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How to change the value of an asp label with JavaScript?

I'm trying to add an error label to the top of a panel I have. I have a button created in C# on page load that calls a JavaScript function that I want to display an error message on my panel when clicked.
C#:
private void CreateButton(int pID, string changeType)
{
ASPxButton btn = new ASPxButton();
btn.Text = "Execute Request";
btn.ID = "btn" + changeType;
btn.AutoPostBack = false;
btn.ClientSideEvents.Click = GetClientSideEventHandler(string.Format("OnProcessRequest(s, e, '{0}','{1}')", pID.ToString(), changeType));
TableRow oRow = new TableRow();
TableCell oCell = new TableCell();
oCell.CssClass = "table-cell";
oCell.Controls.Add(btn);
oRow.Cells.Add(oCell);
tblButtons.Rows.Add(oRow);
}
JS:
function OnProcessRequest$(pID, pChangeType) {
document.getElementById('errLabel').value = "Test";
}
ASPX:
<asp:Label ID="errLabel" runat="server"/>
When this code runs, it always throws the following error:
Error: Unable to set property 'value' of undefined or null reference.
I have tried also using:
document.getElementById('<%=errLabel.ClientID%>').value = "Test";
but this also throws the error.
How can I change the value of this label when this button is clicked in JS?

Ok, to change a asp.net label in JavaScript, you can do this:
(we assume you set the label client id mode>
So, if we have label on the page, you can do this:
<asp:Label ID="Label1" runat="server" Text="" ClientIDMode="Static"></asp:Label>
JavaScript to change above is this:
var lbl = document.getElementById('<%=Label1.ClientID%>');
lbl.innerText = "Js lable text changed";
Or
lbl.innerHTML = "<h2>this is some big text by js</h2>"
Be VERY careful with case, and VERY careful with extra spaces etc. in the get Element.
Also, do NOT forget to include the Text="" in your label!!!! (you are missing this!!!).
JavaScript is VERY flakey - one small wrong move, and it just rolls over and goes home. (and the debugger in browsers is on par with a trip to the dentist).
You can also use jQuery.
The above thus becomes this:
var lbl = $('#Label1');
lbl1.text("js jquery text change");
Now, lets do the same for a text box.
our asp.net text box:
<asp:TextBox ID="TextBox1" runat="server" ClientIDMode="Static" ></asp:TextBox>
JavaScript:
var txt = document.getElementById('<%=TextBox1.ClientID%>');
txt.value = "This is js text for text box";
And as jQuery:
var txt = $('#TextBox1');
txt.val("js jquery text for the text box");
So, for a asp.net label? You use innerText, or innerHTML.
(or text("your text here") with jQuery)
and with jQuery, you use .value without ()

Try adding ClientIDMode="Static" to your label if that property is available to you. Or you could add ClientID="errLabel" as an alternative. What's happening is asp.net will automatically give your field a generated id for closure on the client side so it will not match your id "errLabel".
<asp:Label runat="server" ID="errLabel" ClientIDMode="Static"></asp:Label>
OR
<asp:Label runat="server" ID="errLabel" ClientID="errLabel"></asp:Label>
https://learn.microsoft.com/en-us/dotnet/api/system.web.ui.control.clientidmode?view=netframework-4.8#System_Web_UI_Control_ClientIDMode

how to pass selected row of gridview in popup window into parent page

I have 2 pages in asp.net with c# .
a parent.aspx and popup.aspx.
I passed a querystring(id) into page load of popup.aspx and used function to call row of table base on id and show gridview on popup.aspx.
now I want to select this row, and pass details into text boxes of parent.aspx that is open now.
Everything is ok and row of table is passed into text boxes, but it is into new window popup of parent.aspx page, that I don't want this.
I want pass details into this page(parent.aspx) that now is open.
How can I do that.thanks.
below is my code for pass id to pop-up window
protected void btn_search_id_Click(object sender, ImageClickEventArgs e)
{
string str1 = Encrypt(txt_sh_p.Text);
btn_search_id.Attributes.Add("onclick", "window.open('popup.aspx?sh_p_=" + str1 + "','Report','width=750,height=500,toolbar=no,status=no,menubar=no,scrollbars=yes,resizable=yes,copyhistory=no,left=200,top=50'); return false;");
}
my code for reading id and select row and display row in gridview on page load event of popup.aspx:
protected void Page_Load(object sender, EventArgs e)
{
DAL.Sab_Ashkh sabt_ashkh = new DAL.Sab_Ashkh();
List<DAL.Sab_Ashkh> sabt_ashkh_list;
sabt_ashkh.sh_p = Decrypt(Request.QueryString["sh_p_"]);
sabt_ashkh_list = sabt_ashkhDB.GetShakh_find(sabt_ashkh.sh_p);
grid_ashkh.Visible = true;
grid_ashkh.DataSource = grid_ashkh_list;
grid_ashkh.DataBind();
}
and html code for pass row to parent page:
<Columns>
<asp:HyperLinkField DataTextField="id_shakh" DataNavigateUrlFields="id_shakh" DataNavigateUrlFormatString="parent.aspx?id_shakh={0}"
HeaderText="id" ItemStyle-Width = "150" />
<asp:TemplateField HeaderText="select">
<ItemTemplate>
<asp:CheckBox ID="CheckBox1" runat="server" />
</ItemTemplate>
</asp:TemplateField>
<asp:BoundField ItemStyle-Width = "150px" DataField = "sh_p" HeaderText ="kod"
>
<ItemStyle Width="150px"></ItemStyle>
</asp:BoundField>
</Columns>

I am just telling you how you can change the variable in Main page in popup page like:
Suppose Parent.aspx have:
<script type="text/javascript">
var items =[];
</script>
and in popup.aspx you can do something like :
window.opener.items.push(yourSelectedRows);
but as another workaround you can also use local storage like:
localStorage.setItem("selectedRecords", JSON.stringify(selectedRows));
suppose selectedRows are your array of object or anything else but as my experience the selectedRecords would be accessible in all HTML pages.
hope this help you.

Hi Aria Thanks for reply. I pass id by query string into popup window and show gridview . and this script for popup win :
<script type="text/javascript">
function SetName() {
if (window.opener != null && !window.opener.closed) {
var txtName = window.opener.document.getElementById("txt_id_mah");
grid = document.getElementById("grid_ashkh");
var cellPivot;
if (grid.rows.length > 0) {
for (i = 1; i < grid.rows.length; i++) {
cellPivot = grid.rows[i].cells[1];
TXT.value = cellPivot;
}
}
}
window.close();
}
</script>
but does not work.

Asp.Net Control Value return undefined in Jquery

In my project i need change visible of dynamic asp control when click label based on textbox values. So i first tried to get textbox value when click label but its return undefined. I am search and get two methods i tried that also it return same.
My Try :
<script language="javascript" type="text/javascript">
$(document).ready(function() {
$(document).on("click", "#lblShow", function() {
alert($('#<%=txtTotalVersion.ClientID%>').val());
alert($('input[id$=txtTotalVersion]').val());
var xTotlal =$('#<%=txtTotalVersion.ClientID%>').val()
var i = 0;
for (i = 0; i < xTotlal; i++) {
$('#createDiv' + i).style.display = "blcok";
$('#createDiv1' + i).style.display = "block";
$('#createDiv2' + i).style.display = "block";
$('#createDiv3' + i).style.display = "block";
}
});
});
</script>
HTML
<div id="DivCompName">
<asp:TextBox runat=server ID="txtTotalVersion" Visible="false"></asp:TextBox>
<asp:TextBox runat=server ID="txtCurrentDisplay" Visible="false"></asp:TextBox>
</div>
First two alert return undefined.

Visible="false" is asp.net attribute, in this case your control will not be rendered at the client side. So your client script won't find the control as it doesn't exists!
If you want to store some value at client side and don't want to display it then you can use HiddenFields or you can make the same control hidden by using css style display:none;. (Don't use Visible="false" for this)

you can add ClientIDMode=Static and call it from your jquery
<asp:TextBox runat=server ID="txtTotalVersion" Visible="false" ClientIDMode="Static"></asp:TextBox>
<script>
$(document).ready(function () {
alert("#txtTotalVersion").val();
})
</script>
reason is, the client id for your control might not be as it is assigned with ID="xxx", if the control is inside of another asp.net server control, after adding the ClientIDMode, you are telling your server to treat this control with a static ID
to learn more: msdn

How to change a server side label display to Block in javascript?

How can I change the display property of my server side label to Block?
<asp:Label id="lblError" runat="server" style="display:none;"></asp:Label>
function block()
{
// change display property to block
}
I tried
document.getElementById('lblError').style.display = "block";
but it's not working, please help me.

use ClientIDMode :
<asp:Label id="lblerror" runat="server" ClientIDMode="Static" style="display:none;"></asp:Label>
and in client:
document.getElementById('lblerror').style.display = "block";

Should be:
document.getElementById('lblError').style.display = "block";
Or you can use a safer way:
var buttonID = '<%= lblError.ClientID %>';
var button = document.getElementById(buttonID);
if (button) {
button.style.display = 'block';
}
It also looks like youv'e missed a double quote after style=. It should be :
<asp:Label id="lblError" runat="server" style="display:none;"></asp:Label>

One way to access a control from client script is to pass the value of the ClientID property of the server control to the document.getElementById method. Like this:
document.getElementById('<%= lblerror.ClientID %>').style.display = "block";
Have a look at this: How to: Access Controls from JavaScript by ID.

couldn't get value into javascript from a asp Label which value bind from code behind

Can't get value from getValue in JS and can't show value in showValue field.
Thanks in advance
ASPX PAGE CODE
<script>
var x = document.getElementById("<%=getValue.ClientID %>").innerHTML;
document.getElementById('showValue').innerHTML = x;
</script>
<asp:Label runat="server" ID="getValue" />
<asp:Label runat="server" ID="showValue" />
C# CODE
getValue.Text = "value from code";

You need to call javascript once all the controls are rendered, then only you can get the proper id of the <asp:label> control, so you need to write that javascript code under window.onload().
Also for showValue, you need to use ClientID.
window.onload= function(){
var x = document.getElementById("<%=getValue.ClientID%>").innerHTML;
document.getElementById('<%=showValue.ClientID %>').innerHTML = x;
};