Document, Events, Interfaces

An event is a signal that something has happened. All DOM nodes generate such signals (but events are not limited to DOM).

Here's a list of the most useful DOM events, just to take a look at:

Mouse events:

click -- when the mouse clicks on an element (touchscreen devices generate it on a tap).
contextmenu -- when the mouse right-clicks on an element.
mouseover / mouseout -- when the mouse cursor comes over / leaves an element.
mousedown / mouseup -- when the mouse button is pressed / released over an element.
mousemove -- when the mouse is moved.

Keyboard events:

keydown and keyup -- when a keyboard key is pressed and released.

Form element events:

submit -- when the visitor submits a <form>.
focus -- when the visitor focuses on an element, e.g. on an <input>.

Document events:

DOMContentLoaded -- when the HTML is loaded and processed, DOM is fully built.

CSS events:

transitionend -- when a CSS-animation finishes.

There are many other events. We'll get into more details of particular events in next chapters.

Event handlers

To react on events we can assign a handler -- a function that runs in case of an event.

Handlers are a way to run JavaScript code in case of user actions.

There are several ways to assign a handler. Let's see them, starting from the simplest one.

HTML-attribute

A handler can be set in HTML with an attribute named on<event>.

For instance, to assign a click handler for an input, we can use onclick, like here:

<input value="Click me" *!*onclick="alert('Click!')"*/!* type="button">

On mouse click, the code inside onclick runs.

Please note that inside onclick we use single quotes, because the attribute itself is in double quotes. If we forget that the code is inside the attribute and use double quotes inside, like this: onclick="alert("Click!")", then it won't work right.

An HTML-attribute is not a convenient place to write a lot of code, so we'd better create a JavaScript function and call it there.

Here a click runs the function countRabbits():

<script>
  function countRabbits() {
    for(let i=1; i<=3; i++) {
      alert("Rabbit number " + i);
    }
  }
</script>

<input type="button" *!*onclick="countRabbits()"*/!* value="Count rabbits!">

As we know, HTML attribute names are not case-sensitive, so ONCLICK works as well as onClick and onCLICK... But usually attributes are lowercased: onclick.

DOM property

We can assign a handler using a DOM property on<event>.

For instance, elem.onclick:

<input id="elem" type="button" value="Click me">
<script>
*!*
  elem.onclick = function() {
    alert('Thank you');
  };
*/!*
</script>

If the handler is assigned using an HTML-attribute then the browser reads it, creates a new function from the attribute content and writes it to the DOM property.

So this way is actually the same as the previous one.

These two code pieces work the same:

Only HTML:

<input type="button" *!*onclick="alert('Click!')"*/!* value="Button">

HTML + JS:

<input type="button" id="button" value="Button">
<script>
*!*
  button.onclick = function() {
    alert('Click!');
  };
*/!*
</script>

In the first example, the HTML attribute is used to initialize the button.onclick, while in the second example -- the script, that's all the difference.

As there's only one onclick property, we can't assign more than one event handler.

In the example below adding a handler with JavaScript overwrites the existing handler:

<input type="button" id="elem" onclick="alert('Before')" value="Click me">
<script>
*!*
  elem.onclick = function() { // overwrites the existing handler
    alert('After'); // only this will be shown
  };
*/!*
</script>

To remove a handler -- assign elem.onclick = null.

Accessing the element: this

The value of this inside a handler is the element. The one which has the handler on it.

In the code below button shows its contents using this.innerHTML:

<button onclick="alert(this.innerHTML)">Click me</button>

Possible mistakes

If you're starting to work with events -- please note some subtleties.

We can set an existing function as a handler:

function sayThanks() {
  alert('Thanks!');
}

elem.onclick = sayThanks;

But be careful: the function should be assigned as sayThanks, not sayThanks().

// right
button.onclick = sayThanks;

// wrong
button.onclick = sayThanks();

If we add parentheses, then sayThanks() becomes is a function call. So the last line actually takes the result of the function execution, that is undefined (as the function returns nothing), and assigns it to onclick. That doesn't work.

...On the other hand, in the markup we do need the parentheses:

<input type="button" id="button" onclick="sayThanks()">

The difference is easy to explain. When the browser reads the attribute, it creates a handler function with body from the attribute content.

So the markup generates this property:

button.onclick = function() {
*!*
  sayThanks(); // <-- the attribute content goes here
*/!*
};

Don't use setAttribute for handlers.

Such a call won't work:

// a click on <body> will generate errors,
// because attributes are always strings, function becomes a string
document.body.setAttribute('onclick', function() { alert(1) });

DOM-property case matters.

Assign a handler to elem.onclick, not elem.ONCLICK, because DOM properties are case-sensitive.

addEventListener

The fundamental problem of the aforementioned ways to assign handlers -- we can't assign multiple handlers to one event.

Let's say, one part of our code wants to highlight a button on click, and another one wants to show a message on the same click.

We'd like to assign two event handlers for that. But a new DOM property will overwrite the existing one:

input.onclick = function() { alert(1); }
// ...
input.onclick = function() { alert(2); } // replaces the previous handler

Developers of web standards understood that long ago and suggested an alternative way of managing handlers using special methods addEventListener and removeEventListener. They are free of such a problem.

The syntax to add a handler:

element.addEventListener(event, handler, [options]);

event
: Event name, e.g. "click".

handler
: The handler function.

options
: An additional optional object with properties:
- once: if true, then the listener is automatically removed after it triggers.
- capture: the phase where to handle the event, to be covered later in the chapter info:bubbling-and-capturing. For historical reasons, options can also be false/true, that's the same as {capture: false/true}.
- passive: if true, then the handler will not call preventDefault(), we'll explain that later in info:default-browser-action.

To remove the handler, use removeEventListener:

element.removeEventListener(event, handler, [options]);

To remove a handler we should pass exactly the same function as was assigned.

This doesn't work:

```js no-beautify
elem.addEventListener( "click" , () => alert('Thanks!'));
// ....
elem.removeEventListener( "click", () => alert('Thanks!'));
```

The handler won't be removed, because `removeEventListener` gets another function -- with the same code, but that doesn't matter, as it's a different function object.

Here's the right way:

```js
function handler() {
  alert( 'Thanks!' );
}

input.addEventListener("click", handler);
// ....
input.removeEventListener("click", handler);
```

Please note -- if we don't store the function in a variable, then we can't remove it. There's no way to "read back" handlers assigned by `addEventListener`.

Multiple calls to addEventListener allow to add multiple handlers, like this:

<input id="elem" type="button" value="Click me"/>

<script>
  function handler1() {
    alert('Thanks!');
  };

  function handler2() {
    alert('Thanks again!');
  }

*!*
  elem.onclick = () => alert("Hello");
  elem.addEventListener("click", handler1); // Thanks!
  elem.addEventListener("click", handler2); // Thanks again!
*/!*
</script>

As we can see in the example above, we can set handlers both using a DOM-property and addEventListener. But generally we use only one of these ways.

````warn header="For some events, handlers only work with addEventListener"
There exist events that can't be assigned via a DOM-property. Only with addEventListener.

For instance, the DOMContentLoaded event, that triggers when the document is loaded and DOM is built.

// will never run
document.onDOMContentLoaded = function() {
  alert("DOM built");
};

// this way it works
document.addEventListener("DOMContentLoaded", function() {
  alert("DOM built");
});

So addEventListener is more universal. Although, such events are an exception rather than the rule.


## Event object

To properly handle an event we'd want to know more about what's happened. Not just a "click" or a "keydown", but what were the pointer coordinates? Which key was pressed? And so on.

When an event happens, the browser creates an *event object*, puts details into it and passes it as an argument to the handler.

Here's an example of getting pointer coordinates from the event object:

```html run
<input type="button" value="Click me" id="elem">

<script>
  elem.onclick = function(*!*event*/!*) {
    // show event type, element and coordinates of the click
    alert(event.type + " at " + event.currentTarget);
    alert("Coordinates: " + event.clientX + ":" + event.clientY);
  };
</script>
```

Some properties of `event` object:

`event.type`
: Event type, here it's `"click"`.

`event.currentTarget`
: Element that handled the event. That's exactly the same as `this`, unless the handler is an arrow function, or its `this` is bound to something else, then we can get the element from  `event.currentTarget`.

`event.clientX / event.clientY`
: Window-relative coordinates of the cursor, for pointer events.

There are more properties. Many of them depend on the event type: keyboard events have one set of properties, pointer events - another one, we'll study them later when we come to different events in details.

````smart header="The event object is also available in HTML handlers"
If we assign a handler in HTML, we can also use the `event` object, like this:

```html autorun height=60
<input type="button" onclick="*!*alert(event.type)*/!*" value="Event type">
```

That's possible because when the browser reads the attribute, it creates a handler like this:  `function(event) { alert(event.type) }`. That is: its first argument is called `"event"`, and the body is taken from the attribute.

Object handlers: handleEvent

We can assign not just a function, but an object as an event handler using addEventListener. When an event occurs, its handleEvent method is called.

For instance:

<button id="elem">Click me</button>

<script>
  let obj = {
    handleEvent(event) {
      alert(event.type + " at " + event.currentTarget);
    }
  };

  elem.addEventListener('click', obj);
</script>

As we can see, when addEventListener receives an object as the handler, it calls obj.handleEvent(event) in case of an event.

We could also use a class for that:

<button id="elem">Click me</button>

<script>
  class Menu {
    handleEvent(event) {
      switch(event.type) {
        case 'mousedown':
          elem.innerHTML = "Mouse button pressed";
          break;
        case 'mouseup':
          elem.innerHTML += "...and released.";
          break;
      }
    }
  }

*!*
  let menu = new Menu();
  elem.addEventListener('mousedown', menu);
  elem.addEventListener('mouseup', menu);
*/!*
</script>

Here the same object handles both events. Please note that we need to explicitly setup the events to listen using addEventListener. The menu object only gets mousedown and mouseup here, not any other types of events.

The method handleEvent does not have to do all the job by itself. It can call other event-specific methods instead, like this:

<button id="elem">Click me</button>

<script>
  class Menu {
    handleEvent(event) {
      // mousedown -> onMousedown
      let method = 'on' + event.type[0].toUpperCase() + event.type.slice(1);
      this[method](event);
    }

    onMousedown() {
      elem.innerHTML = "Mouse button pressed";
    }

    onMouseup() {
      elem.innerHTML += "...and released.";
    }
  }

  let menu = new Menu();
  elem.addEventListener('mousedown', menu);
  elem.addEventListener('mouseup', menu);
</script>

Now event handlers are clearly separated, that may be easier to support.

Summary

There are 3 ways to assign event handlers:

HTML attribute: onclick="...".
DOM property: elem.onclick = function.
Methods: elem.addEventListener(event, handler[, phase]) to add, removeEventListener to remove.

HTML attributes are used sparingly, because JavaScript in the middle of an HTML tag looks a little bit odd and alien. Also can't write lots of code in there.

DOM properties are ok to use, but we can't assign more than one handler of the particular event. In many cases that limitation is not pressing.

The last way is the most flexible, but it is also the longest to write. There are few events that only work with it, for instance transitionend and DOMContentLoaded (to be covered). Also addEventListener supports objects as event handlers. In that case the method handleEvent is called in case of the event.

No matter how you assign the handler -- it gets an event object as the first argument. That object contains the details about what's happened.

We'll learn more about events in general and about different types of events in the next chapters.

Introduction Browser Events