Chapter 10 — Event Bubbling, Capturing & Delegation

📖 Definitions

Event bubbling — an event triggered on a child propagates upward through its ancestors.
Event capturing — the opposite direction; from root down to the target.
Event delegation — attaching one listener on a parent and relying on bubbling to handle events from many descendants.

🔍 Explanation

When you click an element, the event travels through three phases:

Capture phase — root → target (top-down).
Target phase — at the actual clicked element.
Bubble phase — target → root (bottom-up).

By default, addEventListener listens in the bubble phase. Pass true (or { capture: true }) to listen in the capture phase.

💻 Code Example — Bubbling

<div id="outer">
  <div id="inner">
    <button id="btn">Click me</button>
  </div>
</div>

document.getElementById("outer").addEventListener("click", () => console.log("outer"));
document.getElementById("inner").addEventListener("click", () => console.log("inner"));
document.getElementById("btn").addEventListener("click",   () => console.log("btn"));

// Clicking the button logs: btn → inner → outer  (bubbling)

💻 Code Example — Capturing

document.getElementById("outer").addEventListener("click",
  () => console.log("outer"), true);  // capture
document.getElementById("inner").addEventListener("click",
  () => console.log("inner"), true);
document.getElementById("btn").addEventListener("click",
  () => console.log("btn"));

// Output on click: outer → inner → btn  (capture goes top-down)

💻 Code Example — Stopping Propagation

document.getElementById("btn").addEventListener("click", (e) => {
  console.log("btn");
  e.stopPropagation();          // prevents outer/inner from firing
});

// Now clicking the button logs only: btn

e.stopImmediatePropagation() also prevents other listeners on the same element.

💻 Code Example — Event Delegation

<ul id="list">
  <li data-id="1">One</li>
  <li data-id="2">Two</li>
  <li data-id="3">Three</li>
</ul>

// One listener handles all <li> clicks — even ones added later
document.getElementById("list").addEventListener("click", (e) => {
  const li = e.target.closest("li");
  if (!li) return;
  console.log("Clicked id:", li.dataset.id);
});

// Dynamically add — still works without re-binding
document.getElementById("list").insertAdjacentHTML(
  "beforeend", `<li data-id="4">Four</li>`
);

Benefits of Delegation

1 listener instead of N → less memory.
Works for elements added after the listener is attached.
Easier to clean up.

💻 Code Example — `preventDefault`

document.querySelector("a").addEventListener("click", (e) => {
  e.preventDefault();           // stop navigation
  console.log("Link clicked but blocked");
});

document.querySelector("form").addEventListener("submit", (e) => {
  e.preventDefault();           // stop full-page form submit
  // ...submit via fetch
});

💻 Code Example — Once / Passive Options

button.addEventListener("click", handler, { once: true }); // auto-removes after first call

window.addEventListener("scroll", onScroll, { passive: true });
// `passive: true` tells the browser the handler will not call preventDefault,
// allowing it to scroll without waiting → smoother performance.

🌍 Real-World Impact

In React, synthetic events are delegated to the root container automatically.
Tables, lists, and large grids should use event delegation for performance.
passive listeners are crucial for scroll/touch performance on mobile.

🎯 Likely Interview Questions

Event bubbling vs capturing.
What is event delegation and why use it?
Difference between stopPropagation and preventDefault?
- stopPropagation — stops the event from traveling further along the chain.
- preventDefault — cancels the browser's default action (navigation, form submit, etc.).
How does React handle events under the hood? — Synthetic event system, delegated to the root.

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