//checks all checkboxes
function checkAll(field) {
		if (!(field.length)) {
	    field.checked = true;
		}
		else {
    for (var i = 0; i < field.length; i++) {
	    field[i].checked = true;
    }
  }
}
 
//unchecks all checkboxes
function uncheckAll(field) {
		if (!(field.length)) {
        field.checked = false;
		}
		else {
    for (var i = 0; i < field.length; i++) {
        field[i].checked = false;
    }
  }
}	
 
<a href="javascript:checkAll(document.FRM.FIELD);">select</a> / <a href="javascript:uncheckAll(document.FRM.FIELD);">deselect</a> all
 
loop through all checkboxes in a form:
 
function checkAll() {
    for (i=0; i<document.FormName.elements.length; i++) {
       if (document.FormName.elements[i].type=="checkbox")
          document.FormName.elements[i].checked=true;
   }
}
//javascript/3917