[ITEM 4] QUESTION: Как предотвратить действие по умолчанию?

Many web page elements have built-in standard behavior set by the browser by default. Examples of such behavior include submitting a form when clicking a submit button, following a link when clicking an a tag, opening a context menu on a right-click, or scrolling the page when pressing arrow keys. However, in modern web development, there is often a need to intercept these actions and process them using scripts, for example, sending form data via AJAX without reloading the page.

To cancel the browser's standard behavior, the event.preventDefault() method is used, which is called on the event object inside the handler function. As soon as this method is triggered, the browser stops executing the default action associated with the current event. It is important to note that this method cancels precisely the standard behavior, but does not stop the propagation of the event through the DOM tree, so bubbling or capturing will continue unless stopped separately.

In old code and legacy JavaScript standards, returning the boolean value false at the end of the handler function, for example via return false, was often used for the same purposes. This approach simultaneously canceled the default action and stopped event bubbling; however, in modern code, it is recommended to abandon it in favor of explicitly calling event.preventDefault() to improve code readability and predictability.

To check whether the standard behavior has been canceled for a specific event, developers can use the special event.defaultPrevented property. It returns the boolean value true if the preventDefault() method has already been called by any of the previous handlers. This is useful in large, complex applications where several independent modules may listen to the same event and make decisions based on the current state of the interface.