“event.preventDefault()”将阻止的“浏览器默认行为”有哪些示例？

Question

I quite understand that event.preventDefault() will prevent any default behavior triggered by an event on browser, but this definition is too broad for me , for example what are those event default behavior on browser? since it's quite common to find developers use event.preventDefault() but I still don't understand what kind of behavior they're trying to prevent.

Answer 1

If you click on a link, such as to http://example.com , your browser's default behavior will be to take you to http://example.com .

If you preventDefault in a click handler, the browser will no longer change your window location.

 document.querySelector('.prevent-default').addEventListener('click', function (e) { e.preventDefault(); }, false);

 <a href="http://example.com">Normal Example</a> <a class="prevent-default" href="http://example.com">Prevented Example</a>

Other examples include:

• submit has associated form submission
• mousedown has associated text selection
• keydown has associated input
• touchstart may have associated scrolling/zooming behaviors.

Answer 2

one quick example is click event. Let's say you have a tag like

<a href="/my-sub-page.html" class='ajax-link'>

default behavior will be when you click this link it will take you to the /my-sub-page.html page. but if you want to avoid page refresh and instead you want to use ajax. then you will use

$('.ajax-link').on('click', function(event){
  event.preventDefault();
  $.ajax({
     url : $(this).attr('href'),
     success : function(response){

        $('.my-content').html(response);
      }
  })
});

or another example can be form submits. Similar to above, you canprevent default behavior of form submission, and use ajax and get result from post request without loading the page.

Answer 3

There are many default browser actions:

• mousedown – starts the selection (move the mouse to select).
• click on <a href=""> - open the page.
• click on <input type="checkbox"> – checks/unchecks the input.
• submit – clicking an <input type="submit"> or hitting Enter inside a form field causes this event to happen, and the browser submits the form after it.
• keydown – pressing a key may lead to adding a character into a field, or other actions.
• contextmenu – the event happens on a right-click, the action is to show the browser context menu. etc.

Answer 4

The event.preventDefault() method stops the default action of an element from happening.

For example:

• Prevent a submit button from submitting a form
• Prevent a link from following the URL

Use the event.isDefaultPrevented() method to check whether the preventDefault() method was called for the event.