Introduction to browser events

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

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:

1. Only HTML:

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

2. 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 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 is that 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 the special methods addEventListener and removeEventListener which aren’t bound by such constraint.

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 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 Actions par défaut du navigateur.

To remove the handler, use removeEventListener:

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

elem.addEventListener( "click" , () => alert('Thanks!'));
// ....
elem.removeEventListener( "click", () => alert('Thanks!'));

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

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

Multiple calls to addEventListener allow it 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.

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

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

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:

<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 as we move on to the details of different events.

<input type="button" onclick="alert(event.type)" value="Event type">

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 objects of a custom class, like this:

<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:

1. HTML attribute: onclick="...".
2. DOM property: elem.onclick = function.
3. 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.