NOTHING in addEventListener!
});
// ✅ CORRECT - use preventDefault()
button.addEventListener('click', function(event) {
event.preventDefault(); // Actually prevents default action
});
// Note: return false works in jQuery and inline onclick="return false"
// but NOT in addEventListenerOutput
// ❌ Default action still happens (return false ignored) // ✅ Default action prevented successfully
Mistake 7: Accessing event object asynchronously
js example
// ❌ WRONG - event object may be nullified/reused
button.addEventListener('click', function(event) {
setTimeout(function() {
console.log(event.type); // Might be null in some frameworks (React synthetic events)
}, 1000);
});
// ✅ CORRECT - save needed values immediately
button.addEventListener('click', function(event) {
const eventType = event.type;
const targetId = event.target.id;
setTimeout(function() {
console.log(eventType); // Safe - primitive value saved
console.log(targetId); // Safe - primitive value saved
}, 1000);
});Output
// ❌ May log null or throw error (framework-dependent) // ✅ click // ✅ myBtn
🎯 Key Takeaways
- Event Object is automatically provided — The browser passes it as the first parameter to the event handler. You just need to accept the parameter.
- event.target ≠ event.currentTarget —
targetis where the event actually occurred,currentTargetis where the listener is registered. This difference is critical in delegation.
- preventDefault() only stops default behavior — It stops navigation, form submit, etc., but the event continues to bubble.
- stopPropagation() only stops bubbling — The event won't reach parent elements, but the default browser action will still occur.
- Not all events bubble —
focus,blur,mouseenter,mouseleavedon't bubble. Use their bubbling alternatives (focusin,focusout).
- Use event.key, not keyCode —
keyCodeis deprecated. Always useevent.keyorevent.codein modern code.
- MouseEvent has multiple coordinate systems —
clientX/Y(viewport),pageX/Y(document),screenX/Y(monitor),offsetX/Y(element). Choose based on your needs.
- Custom Events can be created — Create your own events with custom data using
new CustomEvent('name', { detail: data }).
- event.isTrusted is important for security — User-triggered events return
true, programmatic events returnfalse. Check this for security-sensitive operations.
- stopImmediatePropagation() is the strictest — It blocks even other listeners on the same element. Use sparingly.
❓ Interview Questions
Q1: What is the Event Object in JavaScript?
Answer: The Event Object is a browser-generated object that is automatically passed as the first parameter to the event handler function. It contains complete information about the event — type, target element, position, keys pressed, timestamp, and methods to control propagation.
Q2: What is the difference between event.target and event.currentTarget?
Answer:
event.target— Actual element on which the event originated (deepest element in DOM)event.currentTarget— Element on which addEventListener was called
In event delegation, currentTarget is the parent (where the listener is attached) and target is the actual clicked child. Inside the handler, this (in non-arrow functions) also equals currentTarget.
Q3: What is the difference between preventDefault() and stopPropagation()?
Answer:
preventDefault()— Stops the browser's default action (e.g., form submit, link navigate). Has no effect on event propagation.stopPropagation()— Stops the event from bubbling up to parent elements. Has no effect on the default action.
Both are independent — they don't affect each other. You can use both simultaneously.
Q4: Which events do NOT bubble?
Answer: Common non-bubbling events:
focus/blur(usefocusin/focusoutfor bubbling)mouseenter/mouseleave(usemouseover/mouseoutfor bubbling)load/unload/error(on elements)resize/scroll(on window)
You can verify by checking the event.bubbles property.
Q5: How do you create and dispatch custom events?
Answer:
js example
// Create custom event with data
const event = new CustomEvent('orderPlaced', {
detail: { orderId: 'ORD-123', amount: 999 },
bubbles: true,
cancelable: true
});
// Listen for it
element.addEventListener('orderPlaced', function(e) {
console.log(e.detail.orderId); // 'ORD-123'
});
// Dispatch it
element.dispatchEvent(event);Output
ORD-123
Custom events always have isTrusted set to false because they are created programmatically.
Q6: What is event.composedPath()?
Answer: event.composedPath() returns an array representing the event's propagation path — including all elements from the target to the window.
Example
button.addEventListener('click', function(event) {
const path = event.composedPath();
console.log(path.map(el => el.tagName || el.constructor.name));
});
Output
["BUTTON", "DIV", "SECTION", "BODY", "HTML", "HTMLDocument", "Window"]
This also shows the path through Shadow DOM, which is useful for debugging and complex component architectures.
Q7: What is the difference between event.key and event.code?
Answer:
event.key— The logical name of the character or key ("a", "A", "Enter", "ArrowUp"). Changes according to the keyboard layout.event.code— Physical key identifier ("KeyA", "Enter", "ArrowUp"). Independent of keyboard layout.
Use case: Use event.code in games (physical position matters), use event.key for text input (actual character matters).
js example
// When pressing the 'A' position on an AZERTY keyboard:
// event.key = "q" (AZERTY layout)
// event.code = "KeyA" (physical key same hai)Output
key: q code: KeyA
📝 FAQ
Q: Can the event object be stored in a variable?
Answer: Yes, in vanilla JavaScript the event object persists. However, in React's synthetic events, they are recycled — if you need to use it in an async operation, extract the values first or call event.persist().
Q: Is it necessary to name the event parameter "event"?
Answer: No, you can name it anything — e, evt, ev, or anything else. It's just a function parameter. By convention, event or e is commonly used:
Example
button.addEventListener('click', (e) => {
console.log(e.type); // Works perfectly
});
Output
click
Q: Is the event object available in arrow functions?
Answer: Yes! Arrow functions also receive the event object as the first parameter. The only difference is that this in an arrow function is lexical (from the parent scope), whereas in a regular function this refers to currentTarget.
Example
button.addEventListener('click', (event) => {
console.log(event.type); // "click" ✅
console.log(this === window); // true (arrow function)
});
button.addEventListener('click', function(event) {
console.log(this === event.currentTarget); // true (regular function)
});
Output
click true true
Q: What other useful properties in the event object are lesser known?
Answer: Some lesser-known but powerful properties:
event.composedPath()— Full propagation path arrayevent.defaultPrevented— Check if preventDefault already calledevent.eventPhase— 1 (capture), 2 (target), 3 (bubble)event.detail— Click count (dblclick = 2) ya custom dataevent.relatedTarget— Related element in Mouse/Focus events
Q: Do multiple event listeners receive the same event object?
Answer: Yes, when a single event is triggered, all listeners receive the same event object. If one listener calls stopImmediatePropagation(), subsequent listeners won't receive this object.
Example
btn.addEventListener('click', (e) => {
e.myCustomProp = "hello";
console.log("First:", e.myCustomProp);
});
btn.addEventListener('click', (e) => {
console.log("Second:", e.myCustomProp); // Same object!
});
Output
First: hello Second: hello
Exam Focus
Revise definitions, diagrams, examples, and short-answer points for JavaScript Event Object – Complete Guide with Examples (2026).
Interview Use
Prepare one clear explanation, one practical example, and one common mistake for this JavaScript Master Course topic.
Search Terms
javascript-complete-guide, javascript master course, javascript, complete, guide, events, event, object
Related JavaScript Master Course Topics
Continue learning this concept
EventsJavaScript Click Event - addEventListener, Event Handler Complete GuideMaster the JavaScript click event with addEventListener, inline handlers, once option, double-click, right-click, and dynamic button handling. Build real interactive UIs with click events.EventsEvent Bubbling in JavaScript – Complete Guide with Examples (2026)Learn Event Bubbling in JavaScript with examples, diagrams & interview questions. What is event bubbling and how it works – complete guide 2026.EventsEvent Capturing in JavaScript – Complete Guide to Capture Phase (2026)Learn JavaScript Event Capturing (Capture Phase) with examples. What is event capturing, how it works, and when to use it. Complete guide 2026.EventsEvent Delegation in JavaScript – Complete Guide with Examples (2026)What is event delegation in JavaScript? Learn event delegation pattern with examples, best practices, and interview questions in comprehensive 2026.EventsJavaScript Form Events – Complete Guide with Examples (2026)Learn JavaScript form events like submit, reset, input, change, focus, blur & invalid with real examples, validation techniques & interview questions.EventsJavaScript Keyboard Events – Complete Guide to keydown, keyup & Key Properties (2026)Master JavaScript keyboard events: keydown, keyup, keypress, key properties, shortcuts, and real-world examples with outputs. Complete guide 2026.