Managing Field Control Visibility - a step towards field level security and much more

Hi all,
Many times I face the same problems with almost all my customers.
In some point it seems they all want more controls over fields in the “edit” and “new” item form.
Some want to hide it from unauthorized users; others want to give a field a default value that changes according to the current user, and even some more interesting things like filtering values in choice fields to answer some complicated logic.
So one way to support all these whims is to develop new field types with field controls to mach – you can find plenty examples for that on the web (like our soon-to-be-released tags field type).
Here I want to show a simpler solution that involves only editing the edit/view form of the list and make use of very simple javascript code to get control over the client-side HTML element of each field.
One more thing – although we can create a custom list form using the SharePoint designer to hide controls there is a major disadvantage in using such a custom list form. If your user will add a new field in the future it will not show automatically in the edited form and you will have to edit these forms every time your user changes the list.
This code I wrote with the help of Sveta (thanks) expects the field display name and look for it in the HTML while it assumes a comment tag with the field name will exist right above the field itself.
Here is the code:

var ctrl = null;
//Place this code right AFTER the list view web part.
//To go into edit mode of the page,
//in case you dont have the edit page menu paste this in the browser's address bar:
//javascript:MSOLayout_ChangeLayoutMode(false)
function foo()
{
var ctrl = null;

var arr = document.getElementsByTagName('!');//get all comments

for(var i=0;i<arr.length;i++)
{
try
{
var html = arr[i].innerHTML;

if(
html.indexOf("field to hide 1") > 0 ||
html.indexOf("field to hide 2") > 0
)
{
ctrl = arr[i].parentElement.parentElement;
ctrl.style.display="none";
}

}
catch(e){alert(e.description);}
}
}
foo();

Doing this allows you to keep the default list form and still get some control of the desired field element. I usually wrap this code in a custom control that analyzes on the server side what needs to be done (hide element, replace with another or filter a combo box) and make use of the code above to complete the job on the client site. For example if I want to achieve a field level security for any SharePoint list field my server control will test the user permissions and will hide the control if the current user does not have enough permissions.
Hope this could help some of you guys,
Shai Petel.