Eliminate the sTotal, dTotal, and deTotal intermediates.js

function byId(id) { 
  return document.getElementById(id); 
}

function byTag(tag) { 
  return document.getElementsByTagName(tag); 
}



var ckShelf     = byId("shelf"),
    shelves     = [ byId("shelf1"), byId("shelf2"), byId("shelf3") ],
    ShelvingCfg = { enabler: ckShelf, items: shelves, total: byId("shelfCost") };

var ckDrawer    = byId("drawer"),
    drawers     = [ byId("drawer1"), byId("drawer2"), byId("drawer3") ],
    DrawerCfg   = { enabler: ckDrawer, items: drawers, total: byId("drawerCost") };

var ckDesk      = byId("desk"),
    desks       = [ byId("desk1"), byId("desk2"), byId("desk3") ],
    DeskCfg     = { enabler: ckDesk, items: desks, total: byId("deskCost") };

var Furniture   = [ShelvingCfg, DrawerCfg, DeskCfg];

var tTotal      = byId("taxCost");

var ckShip      = byId("shiptax"),
    shTotal     = byId("shipCost");

var gTotal      = byId("totalCost");

var totalInputs = [ShelvingCfg.total, DrawerCfg.total, DeskCfg.total, shTotal, tTotal, gTotal];





function clearAll() {

  byTag("input").map(eachInput => eachInput.checked = false);
  totalInputs.map(control => control.value = null);

}





function setDollarValue(Target, DollarValue) {
  Target.value = ("$" + Number(DollarValue).toFixed(2));
}





function ckCheckedRow(Enabler, Controls, Target) {

  if (Enabler.checked) {

    var FirstOn     = Controls.findIndex(function(C) { return C.checked; }),
        DollarValue = (FirstOn !== -1)? Controls[FirstOn].value : 0;
    
    setDollarValue(Target, DollarValue);

  } else {
    Target.value = null;
  }

}





function ckChecked() {

  Furniture.map( Cfg => ckCheckedRow(Cfg.enabler, Cfg.items, Cfg.total) );

  if (ckShip.checked) { setDollarValue(shTotal, 10); } 
  else                { shTotal.value = null;        }

}





function sumInto(Target, Left, Right) {
  Target.value = parseInt(Left.value) + parseInt(Right.value);  
}





function addTax() {

  tTotal.value = ("$" + (0).toFixed(2));

  if (ckShelf.checked)  { sumInto(tTotal, tTotal, ShelvingCfg.total); }
  if (ckDrawer.checked) { sumInto(tTotal, tTotal, DrawerCfg.total);   }
  if (ckDesk.checked)   { sumInto(tTotal, tTotal, DeskCfg.total);     }

}





function grandTotal() {

  gTotal.value = ("$" + (0).toFixed(2));

  if (ckShelf.checked)  { sumInto(gTotal, gTotal, ShelvingCfg.total); }
  if (ckDrawer.checked) { sumInto(gTotal, gTotal, DrawerCfg.total);   }
  if (ckDesk.checked)   { sumInto(gTotal, gTotal, DeskCfg.total);     }
  if (ckShip.checked)   { sumInto(gTotal, gTotal, shTotal); }

}