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Learn how to create interactivity with DOM Events using JavaScript to make some cool websites!

DOM Events with JavaScript

When you use a mouse device on a website, _mouse events_ fire. You've seen the `click` and `wheel` events, but, there are even more mouse events to explore!

The `mousedown` event is fired when the user presses a mouse button down. This is different from a `click` event because `mousedown` doesn't need the mouse button to be released to fire.

<img alt="Mouse Down Event Image" src="https://content.codecademy.com/courses/javascript-dom-events/mousedown.png">

The `mouseup` event is fired when the user releases the mouse button. This is different from the `click` and `mousedown` events because `mouseup` doesn't depend on the mouse button being pressed down to fire.

<img alt="Mouse Up Event Image" src="https://content.codecademy.com/courses/javascript-dom-events/mouseup.png">

The `mouseover` event is fired when the mouse enters the content of an element.

<img alt="Mouse Over Event Image" src="https://content.codecademy.com/courses/javascript-dom-events/mouseover.png">

The `mouseout` event is fired when the mouse leaves an element.

<img src="https://content.codecademy.com/courses/javascript-dom-events/mouseout.gif" alt="Mouse Out Event Image">

Mouse Events

JavaScript stores events as [`Event` objects](https://developer.mozilla.org/en-US/docs/Web/API/Event) with their related data and functionalities as properties and methods. When an event is triggered, the event object can be passed as an argument to the event handler function.

```js
function eventHandlerFunction(event){
   console.log(event.timeStamp);
}

eventTarget.addEventListener('click', eventHandlerFunction);
```

In the example above, when the `'click'` event is triggered on the `eventTarget`, the `eventHandlerFunction` receives `event`, the Event object, which has information related to the `'click'` event. Then, we log the time it took for the event to be triggered since the document was loaded by accessing the `.timeStamp` property of the `event` object.

There are pre-determined properties associated with event objects. You can call these properties to see information about the event, for example:
- the [`.target` property](https://developer.mozilla.org/en-US/docs/Web/API/Event/target) to reference the element that the event is registered to.
- the [`.type` property](https://developer.mozilla.org/en-US/docs/Web/API/Event/type) to access the name of the event.
- the [`.timeStamp` property](https://developer.mozilla.org/en-US/docs/Web/API/Event/timeStamp) to access the number of milliseconds that passed since the document loaded and the event was triggered.

Event Object Properties

The `.removeEventListener()` method is used to reverse the `.addEventListener()` method. This method stops the event target from "listening" for an event to fire when it no longer needs to. `.removeEventListener()` also takes two arguments:
1. The event type as a string
2. The event handler function

Check out the syntax of a `.removeEventListener()` method with a click event:

```js
eventTarget.removeEventListener('click', eventHandlerFunction);
```

Because there can be multiple event handler functions associated with a particular event, `.removeEventListener()` needs both the exact event type name and the name of the event handler you want to remove. If `.addEventListener()` was provided an anonymous function, then that event listener cannot be removed.

Removing Event Handlers

After a specific event fires on a specific element in the [document object model](https://www.codecademy.com/paths/web-development/tracks/build-interactive-websites/modules/web-dev-interactive-websites/lessons/intro-dom/exercises/what-is-the-dom) (or DOM), an _event handler_ function can be created to run as a response. In this lesson, we will learn about event handler functions that modify and update DOM elements after an event fires.

Let's compare the firing of events to something more familiar: a dog trained to eat when they hear the sound of a bell! (This is known as the [Pavlovian conditioning](https://en.wikipedia.org/wiki/Classical_conditioning).)

As you can see in the diagram, the ringing of the bell is analogous to a JavaScript event _firing_. The dog is trained to anticipate the ringing of the bell and this action is analogous to creating an _event listener_. After the dog hears the bell, it'll come over and eat its food! This response is like an _event handler function_ that executes code which, in a website, could change an element’s color, text, and much more!"

Most events in the browser take place without being noticed &mdash; the events are undetected because there is no event handler associated with the event to execute an action. Event handlers are crucial for creating interactive websites with JavaScript.

The dog responds to the detected event by coming over to eat their food! This response is like an event handler function that executes code which could change a DOM element's color, text, and much more!

"Firing" Events

Beyond the `click` event, there are all types of DOM events that can fire in a browser! It's important to know *most* events in the DOM take place without being noticed because there are no event handlers connected to them.

It's also important to know some registered events don't depend on user interactions to fire. For instance, the `load` event fires after website files completely load in the browser. 

Browsers can fire many other events without a user &mdash; you can check out a list of events on the <a href="https://developer.mozilla.org/en-US/docs/Web/Events" target="_blank" rel="noopener noreferrer">MDN Events Reference</a> page.

Many events need user interaction with the DOM to fire. One user interaction event you've become familiar with is the `click` event. A `click` event fires when the user presses and releases a mouse button on an element in the DOM.

<img alt="Click Event Image" src="https://content.codecademy.com/courses/javascript-dom-events/click-event.png">

In the rest of this lesson, you'll explore more user interaction event types like the mouse and keyboard events. To explore more event types, check out the <a href="https://developer.mozilla.org/en-US/docs/Web/Events" target="_blank" rel="noopener noreferrer">MDN Events Reference</a>.

Event Types

You're doing great! Now it's time to dive into using event handler functions to create interactivity.

Using the `.addEventListener()` method, we can have a DOM element listen for a specific event and execute a block of code when the event is detected. The DOM element that listens for an event is called the *event target* and the block of code that runs when the event happens is called the *event handler*.

Let's take a look at the code below:
```js
let eventTarget = document.getElementById('targetElement');

eventTarget.addEventListener('click', function() {
  // this block of code will run when click event happens on eventTarget element
});
```

Let's break this down! 
* We selected our event target from the DOM using `document.getElementById('targetElement')`. 
* We used the `.addEventListener()` method on the `eventTarget` DOM element.
* The `.addEventListener()` method takes two arguments: an event name in _string_ format and an event handler function. We will learn about different values we can use as event names in a later lesson.
* In this example, we used the `'click'` event, which fires when the user clicks on `eventTarget`.
* The code block in the event handler function will execute when the `'click'` event is detected.

Instead of using an anonymous function as the event handler, it's best practice to create a named event handler function. Your code will remain organized and reusable this way, even if your code gets complex. Check out the syntax:
```js
function eventHandlerFunction() {
  // this block of code will run when click event happens
}

eventTarget.addEventListener('click', eventHandlerFunction);
```
The named function `eventHandlerFunction` is passed as the second argument of the `.addEventListener()` method instead of defining an anonymous function within the method!

Event Handler Registration

Congrats, you completed the lesson! Now you've learned about JavaScript events and you can leverage these events on the DOM to create interactivity with event handlers.

 Let's review what you've learned:
- You can register events to DOM elements using the `addEventListener()` method.
- The `addEventListener()` method takes two arguments: an event type and an event handler function.
- When an event is triggered on the event target, the registered event handler function executes.
- Event handler functions can also be registered as values of `onevent` properties of their event target.
- Event object properties like `.target`, `.type`, and `.timeStamp` are used to provide information about the event.
- The `addEventListener()` method can be used to add multiple event handler functions to a single event.
- The `removeEventListener()` method stops specific event handlers from "listening" for specific events firing.

Review

When you refresh your email inbox, double tap on a post, or scroll through a newsfeed &mdash; something cool happens in your browser. These actions are known as _events_! 

Events on the web are user interactions and browser manipulations that you can program to trigger functionality. Some other examples of events are:
- A mouse clicking on a button
- Webpage files loading in the browser
- A user swiping right on an image

When a user does any of the above actions, they're causing the event to be _fired_ or be _triggered_. As in, "a click event fired when the button was clicked". Being able to respond to these events makes your website interactive and therefore _dynamic_.

What is an Event?

We looked at registering event handlers using the `.addEventListener()` method, but there is also another way!

Event Handlers can also be registered by setting an `.onevent` property on a DOM element (event target). The pattern for registering a specific event is to append an element with `.on` followed by the lowercased event type name. For instance, if we want to register a click event with this pattern, we would write: 

```js
eventTarget.onclick = eventHandlerFunction;
```

Here, we give the DOM element `eventTarget` the [`.onclick` property](https://developer.mozilla.org/en-US/docs/Web/API/GlobalEventHandlers/onclick) and set its value as the event handler function `eventHandlerFunction`. 

It's important to know that this `.onevent` property and `.addEventListener()` will both register event listeners. With `.onevent`, it allows for one event handler function to be attached to the event target. However, with the `.addEventListener()` method , we can add multiple event handler functions. In the later exercises, we'll be using the `.addEventListener()` syntax, but we wanted to also introduce the `.onevent` syntax because both are widely used.

Adding Event Handlers

Other popular types of events are keyboard events! _keyboard events_ are triggered by user interaction with keyboard keys in the browser.

The `keydown` event is fired while a user presses a key down.
<img alt="Key Down Event Image" src="https://content.codecademy.com/courses/javascript-dom-events/keydown.png">

The `keyup` event is fired while a user releases a key.
<img alt="Key Up Event Image" src="https://content.codecademy.com/courses/javascript-dom-events/keyup.png">

The `keypress` event is fired when a user presses a key down and releases it. This is different from using `keydown` and `keyup` events together, because those are two complete events and `keypress` is one complete event.
<img alt="Key P ress Event Image" src="https://content.codecademy.com/courses/javascript-dom-events/keypress.png">

Keyboard events have unique properties assigned to their event objects like the `.key` property that stores the values of the key pressed by the user. You can program the event handler function to react to a specific key, or react to any interaction with the keyboard.