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Event handling in JavaScript

An event is something that occurs within the application. For instance, a user clicking on a button element within a web page would register an event.

action => event

An event is not limited to the user doing something and can triggered when the browser performs some action.

JavaScript events

JavaScript has quite a few events, but they vary from device to device and from the element that the event occurs on. For instance, a button does not have an onchange event but a select does.

Create inline event handlers

An inline event handler is one that is contained in HTML.

To create an event handler in HTML it is simply set as an attribute on an element:

HTML
<script>
function runJS(){
  alert("Button pressed");
}
</script>
<button onclick="runJS();">Run some JavaScript</button>
				

Using pure JavaScript

It is possible, and preferred, to create a purely JavaScript implementation of event handlers. It is the preferred method due to the fact that JavaScript is often cached by a browser and therefore event handlers are cached. As a result, less data is required by the user's machine by avoiding downloading event handlers again. As well as this, multiple objects may have the same event handler and HTML cannot do this directly. Thirdly, the keeps to the concept that content should be separated from functionality.

To implement this in JavaScript the addEventListener method is used.:

JavaScript
var special_button = document.getElementById('special_button');
special_button.addEventListener('click', function(){
  alert("It works");
});

This implementation is guaranteed to work in contemporary browsers such as the latest version of Google Chrome or Firefox, but it will not work in old browsers, including Internet Explorer 8. Old browsers such as Internet Explorer 8 did however support a similar method called attachEvent. To check if the current browser supports the more modern and standards-compliant addEventListener method the following code can be used:

JavaScript
var f = function(){
  //Do something on load
};
if(window.addEventListener){
  window.addEventListener('load', f);
} else {
  window.attachEvent('onload', f);
}

Of course, running the previous script will not make any difference since the window has already loaded, but it does demonstrate how to check for the modern implementation.

Some example events

Below is a list of some events available in JavaScript:

  • load
  • click
  • mouseover
  • mouseout
  • keydown
  • keyup

Although this is not a comprehensive list of every event that JavaScript handles, this is a good list to get started with.

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