Reacting to Events
A JavaScript can be executed when an event occurs, like when a user clicks on an HTML element.
To execute code when a user clicks on an element, add JavaScript code to an HTML event attribute:
onclick=JavaScript
Examples of HTML events:
- When a user clicks the mouse
- When a web page has loaded
- When an image has been loaded
- When the mouse moves over an element
- When an input field is changed
- When an HTML form is submitted
- When a user strokes a key
In this example, the content of the <h1> element is changed when a user clicks on it:
Example
<!DOCTYPE html>
<html>
<body>
<h1 onclick="this.innerHTML = 'Ooops!'">Click on this text!</h1>
</body>
</html>
In this example, a function is called from the event handler:
Example
<!DOCTYPE html>
<html>
<body>
<h1 onclick="changeText(this)">Click on this text!</h1>
<script>
function changeText(id) {
id.innerHTML = "Ooops!";
}
</script>
</body>
</html>
HTML Event Attributes
To assign events to HTML elements you can use event attributes.
Example
<button onclick="displayDate()">Try it</button>
In the example above, a function named displayDate will be executed when the button is clicked.
Assign Events Using the HTML DOM
The HTML DOM allows you to assign events to HTML elements using JavaScript:
Example
<script>
document.getElementById("myBtn").onclick = displayDate;
</script>
In the example above, a function named displayDate is assigned to an HTML element with the id="myBtn".
The function will be executed when the button is clicked.
The onload and onunload Events
The onload and onunload events are triggered when the user enters or leaves the page.
The onload event can be used to check the visitor's browser type and browser version, and load the proper version of the web page based on the information.
The onload and onunload events can be used to deal with cookies.
Example
<body onload="checkCookies()">
The oninput Event
The oninput event is often to some action while the user input data.
Below is an example of how to use the oninput to change the content of an input field.
Example
<input type="text" id="fname"
oninput="upperCase()">
The onchange Event
The onchange event is often used in combination with validation of input fields.
Below is an example of how to use the onchange. The upperCase() function will be called when a user changes the content of an input field.
Example
<input type="text" id="fname"
onchange="upperCase()">
The onmouseover and onmouseout Events
The onmouseover and onmouseout events can be used to trigger a function when the user mouses over, or out of, an HTML element:
The onmousedown, onmouseup and onclick Events
The onmousedown, onmouseup, and onclick events are all parts of a mouse-click. First when a mouse-button is clicked, the onmousedown event is triggered, then, when the mouse-button is released, the onmouseup event is triggered, finally, when the mouse-click is completed, the onclick event is triggered.
More Examples
Change an image when a user holds down the mouse button.
Display an alert box when the page has finished loading.
Change the background-color of an input field when it gets focus.
Change the color of an element when the cursor moves over it.
HTML DOM Event Object Reference
For a list of all HTML DOM events, look at our complete HTML DOM Event Object Reference.
