ASP NET document getElementById Control ClientID returns null

Question

I m trying to retrieve a server control in JavaScript  For testing purposes I am calling the JavaScript function from the page load event   protected void Page Load object sender  EventArgs e       ClientScript RegisterClientScriptBlock GetType     js    confirmCallBack      true       And my JavaScript function is   function confirmCallBack         var a   document getElementById   lt    Page Master FindControl  PlaceHolderContent   FindControl  Button1   ClientID   gt         var b   document getElementById   lt   Button1 ClientID  gt         my problem is that both a and b return null  Even when I view the page source the correct ClientID is returned   I should add that I m using master page   Any ideas

User · Answer

Is Button1 visible  I mean  from the server side  Make sure Button1 Visible is true   Controls that aren t Visible won t be rendered in HTML  so although they are assigned a ClientID  they don t actually exist on the client side

User · Answer

Gotcha   You have to use RegisterStartupScript instead of RegisterClientScriptBlock  Here My Example   MasterPage    lt    Master Language  C   AutoEventWireup  true  CodeBehind  MasterPage master cs      Inherits  prueba MasterPage    gt    lt  DOCTYPE html PUBLIC  -  W3C  DTD XHTML 1 0 Transitional  EN   http   www w3 org TR xhtml1 DTD xhtml1-transitional dtd  gt   lt html xmlns  http   www w3 org 1999 xhtml  gt   lt head runat  server  gt       lt title gt  lt  title gt        lt script type  text javascript  gt           function confirmCallBack                 var a   document getElementById   lt    Page Master FindControl  ContentPlaceHolder1   FindControl  Button1   ClientID   gt                  alert a value                    lt  script gt        lt asp ContentPlaceHolder ID  head  runat  server  gt       lt  asp ContentPlaceHolder gt   lt  head gt   lt body gt       lt form id  form1  runat  server  gt       lt div gt           lt asp ContentPlaceHolder ID  ContentPlaceHolder1  runat  server  gt           lt  asp ContentPlaceHolder gt       lt  div gt       lt  form gt   lt  body gt   lt  html gt    WebForm1 aspx   lt    Page Title    Language  C   MasterPageFile    MasterPage Master  AutoEventWireup  true      CodeBehind  WebForm1 aspx cs  Inherits  prueba WebForm1    gt    lt asp Content ID  Content1  ContentPlaceHolderID  head  runat  server  gt    lt  asp Content gt    lt asp Content ID  Content2  ContentPlaceHolderID  ContentPlaceHolder1  runat  server  gt   lt asp Button ID  Button1  runat  server  Text  Button    gt   lt  asp Content gt    WebForm1 aspx cs  using System  using System Collections Generic  using System Linq  using System Web  using System Web UI  using System Web UI WebControls   namespace prueba       public partial class WebForm1   System Web UI Page               protected void Page Load object sender  EventArgs e                        ClientScript RegisterStartupScript this GetType     js    confirmCallBack      true

[javascript] ASP.NET document.getElementById('<%=Control.ClientID%>'); returns null

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