Textbox maxlength validation and display remaining characters

Write below Javascript function in ASPX page as:-

function CountCharactersGeneral(sourceTextBox,displayControl,maxLength)
{
   if(sourceTextBox != null && displayControl != null)
   {
      sourceTextBox = document.getElementById(sourceTextBox);
      displayControl = document.getElementById(displayControl);

      if(sourceTextBox != null)
      {
         var len = sourceTextBox.value.length;

         if (len <= maxLength)
         {
             displayControl.innerHTML = maxLength -len +" Character(s) remaining.";
         }
         else
         {
             sourceTextBox.value = sourceTextBox.value.substring(0,maxLength);
             return false;
         }
      }
   }
}

Here is my controls:-

<asp:Label runat="server" ID = "displayControl" Text = ""></asp:Label>
<asp:TextBox ID ="sourceTextBox" runat="server"/>