javascript - unCaught type error: cannot read property 'null' value for document.getElementByID().value; -

i trying value of input field console says cannot read property null value.i have used document.getelementbyid().value function. view source code displays values javascript variable not getting value. following code:

 <script type="text/javascript">     function addcart(id,name,price)      {        var quantity=document.getelementbyid("quantity_"+id).value;//this line gets error              $.ajax({          type:'post',          url:'addcart.php',          data:{            item_id:id,            item_name:name,            item_quantity:quantity,            item_price:price          },          success:function(response) {            document.getelementbyid("cart").innerhtml=response;          }        });         }   </script>

i tried using jquery val() function, follows...

 <script type="text/javascript">     function addcart(id,name,price)      {      var quantity = $('#quantity_'+id).val(); //this line         $.ajax({          type:'post',          url:'addcart.php',          data:{            item_id:id,            item_name:name,            item_quantity:quantity,            item_price:price          },          success:function(response) {            document.getelementbyid("cart").innerhtml=response;          }        });         }   </script>

look html code:

 <?php while($row = mysqli_fetch_array( $fetch_customer_menu_result))        {          $item_id=$row['item_id'];          $item_name=$row['item_name'];          $item_price=$row['item_price'];                   echo '                        <div class="w3-col l8 w3-col s8">            <input class="form-control w3-left" type="number" placeholder="count" id="quantity_'.$row['item_id'].'" value="1">                  <span class="w3-right"><u>rate (each):</u></span><br>                       <span class="w3-right w3-large"><b>rs. '.$row['item_price'].'</b></span>                                                     </div>              </div>               <div class="w3-col l12 w3-center" style="margin-top: 5px">              <button type="button" class="w3-red btn form-control" onclick="addcart(\' '.$item_id.' \',\' '.$item_name.' \',\' '.$item_price.' \')" >add cart</button>            </div>          </div>        </div>';        }        ?>

uncaught type error: cannot read property 'null' value document.getelementbyid().value;

this error means document.getelementbyid() returns null pointer (i.e. can't find element looking for, returns null), , trying find value of field doesn't exist (document.getelementbyid("quantity_"+id).value becomes null.value, , null not have field named value).

why?

the reason (in case) value pass in id in addcart function incorrect. however, there few causes. follow these debugging steps:

• make sure order of parameters same defined in function , calling (i.e. not passing in name should passing in id)

• add line above line error: console.log("quantity_"+id) , use browser console check if text box has exactly id - if doesn't, problem.

• make sure addcart being called when text field on screen - ensure don't delete text field , call addcart, , addcart doesn't called before element has been created.

• make sure there not 2 functions name addcart.