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How do I pass a JSON array from a text/javascript block to a runat=server block in exacttarget/salesforce marketing cloud?

I've got a page with two <script> blocks on it, one running locally, one running on the ExactTarget server:
<script type="text/javascript">
var results = localStorage.getItem('results');
var results = JSON.parse(results);
</script>
<script runat=server>
Platform.Load("Core","1");
Write(Stringify(results)); //this returns a 'null' value
var campaignsupdate = DataExtension.Init("0DB9904F-CE05-45E7-8052- 7C6935B4B012");
for(var i=0; i < results.length; i++) {
var rowid = results[i]["ROWID"];
var title = results[i]["title"];
campaignsupdate.Rows.Update({title: title},["ROWID"], [rowid]);
}
</script>
At the moment, calling 'results' in the 'runat=server' block returns a null value. How do I access the 'results' array in the 'runat=server' block?

You can't. As the Server Side Java Script runs on the server :), it's executed prior to the javascript which is executed on the client machine, so the var 'results' does not exists when you access to it in the SSJS block.
As far as I know SSJS can't communicate with javascript so you can't use javascript libraries or code written in a javascript block.
As an alternative, you can pass a value in the GET/POST request and get it in SSJS.
Have a look at: http://help.marketingcloud.com/en/documentation/exacttarget/content/server_side_javascript/server_side_javascript_syntax_guide/core_library_server_side_javascript_functions/utilities_server_side_javascript_functions/
Request Object and Functions Area

How to insert javascript variable inside ejs tags

I am doing dynamic filtering ,but i am unable to do if condition check.
i want to insert targetOS var inside <%if(data[i].ListNames == targetOS ){%>
for the above syntax it is giving "targetOS variable is undefined"
Please help on this.
$(document).on("change",'.COTSCurrentOSClass', function(e){
var selectedOS = $(this).val();
var targetOS = "OS::From::"+selectedOS;
<%for(var i=0;i<data.length;i++){%>
<%if(data[i].ListNames == ***targetOS*** ){%>
<%for(var j=0;j<data[i].Values.length;j++){%>
console.log("options are");
console.log(data[i].Values[j]);
<%}}}%>
($(this).parent().parent()).find(".COTSTargetOSClass").append("<option>Select One</option>");
});

What you want to do is not possible!
This is the sequence of what happens:
1 - In the server, the ejs is rendered to HTML
2 - the HTML is then transferred into the browser
3 - The browser reads and processes the javascript
The JavaScript execution happens at a much later stage than the ejs execution.

What #fmsf has replied is correct. However, if you know the targetOS value on the server, while rendering the ejs you can do it like this:
define it like this in your ejs file:
var TARGET_OS = "<%= TARGET_OS %>";
Then when you render the page via ejs, pass parameter to it like this:
res.render(url, {TARGET_OS: "Your desired value here"}, function(err, html) {

How to insert JavaScript variable in JSP String variable

How to insert JavaScript variable in JSP String variable.
<script>
<%!
String host_name = "tst" + location.host;
%>
</script>
<%=host_name%> <!-- getting error -->
Thanks for help.

Javascript is executed in the browser, and JSP on the server. So you can't directly get a value from javascript into JSP (java) : when the javascript is executed, JSP has finished its execution and sent the result.
Depending on your need, you may want to send a value using ajax from the browser to the server but it's impossible to define the applicable strategy without knowing why you tried this.
If what you want is just have your hostname in java/jsp, I'd suggest you to do something along the lines of
<%
InetAddress addr = InetAddress.getLocalHost();
String hostname = addr.getHostName();
%>

assign javascript variable to ruby variable

I am working on one script which has been called from the websocket . This page of code is of html.erb
It pass variable to the javascript, and from that javascript variable i want to assign it to ruby variable ,
Here is the code
function set_color(val1,val2)
{
<%background_color_id = %>
var obj_color_id = '<%=background_color_id ='+val2+'%>' ;
console.log(obj_color_id)
}
The result from console log is +val2+
If I pass var obj_color_id = '<%=background_color_id ='val2'%>' ;
The result from console log is val2
Please help me assign javascript variable to ruby variable

You cannot do this. Javascript runs Client side, Ruby runs Server side.

You can't do that. All values <%= are translated on server side and their values are send to client. There is no ruby on client side. You have to send request to your websocket or http server in order to pass some data to server.

Actually, if I understand your code (your question is not well phrased, unfortunately), the simple solution is:
1- Assign a value via server-side code:
function set_color(val1,val2)
{
var bkgdColorId = "<%= background_color_id %>";
var obj_color_id = bkgdColorId;
console.log(obj_color_id)
}
2- (Or,) Assign a value from client-side code:
function set_color(val1,val2)
{
/** pseudo-code **/
on-click-event: makeAjaxCallToServer(){
urlForWebService, { color: val2 }
}
}
Using some jQuery (if assigning to server from client-side event) would greatly facilitate this process.

how to assign a javascript variable to a jsp string variable?

i m doing a database project using mysql as the database and jsp for my frontend part along with javascript and ajax for interaction.
Now the problem is i need to assign a java script variable which is having a string to a jsp variable.
I did this using the following statement?
<% String str="document.write(s)"; %>
where "s" is already defined as
<script type="text/javascript">
var s="hello world";
<script>
but i m getting error in the assignment statement(which is shown in bold above) as incorrect syntax?
the error i m getting is-
check the manual that corresponds to your MySQL server version for the right syntax to use near '<script>document.write(s)</script>'
what is the error in this stmt or is there any other method in doing this assignment?
Can anyone help in doing this?

You cannot do this. The JSP statement is executed server-side, before the execution of the Javascript statement, that is executed client-side after the browser received the http response.
It is not clear your goal, but if you only need to display in the page the value of a javascript variable, you can use:
trivial javascript:
document.write(s);
targeting existing element:
document.getElementById('myElementId').innerHTML = s;
using jQuery:
$('#myElementId').html(s);

If i understand correctly you are trying to build and execute a sql query based on user input that is handled by javascript. As ADC said this can not be done since jsp is executed server side therefore before browser executes javascript. What you can do is create the sql query and pass this as a parameter a different jsp/servlet (or the same if you can handle this case) which will execute the query

In the first page where sql statement is constructed in variable s you should put something like this
<script>
var s = "hello world";
function createLink(){
document.getElementById('mylink').href= 'page2.jsp?statement='+s;
}
window.onload=createLink;
</script>
<a id="mylink" href=""/>Click to exexute query</a>
which create a link to you second page (page2.jsp) passing the statement as parameter.
Now in page2.jsp you should retreive the parameter value like
<% String statement = request.getParameter("statement") %>
and then execute your query.
Even better you should use a servlet instead of a jsp page to perform the query. You could read a tutorial for jsp/sevlets to see how this can be done
eg. http://www.laliluna.de/articles/posts/first-java-servlets-jsp-tutorial.html