moey moey - 2 months ago 14
Javascript Question

Direct vs. Delegated - jQuery .on()

I am trying to understand this particular difference between the direct and delegated event handlers using the jQuery .on() method. Specifically, the last sentence in this paragraph:


When a
selector
is provided, the event handler is referred to as delegated. The handler is not called when the event occurs directly on the bound element, but only for descendants (inner elements) that match the selector. jQuery bubbles the event from the event target up to the element where the handler is attached (i.e., innermost to outermost element) and runs the handler for any elements along that path matching the selector.


What does it mean by "runs the handler for any elements"? I made a test page to experiment with the concept. But both following constructs lead to the same behavior:

$("div#target span.green").on("click", function() {
alert($(this).attr("class") + " is clicked");
});


or,

$("div#target").on("click", "span.green", function() {
alert($(this).attr("class") + " is clicked");
});


Maybe someone could refer to a different example to clarify this point? Thanks.

Answer

Case 1 (direct):

$("div#target span.green").on("click", function() {...});

== Hey! I want every span.green inside div#target to listen up: when you get clicked on, do X.

Case 2 (delegated):

$("div#target").on("click", "span.green", function() {...});

== Hey, div#target! When any of your child elements which are "span.green" get clicked, do X with them.

In other words...

In case 1, each of those spans has been individually given instructions. If new spans get created, they won't have heard the instruction and won't respond to clicks. Each span is directly responsible for its own events.

In case 2, only the container has been given the instruction; it is responsible for noticing clicks on behalf of its child elements. The work of catching events has been delegated. This also means that the instruction will be carried out for child elements that are created in future.

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