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JavaScript DOM Scripting By Example Adding and Removing Names RSVP Checkbox

Umar Ghouse
Umar Ghouse
14,607 Points

Wouldn't event.target also refer to other elements?

In this video, we add an event listener to the ul element - but this listener isn't for the ul, it's for the checkbox that is a child of the ul (code given below)

HTML:

<ul id="invitedList">
  <li>
    <label>
      <input type="checkbox">
    </label>
  </li>
</ul>

JavaScript:

ul.addEventListener('change', (e) => {
  const checkbox = event.target;
  const checked = checkbox.checked;

  const listItem = checkbox.parentNode.parentNode;

  if (checked) {
    listItem.className = "responded";
  } else {
    listItem.className = "";
  }
});

We have an event.target expression assigned to the variable "checkbox". Let's assume, for the sake of this question, that the event listener was looking for a "click" event, instead of a "change" event. In that case, would it have triggered this block of code if we clicked the label? or the li? (i.e. anything besides the checkbox).

If so, why not target the checkboxes directly, instead of relying on bubbling through the ul? Wouldn't that reduce the margin for error?

Thanks in advance!

1 Answer

Steven Parker
Steven Parker
231,248 Points

You're quite right about how a delegated handler works.

If this handler had been for a click event, then additional could would be needed inside to qualify the target. But no additional filtering is needed for the change event, because the checkboxes are the only elements inside the ul (including itself) that can generate that specific event.

There are two advantages of using the delegated handler: first, it only requires one to handle every checkbox. And second, it will be effective on those added later, when an individual handler would have to be added to any new checkbox element.

Umar Ghouse
Umar Ghouse
14,607 Points

Oh OK. I get it now. Thank you for the brilliant explanation!

wouldn't it be better to write const checkbox = e.target; or does event has some semantic meaning?

Steven Parker
Steven Parker
231,248 Points

The variable "event" is established by the browser, but I agree it's better to use the argument passed in.