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<script>{ "title": "Handling Events", "level": "beginner", "source": "http://jqfundamentals.com/legacy", "attribution": [ "jQuery Fundamentals" ] }</script>

jQuery provides a method .on() to respond to any event on the selected elements. This is called an event binding. Although .on() isn't the only method provided for event binding, it is a best practice to use this for jQuery 1.7+. To learn more, read more about the evolution of event binding in jQuery.

The .on() method provides several useful features:

Examples

Simple event binding

// When any <p> tag is clicked, we expect to see '<p> was clicked' in the console.
$( "p" ).on( "click", function() {
	console.log( "<p> was clicked" );
});

Many events, but only one event handler

Suppose you want to trigger the same event whenever the mouse hovers over or leaves the selected elements. The best practice for this is to use "mouseenter mouseleave". Note the difference between this and the next example.

// When a user focuses on or changes any input element,
// we expect a console message bind to multiple events
$( "div" ).on( "mouseenter mouseleave", function() {
	console.log( "mouse hovered over or left a div" );
});

Many events and handlers

Suppose that instead you want different event handlers for when the mouse enters and leaves an element. This is more common than the previous example. For example, if you want to show and hide a tooltip on hover, you would use this.

.on() accepts an object containing multiple events and handlers.

$( "div" ).on({
	mouseenter: function() {
		console.log( "hovered over a div" );
	},
	mouseleave: function() {
		console.log( "mouse left a div" );
	},
	click: function() {
		console.log( "clicked on a div" );
	}
});

The event object

Handling events can be tricky. It's often helpful to use the extra information contained in the event object passed to the event handler for more control. To become familiar with the event object, use this code to inspect it in your browser console after you click on a <div> in the page. For a breakdown of the event object, see Inside the Event Handling Function.

$( "div" ).on( "click", function( event ) {
	console.log( "event object:" );
	console.dir( event );
});

Passing data to the event handler

You can pass your own data to the event object.

$( "p" ).on( "click", {
	foo: "bar"
}, function( event ) {
	console.log( "event data: " + event.data.foo + " (should be 'bar')" );
});

Binding events to elements that don't exist yet

This is called event delegation. Here's an example just for completeness, but see the page on Event Delegation for a full explanation.

$( "ul" ).on( "click", "li", function() {
	console.log( "Something in a <ul> was clicked, and we detected that it was an <li> element." );
});

Connecting Events to Run Only Once

Sometimes you need a particular handler to run only once — after that, you may want no handler to run, or you may want a different handler to run. jQuery provides the .one() method for this purpose.

// Switching handlers using the `.one()` method
$( "p" ).one( "click", function() {
	console.log( "You just clicked this for the first time!" );
	$( this ).click(function() {
		console.log( "You have clicked this before!" );
	});
});

The .one() method is especially useful if you need to do some complicated setup the first time an element is clicked, but not subsequent times.

.one() accepts the same arguments as .on() which means it supports multiple events to one or multiple handlers, passing custom data and event delegation.

Disconnecting Events

Although all the fun of jQuery occurs in the .on() method, its counterpart is just as important if you want to be a responsible developer. .off() cleans up that event binding when you don't need it anymore. Complex user interfaces with lots of event bindings can bog down browser performance, so using the .off() method diligently is a best practice to ensure that you only have the event bindings that you need, when you need them.

// Unbinding all click handlers on a selection
$( "p" ).off( "click" );
// Unbinding a particular click handler, using a reference to the function
var foo = function() {
	console.log( "foo" );
};

var bar = function() {
	console.log( "bar" );
};

$( "p" ).on( "click", foo ).on( "click", bar );

// foo will stay bound to the click event
$( "p" ).off( "click", bar );

Namespacing Events

For complex applications and for plugins you share with others, it can be useful to namespace your events so you don't unintentionally disconnect events that you didn't or couldn't know about. For details, see Event Namespacing.

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