DHTML - HTML Events
HTML events can trigger actions in a browser.
HTML Events
Every element on an HTML page has events which can trigger a JavaScript.
For example, we can use the onClick event of a button element to indicate that a
function will run when a user clicks on the button. We define the events in the
HTML tags.
Examples of events:
A mouse click
A web page or an image loading
Mousing over a hot spot on the web page
Selecting an input field in an HTML form
Submitting an HTML form
A keystroke
In the following example, the content of the h1 element will change when a user
clicks on it:
Example
<html>
<body>
<h1 onclick="this.innerHTML='Ooops!'">Click on this text</h1>
</body>
</html>
1|Page Disha Verma
Event Listener
Triggering of an Event
Handling the Event through Event
Handler
Handling event using even handler
<html>
<head>
<script>
function myFunction()
{
var x = document.getElementById("fname");
x.value = x.value.toUpperCase();
}
</script>
</head>
<body>
Enter your name: <input type="text" id="fname" onchange="myFunction()">
<p>When you leave the input field, a function is triggered which transforms the input text to upper
case.</p>
</body>
</html>
Linking Event Listener to Event Handler
addEventListener Method
The addEventListener() method attaches an event handler to the specified element.
The addEventListener() method attaches an event handler to an element without overwriting
existing event handlers.
You can add many event handlers to one element.
You can add many event handlers of the same type to one element, i.e two "click" events.
2|Page Disha Verma
Syntax
element.addEventListener(event, function, useCapture);
The first parameter is the type of the event (like "click" or "mousedown").
The second parameter is the function we want to call when the event occurs.
The third parameter is a boolean value specifying whether to use event bubbling or event
capturing. This parameter is optional.
Note that you don't use the "on" prefix for the event; use "click" instead of
"onclick".
Adding Event Listener to handler
<html>
<body>
<p>This example uses the addEventListener() method to add many events on the same button.</p>
<button id="myBtn">Try it</button>
<p id="demo"></p>
<script>
var x = document.getElementById("myBtn");
x.addEventListener("mouseover", myFunction);
x.addEventListener("click", mySecondFunction);
x.addEventListener("mouseout", myThirdFunction);
x.addEventListener("click", myFinalFunction);
function myFunction() {
document.getElementById("demo").innerHTML += "Moused over!<br>";
}
function mySecondFunction() {
document.getElementById("demo").innerHTML += "Clicked!<br>";
}
function myThirdFunction() {
document.getElementById("demo").innerHTML += "Moused out!<br>";
}
function myFinalFunction() {
document.getElementById("demo").innerHTML += "Clicked Again!<br>";
}
</script>
</body>
</html>
Event Bubbling or Event Capturing?
There are two ways of event propagation in the HTML DOM, bubbling and capturing.
3|Page Disha Verma
Event propagation is a way of defining the element order when an event occurs. If you have a <p>
element inside a <div> element, and the user clicks on the <p> element, which element's "click" event
should be handled first?
In bubbling the inner most element's event is handled first and then the outer: the <p> element's click
event is handled first, then the <div> element's click event.In capturing the outer most element's event
is handled first and then the inner: the <div> element's click event will be handled first, then the <p>
element's click event.
With the addEventListener() method you can specify the propagation type by using the "useCapture"
parameter:
addEventListener(event, function, useCapture);
The default value is false, which will use the bubbling propagation, when the value is set to true, the
event uses the capturing propagation.
4|Page Disha Verma
