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What is event bubbling? How do you select and modify an element's text content? What is event delegation and why is it a performance best practice? What's the difference between event.target and event.currentTarget? How do you create and dispatch a custom DOM event? What's the difference between the capturing and bubbling phases of event propagation? How do you properly clean up event listeners to avoid memory leaks? What is the difference between passive and non-passive event listeners for scroll performance? How does MutationObserver let you react to DOM changes made by other code?
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JavaScript › DOM & Events

What does event.preventDefault() do?

Beginner

Answer

It cancels the browser's default action for that event — like following a link's href, submitting a form, or checking a checkbox — while still allowing the event to bubble unless you also stop propagation.
form.addEventListener('submit', (e) => {
  e.preventDefault();
  validateAndSubmitViaFetch();
});
Real-world example Intercepting a form submission to send data via fetch() instead of a full page reload.

Common follow-ups: Does preventDefault() also stop event bubbling? (No — that's stopPropagation().)

Networking: Fetch XHR WebSockets & CORS
All 10 DOM & Events questions