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ddl is not updated in server - asp.net

I work on ASP.NET c#. I have a DropDownList. (runat="server")
On $(document).ready, I updated its value:
$(document).ready(function () {
document.getElementById("ddl").value = "abc";
…
When I get back to the Server (c#), there is no value in the ddl:
ddl.SelectedValue == ""
What could be the problem?
Thanks,
YYY

as far your dropdown runat="server" it has generated ClientID, "ddl" it's serverside id, on client you need to call like shown below
document.getElementById("<%= ddl.ClientID %>").value = "abc";

Welcome to the wonderful world of WebForms: in the list of things this framework screws up is ID's of your runat="server" elements
When the engine processes your server markup it generates own id's based on the place where you declared that particular element. Check it in your browser devtools.
Luckily since ASP.NET 4 you can specify special attribute ClientIDMode and now you can do it like
<asp:TextBox ID="txt" runat="server" ClientIDMode="Static" />
which will render like
<input id="txt" name="ctl00$MasterPageBody$ctl00$txt" />
If you cannot use for some reason ASP.NET 4, you can stick to old solution as #Dan proposed:
document.getElementById("<%= ddl.ClientID %>")
But this is not the recommended way.
EDIT
Probably you should do ddl.SelectedItem.Value instead.

read asp.net control in client side

I'm using the javascript below to read the value of an asp.net control in the client-side. However, it always returns the null value. I'm using similar code at other pages in my website, but now i can't read this specific control. Please suggest anyways I can fix this problem.
<asp:Label ID="srch_data" runat="server" ClientIDMode="Static" ></asp:Label>
var srch_data = document.getElementById("<%= srch_data.ClientID %>");
alert(srch_data);

Try using single quotes:
var srch_data = document.getElementById('<%= srch_data.ClientID %>').value;

Try this
var srch_data =document.getElementById('srch_data').innerHTML;
ASP.Net Label becomes span after rendering so instead of finding a label which can not be recognized by JS better find

Add the defer attribute to your script element. I tested and it worked.
Try something like below -
<%--defer indicates the script to be run after the document is completely parsed.--%>
<script type="text/javascript" language="javascript" defer="true">
var label = document.getElementById("<%= srch_data.ClientID %>");
alert("label : " + label);
</script>
<asp:Label ID="srch_data" runat="server" ClientIDMode="Static"></asp:Label>
This should fix your issue.

Creating a Button Click Event and Validating TextBoxes using Javascript

I am having so much trouble with this. I have looked everywhere so I hope someone can either explain to me how to do this or show me. Here's my problem:
I have a DataGrid filled with DataBound Items (ItemTemplate) which was created using ASP.Net.
My reason for the ItemTemplates over a regular DataBound field is to enable the Edit Mode of the DataGrid. In my ItemTemplates, I have labels to display the data and two option buttons (Edit/Delete). I've got the buttons working in the code behind (C#).
Edit places the DataGrid in Edit Mode. In the EditItemTemplates I've got DropDownLists, TextBoxes, and a Save button in place of the Edit button.
The Save button also works with the code I've written for it. All-in-all, the DataGrid works beautifully and displays everything neatly. However, there is one final job I want the Save button to do: I want it to check the TextBoxes and validate that the values entered match the criteria I have set (keep in mind that these are in EditItemTemplates).
I have Javascript already written that will check for validation. I want a modal window (that I have already set up) to display and I want the CSS of the TextBoxes in question to change.
I want to do this using Javascript but my problem is that I can't check for the Save button to create the Click event and I can't 'locate' the TextBoxes to validate them. Is there a way I can 'find' those elements while the DataGrid is in Edit Mode?
Here's a small bit of the code used to create the DataGrid if it helps:
<asp:DataGrid ID="dgCamsizer" CssClass="data" runat="server" AutoGenerateColumns="False"
GridLines="None" OnItemCommand="dgCamsizer_ItemCommand" ShowHeader="true">
<Columns>
<asp:TemplateColumn HeaderStyle-CssClass="infoHeaderDG">
<HeaderTemplate>
Operator</HeaderTemplate>
<ItemTemplate>
<asp:Label ID="Operator" Text='<%# DataBinder.Eval(Container.DataItem, "Operator") %>'
runat="server" /></ItemTemplate>
<EditItemTemplate>
<asp:TextBox ID="EditOper" Width="40px" Text='<%# DataBinder.Eval(Container.DataItem, "Operator") %>'
runat="server"></asp:TextBox></EditItemTemplate>
<HeaderStyle CssClass="infoHeaderDG"></HeaderStyle>
</asp:TemplateColumn>
<asp:TemplateColumn HeaderStyle-CssClass="infoHeaderDG">
<ItemTemplate>
<asp:Button ID="Edit" Text="Edit" CommandName="Edit" runat="server" /></ItemTemplate>
<EditItemTemplate>
<asp:Button ID="Save" Text="Save" CommandName="Save" runat="server" /></EditItemTemplate>
<HeaderStyle CssClass="infoHeaderDG"></HeaderStyle>
</asp:TemplateColumn>
</Columns>
</asp:DataGrid>
Perhaps I should re-phase my question: I managed to 'find' the TextBoxes thanks to Zetlen. I've also managed to get the values. Now... How do I use those values to test for validation?
Here is the code I used to get the values:
$("#<%=dgCamsizer.ClientID %> :text").each(function () {
alert($(this).val());
});

ASP.NET webforms are hard to work with in JavaScript, because the IDs of your HTML elements change during the page lifecycle! The input you're calling "EditOper" will get an HTML ID of something like "dgCamSizer_ctl102_EditOper". One thing you can do is collect these elements on pageload in a cache of DOM element references. I recommend using jQuery or a similar DOM querying library.
<script type="text/javascript">
$(document).ready(function() {
var $editorElms = {},
idSeparator = "_"; // the underscore is ASP.NET's default ID separator, but this can be changed if you wish with the IDSeparator property.
$('#dgCamSizer input').each(function() {
$editorElms[this.id.split('_').pop()] = $(this);
});
// now you can access every input using the $editorElms hash.
// e.g.
function someValidationRoutine() {
if (!$editorElms.EditOper.val()) {
someErrorDisplayer("EditOper must not be blank.");
return false;
}
}
})
</script>
And, again using jQuery, the Save button shouldn't be too tough to find.
var $saveButton = $('#dgCamSizer :submit[value=Save]');
So you can bind the event that way:
$saveButton.click(function(e) {
if (!someValidationRoutine()) {
e.preventDefault();
}
});
This is not the most high-performance solution--selectors of that complexity are always a bit slower. But it gets the job done without messing too much with the DataGrid.

So I managed to solve my own problems and answer my own question. I was able to get the id's of all of the TextBoxes and I even placed them into an array to be used for my validations. Here's what I used to do all of this:
var i = 0;
var data = [];
$("#<%=dgCamsizer.ClientID %> :text").each(function () {
data[i] = $(this).attr("id");
i++;
});

Populate a TextBox from a DropDownList selection ASP.NET/Javascript

I have a DDL and ASP.NET Textbox. I would like to populate the text box with the option I choose from the DDL. I need this to be instant and not use postbacks so it would seem JavaScript would be the obvious choice here. I have done quite a bit of searching but everything I have found seems to be for standard HTML (Selects and Inputs) and these do not appear to work with ASP objects:
<asp:DropDownList runat="server" ID="DDLSalesPerson" DataValueField="keyid" DataTextField="FullName" />
<asp:TextBox runat="server" id="txtSalesPerson" />
My DDL is populated from SQL in the code-behind page.
Can somebody assist with the appropriate code I should use? Thanks.

ASP.Net controls render as standard HTML elements on the browser. In script, you can get a reference to them by using the ClientID property of the ASP.Net control.
Put this in a script block in your aspx:
var ddl = document.getElementById('<%=DDLSalesPerson.ClientID %>');
var textBox = document.getElementById('<%= txtSalesPerson.ClientID%>');
Now you have references to the DOM objects for the select and input elements that the ASP.Net controls rendered and you can use the techniques you've already learned on the HTML elements.
Additional info
You need to add an onchange attribute to your DropDownList control as such:
<asp:DropDownList runat="server" ID="DDLSalesPerson" DataValueField="keyid" onchange="ddlChange();" DataTextField="FullName" />
and then put this script block in your aspx
<script type="text/javascript">
function ddlChange()
{
var ddl = document.getElementById('<%=DDLSalesPerson.ClientID %>');
var textBox = document.getElementById('<%= txtSalesPerson.ClientID%>');
textBox.value = ddl.options[ddl.selectedIndex].text;
}
</script>
As you change the dropdown list, you'll see the textbox update. Tested in IE and Chrome.

Since you've pointed out that you're a JavaScript beginner, may i suggest you use an updatepanel control. An updatepanel allows you to execute server code without refreshing the page. Simply place the dropdownList and the textbox in the same updatepanel or in two separate updatepanels and write the code for the textbox to update based on the dropdownlist selection. Make sure to set the dropdownlist to do autopostback.
The asp markup is as follows:
<asp:UpdatePanel ID="UpdatePanel1" runat="server">
<ContentTemplate>
<asp:DropDownList ID="ddlList" runat="server"
AutoPostBack="True">
<asp:ListItem>-select-</asp:ListItem>
<asp:ListItem>option 1</asp:ListItem>
<asp:ListItem>option 2</asp:ListItem>
</asp:DropDownList>
<asp:TextBox ID="txtTextBox" runat="server" />
</ContentTemplate>
</asp:UpdatePanel>
The codebehind in vb is as follows:
Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load
If ddlList.Text <> "-select-" then
txtTextBox.Text = ddlList.text
End If
End Sub

If you're new to JavaScript, use the jQuery library (simply provide references to the CDN-hosted jQUery files at google.com) and then you can use the following code:
<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.5/jquery.min.js"></script>
<script language="javascript" type="text/javascript">
$("#DDLSalesPerson").change(function () {
$("#txtSalesPerson").val($(this).val());
});
</script>

getElementById not finding control generated by ASP.net

I am simply trying to store a label in a variable in javascript but for some reason this isn't working with document.getElementById('control');. I know my javascript is linking to my html file fine because everything else works.
Here is my javascript code:
function performEvapCooledCircuit(txt)
{
var error = document.getElementById('lblError');
if (txt.value == null || isNaN(txt.value))
{
error.style.visibility = "visible";
}
}
Here is the html code for my label:
<asp:Label ID="lblError" class="NormLabel" runat="server"
style="color:red; visibility:hidden;" Text="Invalid Input."></asp:Label>
I am getting an error that says object expected??

The ID that ASP.NET will generate will not be "lblError" so you'll need to reference it by its ClientID
document.getElementById('<%=lblError.ClientID %>');
If your javascript file is external I've usually had to write a type of "Init" javascript method to make sure my ID's were set up property
On your ASPX page:
<script type="text/javascript">
var lblError = null;
function InitializeVariables()
{
if (lblError == null) // make sure you only do this once
{
lblError = document.getElementById("<%=lblError.ClientID %>");
}
}
</script>
<asp:Label
ID="lblError"
class="NormLabel"
runat="server"
style="color:red; visibility:hidden;"
Text="Invalid Input."></asp:Label>
Then in your javascript file you'll have to call InitializeVariables() to make sure you've got the variables pointing to the proper asp.net controls
function performEvapCooledCircuit(txt)
{
InitializeVariables();
if (txt.value == null || isNaN(txt.value))
{
lblError.style.visibility = "visible";
}
}

"hunter" gives a pretty solid way of doing things, however a, in my opinion far better method is to use the "CliendIDMode" property on the control and set that property to "Static". This will make the client and server IDs the same. Like this:
<asp:TextBox ID="ServerAndClientId" runat="server" ClientIDMode="Static" />

The ID of the label is not "lblError". The ASP.net engine changed the ID. Check the HTML source code in the browser to find out the real ID.

That's not HTML for the label, that is an ASP.NET Control which will be rendered into HTML before it is sent in the response. ASP.NET WebForms controls sometimes change the id for the HTML they create.
View the source of the webpage to see what the id for the HTML element is on the rendered page.

You can use this:
document.getElementById('<%= lblError.ClientID %>').click()
Starting from ASP.NET 4.0 you can use ClientIDMode property for you element. And if you set it into Static then the ClientID value will be set to the value of the ID property:
<asp:Label ID="lblError" runat="server" ClientIDMode="Static" />
will be rendered as something like this:
<span id="lblError" name="ctl00$MasterPageBody$ctl00$Label1" />