HTML DOM - Events

Events are actions that can be detected by JavaScript.

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Events

Every element on a web page has certain events which can trigger JavaScript functions. 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 elements.

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 box in an HTML form
• Submitting an HTML form
• A keystroke

Note: Events are normally used in combination with functions, and the function will not be executed before the event occurs!

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onload and onUnload

The onload and onUnload events are triggered when the user enters or leaves a page.

The onload event is often used to check the visitor's browser type and version, and load the proper version of the web page based on that information.

Both the onload and onUnload events are also often used to deal with cookies that should be set when a user enters or leaves a page. For example, you could have a popup asking for the user's name upon his first arrival to your page. The name is then stored in a cookie. Next time the visitor arrives at your page, you could have another popup saying something like: "Welcome John Doe!".

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onFocus, onBlur and onChange

The onFocus, onBlur and onChange events are often used in combination with validation of form fields.

Below is an example of how to use an onChange event. The checkEmail() function will be called whenever the user changes the content of the e-mail field:

E-mail: <input type="text" id="email" onchange="checkEmail()" />