Is there a way to use "<%= someObject.ClientID %>" in an external javascript file?

If you really want to do this you can do following

<%@ Page ContentType="text/javascript" Language="C#" AutoEventWireup="false" %>
<%@ OutputCache Duration="86400" Location="Any" VaryByParam="None" %>

var time = "<%= DateTime.Now.ToString() %>";
alert(time);

And then reference it in your page

<script src="Scripts/file.aspx" type="text/javascript"></script>

Note When using mentioned method, the only way to pass target page controls client-ids, is to store client id as string in a public property, and then reference it using new instance of that page

If the only thing that made you to do this is client-id then you can use following ASP.NET 4 feature

<any-tag ID="myCustomId" runat="server" ClientIDMode="Static" />

You can also put all your client-ids in C# class then serialize it using JSON and render it in script tag, could be smart way for ASP.NET prior to version 4.

Note using serialization method you have possibility to change any tag ids without worrying about javascript element usages, remember that this is not even possible with ASP.NET 4 ClientIDMode

See

Page-Code-File

public partial class About : System.Web.UI.Page
{
    ...

    protected string GetTagIds()
    {
        return new JavaScriptSerializer()
                    .Serialize(new
                     {
                            myButton = Button1.ClientID,
                            myCalendar = Calendar1.ClientID
                     });
    } 

    ...
}

Page-ASPX

<script type="text/javascript">
    var IDs = <%= GetTagIds() %>;
</script>

Anywhere

<script type="text/javascript">
    IDs.myCalendar.doSomthing...
</script>

There is another option that you can pass all javascript files to ASP.NET handler but i don't recommend it, because of just a single javascript file you make asp.net handler busy.

Code Blocks

<% inline code %>

This is an inline code definition which you can execute codes in it :

<% Response.Write("Hi"); %>

<% while(i < 0) { %>
      <% 
         Response.Write(i.ToString()); 
         i++;
      %>
<%}%>

Note You have to include ';' on end of each statement when using inline code with C# language, you can change inline language using page directive language attribute

<%= inline expression %>

This one equals to calling Response.Write your self, see:

<%= "Hi" %> equals to <% Response.Write("Hi"); %>

Note You shouldn't include ';' when using inline expression

<%: encoded inline expression %>

This one equals to :

Response.Write(HttpUtility.HtmlEncode("<script type="text/javascript">alert('XSS');</script>"))

And is used for security reasons --XSS, any input HTML to this one outputs HTML encoded text which is safe to display user entered contents in page.

Note You shouldn't include ';' when using encoded inline expression

<%$ expressionPrefix: expressionField %>

This one is expression that you can use to bind values from ConnectionStrings, Resources and AppSettings

expressionPrefix possibilities is

• AppSettings
• ConnectionStrings
• Resources

expressionField is the property of specified expressionPrefix that you need, see:

// AppSettings
<asp:Label ... Text="<%$ AppSettings: version %>" />

// ConnectionStrings
<asp:SqlDataSource ... ConnectionString="<%$ ConnectionStrings:dbConnectionString %>" />

// Resources
<asp:Label ... Text="<%$ Resources: Messages, Welcome %>" />

Note You shouldn't include ';' and you can use expressions only on ASP.Net controls attributes

<%# data-binding expression %>

You can use this anywhere inside controls with data-binding support, and usually is used by Eval and Bind methods.

<asp:DropDownList SelectedValue='<%# Bind("CategoryID") %>' 
                  DataSourceID="CategoriesDataSource"
                  DataTextField="CategoryName"
                  DataValueField="CategoryID"
                  runat="Server" />

Which one Eval or Bind?

Using Bind you have two-way binding set over specified attribute of ASP.NET control see the mentioned drop-down, it is using Bind that means if end-user selects a value and then submit the page, drop-down will not loose its selected value.

Use Eval just for displaying data.

<asp:FormView ...>
     <ItemTemplate>
          <a href='Eval("Link")'>Eval("LinkText")</a>
     </ItemTemplate>
</asp:FormView>

<%@ text template directive %>

<%@ Page ...%>
This one is Page Directive

<%@ OutputCache...%>
This one is OutputCache Directive and so on...

This is totally possible.

In your .aspx page, create a script reference to an aspx page that contains your javascript code:

<script src="../MyJavaScriptFile.js.aspx" type='text/javascript'></script>

Then, in MyJavaScriptFile.js.aspx you can write the following:

<%@ Page Language="C#" AutoEventWireup="false"  ContentType="text/javascript" %>

<% 
    var foo = new Whatever();
    foo.ClientId = 123;
%>

// Start Javascript
var clientId = <% HttpContext.Current.Response.Write(foo.ClientId); %>;

.

Also helpful - this technique supports querystring parameters:

 <script src="../MyJavaScriptFile.js.aspx?username=<% somevalue %>" 
       type='text/javascript'></script>

Then, in MyJavaScriptFile.js.aspx, I can reference the value with

var username = '<% Request.QueryString["username"] %>';

It's not the "best practice" way to do things, but it gets the job done in a way that my caveman brain can understand without resorting to fancy workarounds.

I like to embed a single line of javascript on my page.

<script type="text/javascript">
   Application.init({variable1: "value1", variable2: "value2"...});
</script>

It's poor practice to have a ton of javascript rendered on your page, but it is common to need to pass initialization values to your javascript from the server. This allows you to do it without a whole bunch of unnecessary code and without polluting the global namespace with callback functions. I usually replace Application with some project specific global wrapper.