eventpreventDefault

Event.preventDefault is a method of the Event interface in web browsers. It cancels the default action that the browser would normally perform for a given event, provided the event is cancelable. The call signals the user agent to refrain from its standard behavior, allowing developers to handle the action with custom logic instead.

Common use cases include forms and links. In a form submit handler, preventDefault stops the browser from sending the form data to the server, enabling client-side validation or an AJAX submission. For anchor elements, it prevents navigation to the href, useful in single-page applications where navigation is handled via routing. Preventing default actions can also apply to certain keyboard or drag-and-drop interactions.

Important notes: preventDefault only affects the default action if the event’s cancelable flag is true. It does not stop event propagation; to halt propagation, other methods such as stopPropagation or stopImmediatePropagation are needed. Additionally, in modern browsers, event listeners can be registered as passive to improve performance; if a listener is marked as passive, calling preventDefault may have no effect.

Compatibility: Event.preventDefault is widely supported in modern browsers. In very old environments, alternative techniques such as setting event.returnValue = false may be used.

Best practices: call preventDefault within an event handler to implement custom behavior instead of the browser’s default action. Use it judiciously and avoid suppressing default behavior when it would degrade usability or accessibility.