Hi
The problem is that the whole array is included in the html and users can see it if they look at the html source. I want to hide this information from the user for 2 reasons: 1) it makes the html source huge
One way would be to simply define jsarray as a global variable in the head of your HTML file on first page load. Then populate it with a second ajax call after the page loads. I don't know how the Autocompleter handles that variable (I've never used Autocompleter.Local before), it looks as though updating jsarray after the page loads (with the Autocompleter already initialized) won't do much. If this is the case, you could try setting the Autocompleter with the same rjs you're using to set jsarray.
Sadly, this approach will mean that you will have to spend two requests per page load (the first being the actual page load, and the second an ajax call to populate the array) If this is an in-house project or will otherwise be receiving relatively little traffic this shouldn't be much of a problem. Also, this ajax call will take as long as typing something in a regular Autocompleter would have taken to respond (if I'm correct) so its not exactly blasphemous...
This could be done with something like the following
#view <%=javascript_tag("var jsarray = new Array();")%> <%=javascript_tag(remote_function(:url => {...}))%>
#controller def foo some_array = #populate some_array
render(:update) do |page| page.assign('jsarray', some_array.inspect) #cool use of inspect btw, #rails has 'array_or_string_for_javascript', #but inspect is pretty cool
page << "new Autocompleter.Local(..., ..., jsarray , {...} ); " #the line from your view end end
2) more important I want to only let users access it through the autocomplete functionality. I don't want them to see all the options.
If this is your greatest issue, you could try looking at encoding the array so humans will have a difficult time seeing the values?
jsarray = ['f^o^o', 'b^a^r'] for (int i=0; i < jsarray.length; i++) jsarray[i].replace(/\^/, '');
Of course you should use slightly better encryption 
You could have a hash that maps encoded letters to their real counterparts?
enc_hash = {b: "a", c: "b", ....}
jsarray = new Array(); //these are the 'encrypted' values that the original jsarray would have had encoded_array = ['bc', 'cb']
for(int word=0; word < encoded_array.length; word++) { endstr = ""; for(int letter=0; letter < encoded_array[word].length; letter++) endstr += enc_hash[encoded_array[word][letter]]; jsarray.push(enstr) }
jsarray == ['ab', 'ba'] // true
It's been a while since I did anything with javascript so the above
might not work, but I got carried away and couldn't stop
A third possibility would be to look at the Unobtrusive Javascript Plugin (http://ujs4rails.com). I've never used it, but it sounded like they magically got all your regular JS out of the html file being served to the client? I might have misunderstood what they do but it should still be a good source of inspiration? This could let you define the jsarray exactly as you do now, but have it somewhere (an automatically generated .js file that gets served alongside the html without you telling it to? I don't know how they do it) other than the html source of the page.
I can't think of any other ways at the moment, but maybe someone else in here has faced a similar problem?
Cheery-o Gustav Paul