sinatra-rack-static.rb

require 'rack'
require 'sinatra/base'

class App < Sinatra::Base
  use Rack::Static, :urls => ["/js"], :root => '.'

  not_found do
    halt 404, "nawp"
  end

  error 404 do
    halt 404, "nawp"
  end

  error do
    halt 404, "nawp"
  end
end

test

code λ curl -v localhost:9292/js/not-found.js
* About to connect() to localhost port 9292 (#0)
*   Trying ::1...
* Connection refused
*   Trying 127.0.0.1...
* connected
* Connected to localhost (127.0.0.1) port 9292 (#0)
> GET /js/not-found.js HTTP/1.1
> User-Agent: curl/7.24.0 (x86_64-apple-darwin12.0) libcurl/7.24.0 OpenSSL/0.9.8r zlib/1.2.5
> Host: localhost:9292
> Accept: */*
> 
< HTTP/1.1 404 Not Found
< Content-Type: text/plain
< Content-Length: 33
< X-Cascade: pass
< X-Content-Type-Options: nosniff
< Connection: keep-alive
< Server: thin 1.5.0 codename Knife
< 
File not found: /js/not-found.js
* Connection #0 to host localhost left intact
* Closing connection #0