Sacar un Valor de un input y ponerlo en javascript[No resuelta]

div class="content-wrapper">
  <!-- Content Header (Page header) -->
  <section class="content-header">
   <!-- Main content -->
   <section class="content">
    <div class="row">
      <div class="col-xs-12">
        <div class="box">
          <div class="box-header">
           <h3 class="box-title">Detalle Orden</h3>
         </div>
         <!-- /.box-header -->
         <div class="box-body">
          <form action="<?php echo base_url(); ?>detalle/adicionar_detalle" method="post">



<div class="box-body">
            <div class="row">
             <div class="col col-xs-6">
              <div class="input-group form-group">
                <label class="input-group-addon" >No.Orden</label>
                <select class ="form-control select2" class="form-control" class="form-control" name="fk_nro_orden" id="fk_nro_orden"/>
                <?php  
                if(count($fk_nro_orden)){
                  foreach($fk_nro_orden as $f){
                    echo "<option value'".$f['nro_orden']. "'>".$f['nro_orden']."</option>";
                  }
                }
                ?>
              </select> 
            </div>
          </div>
          </div>
        </div>

<div class="box-body">
            <div class="row">
          
              <div class="col col-xs-6">
               <div class="input-group form-group">
                <label class="input-group-addon">Ref.Vehiculo</label>
                <select class= "form-control select2"  class="form-control" name="fk_id_vehiculo"  id="fk_id_vehiculo"/>
                <?php  
                if(count($fk_id_vehiculo)){
                  foreach($fk_id_vehiculo as $h){
                    echo "<option value'".$h['referencia_v']. "'>".$h['referencia_v']."</option>";
                  }
                }
                ?>
              </select> 
            </div>
          </div>





          <div class="col col-xs-6">
            <div class="input-group form-group">
              <label class= "input-group-addon">Ref.Articulo</label>
              <select  class="form-control select2" class= "form-control" name="fk_id_articulo" id="fk_id_articulo"/>
              <?php  
              if(count($fk_id_articulo)){
                foreach($fk_id_articulo as $f){
                  echo "<option value'".$f['referencia_art']. "'>".$f['referencia_art']."</option>";
                }
              }
              ?>
            </select> 
          </div>
        </div>
      </div>
    </div>







    






      </div>
    </div>
   
    


  </div>
</div> 

            
<div>
  
<label>Detalle</label>
<table class="table">
  <thead>
    <tr>
      <th>Precio</th>
      <th>Cantidad</th>
      <th>Sub-total</th>
    </tr>
  </thead>
  <tbody id="f_tbody">
    
    
  </tbody>
</table>
  <div class="text-right">

      <button  onclick="nuevaFila();" type="button" class="btn btn-info">Agregar</button>          
                
  </div




</div>
</div>
<div class="box-body">
            <div class="row">
    <div class="col col-xs-3">
      <div class="input-group form-group">
        <label class="input-group-addon">Iva</label>
        <input  type="text"  class="form-control" id="iva" name="iva"/ >
      </div>
    </div>
    <div class="col col-xs-3">
      <div class="input-group form-group">
        <label class="input-group-addon">Cantidad</label>
        <input  type="text"  class="form-control" id="cantidad" name="cantidad"/ >


      </div>
    </div>
    
    <div class="col col-xs-3">
      <div class="input-group form-group">
        <label class="input-group-addon">Precio</label>
        <input type="text"  class="form-control" id="precio" name="precio"/ >


      </div>
    </div>
    <div class="col col-xs-3">
      <div class="input-group form-group">
        <label class="input-group-addon">Sub-total</label>
        <input  type="text"  class="form-control" id="subtotal" name="subtotal"/ >


      </div>
    </div>
  </div>
</div>







<!-- /.box-body -->

<div class="box-footer">
  <button type="submit" class="btn btn-primary">Submit</button>
</div>

</form>
<textarea id="text_plantilla" style="display:none;">
  <td>
        <input onkeyup="calcular();" required value = '{precio}' class="form-control" type="text" name="art_precio[]"/>
      </td>
      <td>
        <input onkeyup="calcular();"  required value = '{cantidad}' class="form-control" type="text" name="art_cantidad[]"/>
      </td>
      <td>
        <input required reandoly value = '{subtotal}' class="form-control" type="text" name="art_subtotal[]"/>
      </td>
</textarea>

<script type="text/javascript">

  function my_num(num){
    rs=0;
    tmp=parseFloat(num);
    if(!isNaN(tmp)){
      rs=tmp;

    }


    return rs;


  }


function calcular(){
  total=0;
iva=0;


precio = document.getElementsByName('art_precio[]');
cantidad = document.getElementsByName('art_cantidad[]');
subtotal = document.getElementsByName('art_subtotal[]');



for(x=0; x<precio.length;x++){
  sub=(my_num(precio[x].value)*my_num(cantidad[x].value)).toFixed(3);
  subtotal[x].value=sub;
  total+=my_num(sub);

}
document.getElementById('subtotal').value=total;
iva = total * 0.18;
document.getElementById('iva').value=iva;

total =total +iva;
document.getElementById('precio').value=total;


cantidad=cantidad+cantidad;
document.getElementById('cantidad').value=cantidad;






}




function nuevaFila(){
obj= {};
obj.precio="";
obj.cantidad="";
obj.subtotal="";

agregarFila(obj);

}





function agregarFila(obj){
tr = document.createElement('tr');
cont =document.getElementById('text_plantilla').value;


for(prop in obj){
 cont=cont.replace('{'+prop+'}',obj[prop]);
}


tr.innerHTML =cont;

document.getElementById('f_tbody').appendChild(tr);




}  


</script>