JavaScript – Events

JavaScript could also be called event-driven. Hence it can respond to events. However, let’s first understand what events are.

  1. What are Events
  2. The onclick() Event
  3. The onsubmit() Event
  4. The mouseover() and mouseout() event
  5. List of HTML5 Events

 

1. What are Events

JavaScript code interacts with the HTML via events that are generated when a user or the browser manipulates the page.

For example, when the browser loads a page, this is an event. When a user clicks a button in a html page, this is also an event. Other events includes: pressing a key, resizing a window, closing a window etc.

Programmers can use these events as a trigger to execute a JavaScript code as a response. So when a user clicks on a button, the programmer specifies the codes to run. Generally this is a function as we learned in Tutorial 10.

Events in JavaScript are part of  the Document Object Model (DOM) Level 3. Therefore, every HTML element contains a collection of events which can be used to trigger a JavaScript code.

Let’s start with an onclick() event

 

2. onclick() Event

The onclick() event happens when the mouse is clicked. This is the most frequently used event. You can actually put some validation code to run before the actual action is performed.

The example below demonstrates the onclick event

<html>
   <head>  
   <title>Events Demo</title>   
      <script type = "text/javascript">
         <!--
            function greetings() {
               alert("Good Morning")
            }
         //-->
      </script>      
   </head>
   
   <body>
      <p>Click the following button and see result</p>      
      <form>
         <input type = "button" onclick = "greetings()" value = "Good Morning" />
      </form>      
   </body>
</html>

 

3. onsubmit() Event

This is is an event that occurs when you try to submit a form. Therefore, you can put some validation codes before the actual actions is performed.

For example, the code in the example below show the validation of a form when a submit button is clicked. Remember that if you click the submit button in a form, the data entered in the form is submitted to the server. Won’t it be great to validate the content of the form before you submit.

The example below illustrates this.

<html>
   <head>  
   <title>Events Demo</title>   
      <script type = "text/javascript">
         <!--
            function validation() {
               all validation goes here
               .........
               return either true or false
            }
         //-->
      </script>     
   </head>
   
   <body>
      <p>Click the following button and see result</p>      
      <form method = "POST" action = "#" onsubmit = "return validate()">
         .......
         <input type = "submit" value = "Submit" />
      </form>        
   </body>
</html>

 

4. onmouseover() and onmouseout() events

These are events used to create a hover effect when the mouse moves over a control on a html page. For example, you may want to brighten an images color when the mouse moves over it.

The onmouseover() event triggers when the moves over an element while onmouseout() triggers when the mouse moves out of the that element.

The example below illustrates this

<html>
   <head>  
   <title>moouseover and mouseout Demo</title>   
      <script type = "text/javascript">
         <!--
            function over() {
               document.write ("Mouse Over");
            }            
            function out() {
               document.write ("Mouse Out");
            }            
         //-->
      </script>     
   </head>
   
   <body>
      <p>Take your mouse inside this section to see the outcome:</p>      
      <div onmouseover = "over()" onmouseout = "out()">
         <h2> This is inside the section </h2>
      </div>       
   </body>
</html>

 

5. List of HTML 5 Events

The table below provides a list of standard events in HTML5

Event/Attribute Value Brief Description
Offline script Triggers when the document goes offline.
Onabort script Triggers on an abort event.
onafterprint script Triggers after the document is printed
onbeforeonload script Triggers before the document loads
onbeforeprint script Triggers before the document is printed
onblur script Triggers when the window loses focus
oncanplay script Triggers when media can start play, but might has to stop for buffering
oncanplaythrough script Triggers when media can be played to the end, without stopping for buffering
onchange script Triggers when an element changes
onclick script Triggers on a mouse click
oncontextmenu script Triggers when a context menu is triggered
ondblclick script Triggers on a mouse double-click
ondrag script Triggers when an element is dragged
ondragend script Triggers at the end of a drag operation
ondragenter script Triggers when an element has been dragged to a valid drop target
ondragleave script Triggers when an element is being dragged over a valid drop target
ondragover script Triggers at the start of a drag operation
ondragstart script Triggers at the start of a drag operation
ondrop script Triggers when dragged element is being dropped
ondurationchange script Triggers when the length of the media is changed
onemptied script Triggers when a media resource element suddenly becomes empty.
onended script Triggers when media has reach the end
onerror script Triggers when an error occur
onfocus script Triggers when the window gets focus
onformchange script Triggers when a form changes
onforminput script Triggers when a form gets user input
onhaschange script Triggers when the document has change
oninput script Triggers when an element gets user input
oninvalid script Triggers when an element is invalid
onkeydown script Triggers when a key is pressed
onkeypress script Triggers when a key is pressed and released
onkeyup script Triggers when a key is released
onload script Triggers when the document loads
onloadeddata script Triggers when media data is loaded
onloadedmetadata script Triggers when the duration and other media data of a media element is loaded
onloadstart script Triggers when the browser starts to load the media data
onmessage script Triggers when the message is triggered
onmousedown script Triggers when a mouse button is pressed
onmousemove script Triggers when the mouse pointer moves
onmouseout script Triggers when the mouse pointer moves out of an element
onmouseover script Triggers when the mouse pointer moves over an element
onmouseup script Triggers when a mouse button is released
onmousewheel script Triggers when the mouse wheel is being rotated
onoffline script Triggers when the document goes offline
onoine script Triggers when the document comes online
ononline script Triggers when the document comes online
onpagehide script Triggers when the window is hidden
onpageshow script Triggers when the window becomes visible
onpause script Triggers when media data is paused
onplay script Triggers when media data is going to start playing
onplaying script Triggers when media data has start playing
onpopstate script Triggers when the window’s history changes
onprogress script Triggers when the browser is fetching the media data
onratechange script Triggers when the media data’s playing rate has changed
onreadystatechange script Triggers when the ready-state changes
onredo script Triggers when the document performs a redo
onresize script Triggers when the window is resized
onscroll script Triggers when an element’s scrollbar is being scrolled
onseeked script Triggers when a media element’s seeking attribute is no longer true, and the seeking has ended
onseeking script Triggers when a media element’s seeking attribute is true, and the seeking has begun
onselect script Triggers when an element is selected
onstalled script Triggers when there is an error in fetching media data
onstorage script Triggers when a document loads
onsubmit script Triggers when a form is submitted
onsuspend script Triggers when the browser has been fetching media data, but stopped before the entire media file was fetched
ontimeupdate script Triggers when media changes its playing position
onundo script Triggers when a document performs an undo
onunload script Triggers when the user leaves the document
onvolumechange script Triggers when media changes the volume, also when volume is set to “mute”
onwaiting script Triggers when media has stopped playing, but is expected to resume

We recommend you try out some of these events just to see how they work!