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Get familiar with adding mouse event handlers.

Mouse Events

Have you noticed in our Sole Shoes website that when you mouse over one photo, all of the images zoom. That's because `.product-photo` is a class on all the product photos.

One way to solve this issue is to give each HTML element a unique class and to write three `mouseenter` and `mouseleave` functions. That, however, would result in a lot of repetitive code. Luckily there's a better way.

The solution is in the callback function and selector we're using when we add a new class. Instead of selecting `$('.product-photo')` in each callback function, we need to pass event information into the function and call the `currentTarget` attribute:

```javascript
$('.example-class').on('mouseenter', event => {
  $(event.currentTarget).addClass('photo-active');
});
```
The points below explain some of the features of the `event.currentTarget` selector in the example above.
- When a user triggers the `mouseenter` event listener above, the `.on()` method generates an `event` object that we pass into the callback function.
- Inside the callback function, we select `event.currentTarget`. The `currentTarget` attribute refers to only the `.example-class` element that the learner has moused over.  
- Since `$(event.currentTarget)` refers to the element that an event is detected on, you will only use it inside a callback function.

In our Sole Shoes website, we will use `$(event.currentTarget)` to target the one image a user mouses over.






currentTarget

The jQuery library provides a collection of methods that serve one of two purposes.  
- To listen for an event &mdash; an event (e.g. clicking a button) is something the user does to trigger an action.
- To add a visual effect &mdash; a visual effect (e.g. a drop-down menu appearing when a user clicks a button) is something that changes the appearance of the web page. Events are often responsible for triggering a visual effect.

In this lesson, you will learn how to link a user event to a visual effect using _event handlers_. 

There are two parts to an event handler: an _event listener_ and a _callback function_.
- An _event listener_ is a method that listens for a specified event to occur, like a click event.
- A _callback function_ is a function that executes when something triggers the event listener.

Both the event listener and callback function make up an event handler.

In code, this looks like:

```javascript
$('.example-class').on('click', () => {
	// Execute code here when .example-class is clicked
});
```
Let's consider the example above step-by-step:
- `$('.example-class')` selects all HTML elements with a class of `example-class`.
- The `on('click')` method is the event listener. It checks if the user has clicked an `.example-class` HTML element.
- The second argument to `.on()` is a callback function. When a `'click'` occurs on an `example-class` element, this function executes.



Introduction to Mouse Events

Another popular jQuery event listener is `mouseenter`. The `mouseenter` event triggers a callback function when a user enters the area that a targeted element occupies.

To listen for a mouse enter event, we can use the `mouseenter` event listener. Take a look at this code and compare it with the click function you learned in the previous exercises:

```javascript
$('.example-class').on('mouseenter',  () => {
	// Execute code here when mouse enters .example-class
});
```

- As before, `.on()` is called on a jQuery object that selects `.example-class` elements. 
- The first argument for `.on()` is the `'mouseenter'` event handler. The code above will trigger the callback function when the mouse enters an `.example-class` element's area. 

On 'mouseenter'

In this lesson, you learned a few of the most common mouse events in the jQuery library.

- Event handlers are comprised of an event listener and a callback function. An event listener specifies the type of event that will be detected. The callback function executes when the event happens. Everything together is the event handler.
- An event listener is set up using the `.on()` method.
- The events listened for included: `'click'`, `'mouseenter'`, and `'mouseleave'`.
- In addition to event handlers, you learned how to use `event.currentTarget` to refer to the individual element that an event occurred on. 

Well done! At this point, you understand the purpose of an event, and how to link it to a callback function. Use <a href="http://jqapi.com/" target="_blank">jqapi.com</a> to learn about some other events that jQuery has to offer. Select **Events** in the left navigation bar to browse these other events.



Review: Event Handlers

jQuery also allows us to _chain_ multiple methods. Instead of re-declaring the HTML element you're selecting, you can append one event to another. Let's see how we can use chaining to add hover functionality to `.example-class` elements. 

```javascript
$('.example-class').on('mouseenter', () => {
  $('.example-class-one').show();
}).on('mouseleave', () => {
  $('.example-class-one').hide();
});
```
In the example above, we chain a mouse enter event to a mouse leave event. Both of the event handlers target `.example-class` elements. 

When a user's mouse enters an `.example-class` element's area we show `.example-class-one` elements. When a user's mouse leaves an `.example-class` element's area,  we hide `.example-class-one` elements.

Chaining Events

One issue with the behavior we added to our Sole Shoes website in the last exercise is that the menu remains in the DOM after the mouse leaves the menu area.

The `mouseleave` event listener can detect when a user's mouse leaves the area that an element occupies. The syntax looks like:

```javascript
$('.example-class').on('mouseleave', function() {
	// Execute code here when mouse leaves .example-class
});
```
In the example code above, a user will trigger the callback function when their mouse leaves the `.example-class` area.

