Why use `event.currentTarget` instead of `this` inside an arrow function in JavaScript event handlers?

.this in Arrow Functions: Event Handler Considerations

In working with JavaScript, developers may encounter challenges when utilizing arrow functions within event handlers. By default, when an arrow function is employed in an event callback, obtaining this to represent the element to which the event handler is bound becomes problematic. This occurs because the value of this in an arrow function is determined during its definition rather than at the time of its usage.

To address this issue, it's advisable to use event.currentTarget instead of this when using arrow functions within callbacks. event.currentTarget consistently designates the DOM element associated with the event listener currently being executed, ensuring that the desired element can be referenced accurately.

Understanding .currentTarget

The usage of event.currentTarget stems from the concepts of event capturing and bubbling. Understanding these mechanisms is crucial for effectively navigating event handling in JavaScript.

• event.currentTarget represents the element on which the event listener is defined, known as the event target.
• event.target signifies the element that initially triggered the event.

Event Bubbling and Capturing

During event propagation, browser behavior can be influenced by the use of event capturing and bubbling. Both involve the flow of events within the DOM tree, but in different directions.

• Event Bubbling: Events move up the DOM tree, from the target element to the document's root.
• Event Capturing: Events move down the DOM tree, from the document's root to the target element.

In the context of event handling, the distinction between currentTarget and target becomes apparent. When an event is delegated to a parent element and triggers an event on a child element, currentTarget remains the parent element, while target refers to the child element. This distinction becomes especially important when dealing with elements dynamically created or inserted into the DOM after event listeners have been attached.

Example: Differentiating .currentTarget and .target

Consider the snippet below:

<code class="javascript">var parent = document.getElementById('parent');
parent.addEventListener('click', function(e) {
  document.getElementById('msg').innerHTML = "this: " + this.id + "<br>" +
    "currentTarget: " + e.currentTarget.id + "<br>" +
    "target: " + e.target.id;
});

$('#parent').on('click', function(e) {
  $('#jQmsg').html("*jQuery<br>" + "this: " + $(this).prop('id')
                   + "<br>currenTarget: " + $(e.currentTarget).prop('id')
                   + "<br>target: " + $(e.target).prop('id'));
});

$('#parent').on('click', e => $('#arrmsg').html('*Arrow function <br> currentTarget: ' + e.currentTarget.id));</code>

When this snippet is executed, clicking on the child or grand-child elements triggers event bubbling. The resulting behavior is demonstrated in the following table:

Event Listener	this	currentTarget	target
function	child	parent	child/grand-child
jQuery	parent	parent	child/grand-child
Arrow function	undefined	parent	child/grand-child

As evident from the table, this returns undefined when an arrow function is employed, highlighting the need to use event.currentTarget instead. By using event.currentTarget, the correct element can always be referenced within arrow functions, ensuring effective event handling in JavaScript.

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