The .on() method attaches event handlers to the currently selected set of elements in the jQuery object. As of jQuery 1.7, the .on() method provides all functionality required for attaching event handlers. For equivalents to older jQuery event methods, see .bind(), .delegate(), and .live(). To remove events bound with .on(), see .off(). To attach an event that runs only once and then removes itself, see .one()

Event names and namespaces

Any event names can be used for the events argument. jQuery will pass through the browser's standard JavaScript event types, calling the handler function when the browser generates events due to user actions such as click. In addition, the .trigger() method can trigger both standard browser event names and custom event names to call attached handlers.

An event name can be qualified by event namespaces that simplify removing or triggering the event. For example, "click.myPlugin.simple" defines both the myPlugin and simple namespaces for this particular click event. A click event handler attached via that string could be removed with .off("click.myPlugin") or .off("click.simple") without disturbing other click handlers attached to the elements. Namespaces are similar to CSS classes in that they are not hierarchical; only one name needs to match. Namespaces beginning with an underscore are reserved for jQuery's use.

In the second form of .on(), the events-map argument is a JavaScript Object, or "map". The keys are strings in the same form as the events argument with space-separated event type names and optional namespaces. The value for each key is a function (or false value) that is used as the handler instead of the final argument to the method. In other respects, the two forms are identical in their behavior as described below.

Direct and delegated events

The majority of browser events bubble, or propagate, from the deepest, innermost element (the event target) in the document where they occur all the way up to the body and the document element. In Internet Explorer 8 and lower, a few events such as change and submit do not natively bubble but jQuery patches these to bubble and create consistent cross-browser behavior. The focus and blur events are specified by the W3C to not bubble, but jQuery defines cross-browser focusin and focusout events that do bubble.

If selector is omitted or is null, the event handler is referred to as direct or directly-bound. The handler is called every time an event occurs on the selected elements, whether it occurs directly on the element or bubbles from a descendant (inner) element.

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.

Event handlers are bound only to the currently selected elements; they must exist on the page at the time your code makes the call to .on(). To ensure the elements are present and can be selected, perform event binding inside a document ready handler for elements that are in the HTML markup on the page. If new HTML is being injected into the page, select the elements and attach event handlers after the new HTML is placed into the page. Or, use delegated events to attach an event handler, as described next.

Delegated events have the advantage that they can process events from descendant elements that are added to the document at a later time. By picking an element that is guaranteed to be present at the time the delegated event handler is attached, you can use delegated events to avoid the need to frequently attach and remove event handlers. This element could be the container element of a view in a Model-View-Controller design, for example, or document if the event handler wants to monitor all bubbling events in the document. The document element is available in the head of the document before loading any other HTML, so it is safe to attach events there without waiting for the document to be ready.

In addition to their ability to handle events on descendant elements not yet created, another advantage of delegated events is their potential for much lower overhead when many elements must be monitored. On a data table with 1,000 rows in its tbody, this example attaches a handler to 1,000 elements:

$("#dataTable tbody tr").on("click", function(event){
	alert($(this).text());
});

A delegated-events approach attaches an event handler to only one element, the tbody, and the event only needs to bubble up one level (from the clicked tr to tbody):

$("#dataTable tbody").on("click", "tr", function(event){
	alert($(this).text());
});

The event handler and its environment

The handler argument is a function (or the value false, see below), and is required unless the events-map form is used. You can provide an anonymous handler function at the point of the .on() call, as the examples have done above, or declare a named function and pass its name:

function notify() { alert("clicked"); }
$("button").on("click", notify);

When the browser triggers an event or other JavaScript calls jQuery's .trigger() method, jQuery passes the handler an event object it can use to analyze and change the status of the event. This object is a normalized subset of data provided by the browser; the browser's unmodified native event object is available in event.originalEvent. For example, event.type contains the event name (e.g., "resize") and event.target indicates the deepest (innermost) element where the event occurred.

By default, most events bubble up from the original event target to the document element. At each element along the way, jQuery calls any matching event handlers that have been attached. A handler can prevent the event from bubbling further up the document tree (and thus prevent handlers on those elements from running) by calling event.stopPropagation(). Any other handlers attached on the current element will run however. To prevent that, call event.stopImmediatePropagation(). (Event handlers bound to an element are called in the same order that they were bound.)

Similarly, a handler can call event.preventDefault() to cancel any default action that the browser may have for this event; for example, the default action on a click event is to follow the link. Not all browser events have default actions, and not all default actions can be canceled. See the W3C Events Specification for details.

Returning false from an event handler will automatically call event.stopPropagation() and event.preventDefault(). A false value can also be passed for the handler as a shorthand for function(){ return false; }. So, $("a.disabled").on("click", false); attaches an event handler to all links with class "disabled" that prevents them from being followed when they are clicked and also stops the event from bubbling.

When jQuery calls a handler, the this keyword is a reference to the element where the event is being delivered; for directly bound events this is the element where the event was attached and for delegated events this is an element matching selector. (Note that this may not be equal to event.target if the event has bubbled from a descendant element.) To create a jQuery object from the element so that it can be used with jQuery methods, use $(this).

Passing data to the handler

If a data argument is provided to .on() and is not null or undefined, it is passed to the handler in the event.data property each time an event is triggered. The data argument can be any type, but if a string is used the selector must either be provided or explicitly passed as null so that the data is not mistaken for a selector. Best practice is to use an object (map) so that multiple values can be passed as properties.

The same event handler can be bound to an element multiple times. This is especially useful when the event.data feature is being used, or when other unique data resides in a closure around the event handler function. For example:

function greet(event) { alert("Hello "+event.data.name); }
$("button").on("click", { name: "Karl" }, greet);
$("button").on("click", { name: "Addy" }, greet);

The above code will generate two different alerts when the button is clicked.

As an alternative or in addition to the data argument provided to the .on() method, you can also pass data to an event handler using a second argument to .trigger() or .triggerHandler().

Event performance

In most cases, an event such as click occurs infrequently and performance is not a significant concern. However, high frequency events such as mousemove or scroll can fire dozens of times per second, and in those cases it becomes more important to use events judiciously. Performance can be increased by reducing the amount of work done in the handler itself, caching information needed by the handler rather than recalculating it, or by rate-limiting the number of actual page updates using setTimeout.

Attaching many delegated event handlers near the top of the document tree can degrade performance. Each time the event occurs, jQuery must compare all selectors of all attached events of that type to every element in the path from the event target up to the top of the document. For best performance, attach delegated events at a document location as close as possible to the target elements. Avoid excessive use of document or document.body for delegated events on large documents.

jQuery can process simple selectors of the form tag#id.class very quickly when they are used to filter delegated events. So, "#myForm", "a.external", and "button" are all fast selectors. Delegated events that use more complex selectors, particularly hierarchical ones, can be several times slower--although they are still fast enough for most applications. Hierarchical selectors can often be avoided simply by attaching the handler to a more appropriate point in the document. For example, instead of $("body").on("click", "#commentForm .addNew", addComment) use $("#commentForm").on("click", ".addNew", addComment).

Additional notes

There are shorthand methods for some events such as .click() that can be used to attach or trigger event handlers. For a complete list of shorthand methods, see the events category.

Although not recommended, the pseudo-event-name "hover" can be used as a shorthand for "mouseenter mouseleave". Do not confuse it with the .hover() method, which accepts two functions. There is only one handler function attached by the pseudo-event-name "hover"; the handler should examine event.type to determine whether the event is mouseenter or mouseleave.

jQuery's event system requires that a DOM element allow attaching data via a property on the element, so that events can be tracked and delivered. The object, embed, and applet elements cannot attach data, and therefore cannot have jQuery events bound to them.

The error event on the window object uses nonstandard arguments and return value conventions, so it is not supported by jQuery. Instead, assign a handler function directly to the window.onerror property.

In Internet Explorer 8 and lower, the paste and reset events do not bubble and are not supported for use with delegation. They can be used when the event handler is directly attached to the element generating the event.