Javascript recipes

This is a repository for snippets and methods that help web pages interact with viewers.

Let a user know a form-element has changed

Sometimes we're looking at a form and don't know whether we've changed anything. Javascript allows different methods, including a couple shortcuts that get the job done, if not perfectly. You can add an attribute to any form-input that makes either a key-press or a change in content activate a function. I like to hide a "submit" button until the content of a form has changed—A reminder that there's something to submit.

onchange: This attribute will activate a command, given as the value of the attribute, when the content of the input changes. One problem is the action only happens after the focus has left the input field, so one can type into an input and still not see the expected event.
onkeypress: This attribute will activate as soon as any key is pressed while the input is in focus. This solves the problem mentioned with onchange. The only snag is that it is possible to change a field without the keyboard, for example with the mouse and menu commands.
onfocus: This attribute will activate as soon as the field comes into focus. This would be jumping the gun, because one might select a field without making any changes.
onpaste, oncut: These attributes can complement onkeypress to account for mouse actions that might change text.

Example

This text input, pre-populated with the PHP variable $urlnotes, will activate the custom javascript function showhide() when a key is pressed while the user's cursor is inside the text area.

<script type="text/javascript">
function showhide(whichLayer,action2){
 var elem;
 if( document.getElementById ) // the standard way
   elem = document.getElementById( whichLayer );
 else if( document.all ) // the old MS Internet Explorer way
     elem = document.all[whichLayer];
 else if( document.layers ) // the Netscape Navigator 4 way
   elem = document.layers[whichLayer];
 elem.style.display = action2;
}
</script>
<textarea name="urlnotes" cols="60" rows="10"
 onkeypress="showhide('updatebutton','block')">
 <?php echo $urlnotes; ?></textarea>

Javascript recipes