Check / Uncheck all checkboxes in group 

Javascript Code:

function SetGroup(GrpState, _Group)
{
// Check if group has more than one element
	if(_Group.length > 0)
	{
		for (i = 0; i < _Group.length; i++)
			{ _Group[i].checked = GrpState.checked;	}
	} 

		else	

        { _Group.checked = GrpState.checked; } // Single element , is not array group

}

HTML Code:

<input type=checkbox name=checkall onclick="SetGroup(this, grup1);">
<input type=checkbox name=grup1 id=boxa1>
<input type=checkbox name=grup1 id=boxa2>
<input type=checkbox name=grup1 id=boxa3>
<input type=checkbox name=grup1 id=boxa4>
<input type=checkbox name=grup1 id=boxa5>
<br />
<input type=checkbox name=checkall onclick="SetGroup(this, grup2);">
<input type=checkbox name=grup2 id=boxb1>
<input type=checkbox name=grup2 id=boxb2>
<input type=checkbox name=grup2 id=boxb3>
<input type=checkbox name=grup2 id=boxb4>
<input type=checkbox name=grup2 id=boxb5>