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//Created 2004-12-13, Last Modified 2006-11-10, © Shailesh N. Humbad
//This function is specially designed for dynamic pages with varying numbers of checkboxes. Unlike other functions out there, it will work without error even if there are no checkboxes or there is only one checkbox. It is also significantly faster for long lists, because it saves the length of the checkbox in a variable, instead of recalculating it in the loop. Finally, the function is granted to the public domain--use it as you wish. 

function SetAllCheckBoxes(FormName, FieldName, CheckValue)
{
	if(!document.forms[FormName])
		return;
	var objCheckBoxes = document.forms[FormName].elements[FieldName];
	if(!objCheckBoxes)
		return;
	var countCheckBoxes = objCheckBoxes.length;
	if(!countCheckBoxes)
		objCheckBoxes.checked = CheckValue;
	else
		// set the check value for all check boxes
		for(var i = 0; i < countCheckBoxes; i++)
			objCheckBoxes[i].checked = CheckValue;
}