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=JavaScriptExamples 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
<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
<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
Assign an onclick event to a button element:
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
Assign an onclick event to a button element:
document.getElementById("myBtn").onclick = displayDate;
</script>
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
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
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
onmousedown and onmouseup
Change an image when a user holds down the mouse button.
onload
Display an alert box when the page has finished loading.
onfocus
Change the background-color of an input field when it gets focus.
Mouse Events
Change the color of an element when the cursor moves over it.
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