javascript - Why does preventDefault on checkbox click event returns true for the checked attribute?

Question

I am just curious and need some explanation on the following situation.

Let's say i have an input element of type checkbox with an eventlistener attached to it, listening for the click event. I prevent the default behavior of the checkbox and log the checked state of the checkbox, which will always return true.

The visual representation of the checkbox is telling me that it is not checked. So i would assume that the checked state would return false. I am sure this must be something silly and i am definitively misunderstanding something here. The funny thing is, i am logging the event itself as well. Inside of the target property the checked property is set to false, just as i would have expected.

From my understanding, prevent default will cancel the event without stopping propagation, so what exactly is happening here?

It would be great if someone could enlighten me on this one. Here is the example.

var checkbox = document.getElementsByTagName('input')[0],
    output = document.getElementById('output');

checkbox.addEventListener('click', function(evt) {
  evt.preventDefault();
  output.innerHTML = "Checkbox checked attribute is " + this.checked;
  console.log(this.checked, evt.target.checked);
  console.log(evt);
}, false);

<input type="checkbox" id="my-checkbox" />
<label for="my-checkbox">Checkbox with preventDefault().</label>
<div id="output"></div>

Answer 1

Actually, the result of the checked value in a click handler is implementation dependent.

As I tested in several browsers, Chrome/Firefox/Safari/Opera will always return true in this case, but IE/Edge's behavior gets a bit weird if you keep clicking on that checkbox element.

And I found this paragraph in the spec, which might be a explanation of this inconsistency:

Note: During the handling of a click event on an input element with a type attribute that has the value "radio" or "checkbox", some implementations may change the value of this property before the event is being dispatched in the document. If the default action of the event is canceled, the value of the property may be changed back to its original value. This means that the value of this property during the handling of click events is implementation dependent.

But when I removed the preventDefault statement, the results in IE/Edge are consistent with the other browsers, which confuses me.

So I don't think it is IE/Edge's intended behavior… Therefore I filed a bug on Microsoft Connect.

---

After all, if we presume Chrome's behavior to be standard-compliant, then the following might be a suitable explanation:

According to the HTML Spec, an input[type=checkbox] element's checkedness is restored at the canceled activation process, which comes after the event handlers according to the Activation section. So during the execution of event handlers, the element's checkedness hasn't been restored yet.

(The spec doesn't explicitly state that canceled activation steps must come after all the event handlers; but it's easy to infer because otherwise there's no way to determine the event's canceled state)