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DOM Events

Events are actions or occurrences that happen in the browser, which you can respond to with JavaScript. Understanding events is crucial for creating interactive web applications.

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Adding Event Listeners

In order to add an event listener to an element, you can use the EventTarget.addEventListener() method. Yet, in some cases, adding a listener for every single element can be a bit of a performance hit.

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Basic Event Listener

const button = document.getElementById('my-button');

button.addEventListener('click', (event) => {
  console.log('Button clicked!');
  console.log('Event:', event);
});

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Event Delegation

In these cases, you can use event delegation to add a single event listener to a parent element and then check if the event target matches the target you’re looking for. 
const on = (el, evt, fn, opts = {}) => {
  const delegatorFn = e =>
    e.target.matches(opts.target) && fn.call(e.target, e);
  el.addEventListener(
    evt,
    opts.target ? delegatorFn : fn,
    opts.options || false
  );
  if (opts.target) return delegatorFn;
};

const fn = () => console.log('!');

on(document.body, 'click', fn);
// logs '!' upon clicking the `body` element

on(document.body, 'click', fn, { target: 'p' });
// logs '!' upon clicking a `p` element child of the `body` element

on(document.body, 'click', fn, { options: true });
// logs '!' upon clicking on the `body` element,
//   but uses capturing instead of bubbling

on(document.body, 'click', fn, { target: 'p', options: { once: true} });
// logs '!' upon clicking a `p` element child of the `body` element,
//   but only once
Event delegation is more efficient when you have many elements with the same event listener, as it requires only one listener instead of many.

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Removing Event Listeners

Removing an event listener from an element is as easy as adding one. You can use the EventTarget.removeEventListener() method to remove an event listener from an element. 
const off = (el, evt, fn, opts = false) =>
  el.removeEventListener(evt, fn, opts);

const fn = () => console.log('!');

document.body.addEventListener('click', fn);
off(document.body, 'click', fn);
// no longer logs '!' upon clicking on the page

const delegatorFn =
  on(document.body, 'click', fn, { target: 'p' });
off(document.body, 'click', delegatorFn);
// no longer logs '!' upon clicking a `p` element child of the `body` element

const delegatorFnCapturing =
  on(document.body, 'click', fn, { options: true });
off(document.body, 'click', delegatorFnCapturing, { options: true });
// no longer logs '!' upon clicking on the page
//   (capturing instead of bubbling example)
You must pass the same function reference to removeEventListener that you passed to addEventListener. Arrow functions and anonymous functions won’t work for removal.

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Common Events

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Mouse Events

const element = document.getElementById('my-element');

element.addEventListener('click', (e) => {
  console.log('Clicked at:', e.clientX, e.clientY);
});

element.addEventListener('dblclick', (e) => {
  console.log('Double clicked');
});

element.addEventListener('mouseenter', (e) => {
  console.log('Mouse entered');
});

element.addEventListener('mouseleave', (e) => {
  console.log('Mouse left');
});

element.addEventListener('mousemove', (e) => {
  console.log('Mouse position:', e.clientX, e.clientY);
});

element.addEventListener('contextmenu', (e) => {
  e.preventDefault(); // Prevent default context menu
  console.log('Right clicked');
});

​
Keyboard Events

document.addEventListener('keydown', (e) => {
  console.log('Key pressed:', e.key);
  console.log('Key code:', e.code);
  
  // Check for modifier keys
  if (e.ctrlKey && e.key === 's') {
    e.preventDefault();
    console.log('Ctrl+S pressed');
  }
});

document.addEventListener('keyup', (e) => {
  console.log('Key released:', e.key);
});

input.addEventListener('keypress', (e) => {
  console.log('Character entered:', e.key);
});

​
Form Events

const form = document.getElementById('my-form');
const input = document.getElementById('my-input');

form.addEventListener('submit', (e) => {
  e.preventDefault(); // Prevent form submission
  console.log('Form submitted');
});

input.addEventListener('input', (e) => {
  console.log('Value changed:', e.target.value);
});

input.addEventListener('change', (e) => {
  console.log('Input changed:', e.target.value);
});

input.addEventListener('focus', (e) => {
  console.log('Input focused');
});

input.addEventListener('blur', (e) => {
  console.log('Input blurred');
});

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Window Events

window.addEventListener('load', () => {
  console.log('Page fully loaded');
});

window.addEventListener('resize', () => {
  console.log('Window resized:', window.innerWidth, window.innerHeight);
});

window.addEventListener('scroll', () => {
  console.log('Scrolled to:', window.scrollY);
});

window.addEventListener('beforeunload', (e) => {
  e.preventDefault();
  e.returnValue = ''; // Required for showing confirmation dialog
});

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Triggering Custom Events

JavaScript’s EventTarget.dispatchEvent() method allows you to trigger an event programmatically. This method accepts an Event object as its only argument, which can be created using the CustomEvent constructor.

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Basic Custom Event

const triggerEvent = (el, eventType, detail) =>
  el.dispatchEvent(new CustomEvent(eventType, { detail }));

const myElement = document.getElementById('my-element');
myElement.addEventListener('click', e => console.log(e.detail));

triggerEvent(myElement, 'click');
// The event listener will log: null

triggerEvent(myElement, 'click', { username: 'bob' });
// The event listener will log: { username: 'bob' }
The CustomEvent constructor allows you to pass custom data to the event listener through the detail property.

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Custom Event with Options

const customEvent = new CustomEvent('userLogin', {
  detail: { username: 'alice', timestamp: Date.now() },
  bubbles: true,
  cancelable: true
});

document.dispatchEvent(customEvent);

// Listen anywhere in the document
document.addEventListener('userLogin', (e) => {
  console.log('User logged in:', e.detail.username);
});

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Event Object Properties

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Common Properties

element.addEventListener('click', (event) => {
  console.log('Type:', event.type);                    // 'click'
  console.log('Target:', event.target);                // Element that triggered event
  console.log('Current target:', event.currentTarget); // Element with listener
  console.log('Timestamp:', event.timeStamp);          // When event occurred
  console.log('Bubbles:', event.bubbles);              // Whether event bubbles
  console.log('Cancelable:', event.cancelable);        // Whether event can be canceled
});

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Mouse Event Properties

element.addEventListener('click', (event) => {
  console.log('Client X/Y:', event.clientX, event.clientY); // Relative to viewport
  console.log('Page X/Y:', event.pageX, event.pageY);       // Relative to document
  console.log('Screen X/Y:', event.screenX, event.screenY); // Relative to screen
  console.log('Button:', event.button);                     // Which mouse button (0=left, 1=middle, 2=right)
  console.log('Ctrl key:', event.ctrlKey);                  // Whether Ctrl was pressed
  console.log('Shift key:', event.shiftKey);                // Whether Shift was pressed
  console.log('Alt key:', event.altKey);                    // Whether Alt was pressed
});

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Keyboard Event Properties

document.addEventListener('keydown', (event) => {
  console.log('Key:', event.key);           // 'a', 'Enter', 'ArrowUp', etc.
  console.log('Code:', event.code);         // 'KeyA', 'Enter', 'ArrowUp', etc.
  console.log('Key code:', event.keyCode);  // Deprecated, use 'code' instead
  console.log('Ctrl:', event.ctrlKey);      // Whether Ctrl was pressed
  console.log('Shift:', event.shiftKey);    // Whether Shift was pressed
  console.log('Alt:', event.altKey);        // Whether Alt was pressed
  console.log('Repeat:', event.repeat);     // Whether key is being held down
});

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Event Methods

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preventDefault()

Prevents the default action of the event: 
link.addEventListener('click', (e) => {
  e.preventDefault();
  console.log('Link click prevented');
});

form.addEventListener('submit', (e) => {
  e.preventDefault();
  console.log('Form submission prevented');
});

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stopPropagation()

Stops the event from bubbling up the DOM tree: 
child.addEventListener('click', (e) => {
  e.stopPropagation();
  console.log('Child clicked, event won\'t bubble to parent');
});

parent.addEventListener('click', () => {
  console.log('This won\'t be called when child is clicked');
});

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stopImmediatePropagation()

Stops event bubbling and prevents other listeners on the same element: 
element.addEventListener('click', (e) => {
  e.stopImmediatePropagation();
  console.log('First listener');
});

element.addEventListener('click', () => {
  console.log('This won\'t be called');
});

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Event Options

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Capture Phase

// Event fires during capture phase (top to bottom)
element.addEventListener('click', handler, true);

// Or with options object
element.addEventListener('click', handler, { capture: true });

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Once

// Event listener is removed after first invocation
element.addEventListener('click', handler, { once: true });

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Passive

// Tells browser the listener won't call preventDefault()
// Improves scroll performance
element.addEventListener('scroll', handler, { passive: true });

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Practical Examples

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Debounced Scroll Handler

let scrollTimeout;
window.addEventListener('scroll', () => {
  clearTimeout(scrollTimeout);
  scrollTimeout = setTimeout(() => {
    console.log('Scroll ended at:', window.scrollY);
  }, 150);
});

​
Click Outside to Close

const modal = document.getElementById('modal');
const modalContent = modal.querySelector('.modal-content');

modal.addEventListener('click', (e) => {
  if (!modalContent.contains(e.target)) {
    modal.style.display = 'none';
  }
});

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Keyboard Shortcuts

const shortcuts = {
  'ctrl+s': () => console.log('Save'),
  'ctrl+o': () => console.log('Open'),
  'ctrl+p': () => console.log('Print'),
};

document.addEventListener('keydown', (e) => {
  const key = [
    e.ctrlKey && 'ctrl',
    e.shiftKey && 'shift',
    e.altKey && 'alt',
    e.key.toLowerCase()
  ].filter(Boolean).join('+');

  const handler = shortcuts[key];
  if (handler) {
    e.preventDefault();
    handler();
  }
});

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Best Practices

When elements are added/removed dynamically, event delegation ensures events work without re-attaching listeners.
Always remove event listeners when elements are destroyed or components unmount to prevent memory leaks.
Mark scroll and touch event listeners as passive to improve performance, unless you need preventDefault().
Avoid onclick="..." in HTML. Use addEventListener in JavaScript for better separation of concerns.
For events that fire frequently (scroll, resize, mousemove), use throttling or debouncing to limit execution.

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