diff --git a/servant-client/src/Servant/Client/PerformRequest/GHCJS.hs b/servant-client/src/Servant/Client/PerformRequest/GHCJS.hs
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+{-# LANGUAGE OverloadedStrings #-}
+{-# LANGUAGE ViewPatterns #-}
+
+{-# OPTIONS_GHC -fno-warn-name-shadowing #-}
+
+module Servant.Client.PerformRequest.GHCJS (
+  ServantError(..),
+  performHttpRequest,
+
+  -- exported for testing
+  parseHeaders,
+  ) where
+
+import           Control.Arrow
+import           Control.Concurrent
+import           Control.Exception
+import           Control.Monad
+import qualified Data.ByteString.Char8 as BS
+import qualified Data.ByteString.Lazy as LBS
+import           Data.CaseInsensitive
+import           Data.Char
+import           Data.String.Conversions
+import           Foreign.StablePtr
+import           GHCJS.Foreign.Callback
+import           GHCJS.Prim
+import           Network.HTTP.Client
+import           Network.HTTP.Client.Internal as HttpClient
+import           Network.HTTP.Types
+import           Servant.Client.PerformRequest.Base
+
+newtype JSXMLHttpRequest = JSXMLHttpRequest JSVal
+
+newtype JSXMLHttpRequestClass = JSXMLHttpRequestClass JSVal
+
+performHttpRequest :: Manager -> Request -> IO (Either ServantError (Response LBS.ByteString))
+performHttpRequest _ request = do
+  xhr <- initXhr
+  performXhr xhr request
+  toResponse xhr
+
+-- * initialization
+
+initXhr :: IO JSXMLHttpRequest
+initXhr = do
+  lib <- requireXMLHttpRequestClass
+  newXMLHttpRequest lib
+
+foreign import javascript unsafe
+  -- branching between node (for testing) and browsers
+  "(function () {if (typeof require !== 'undefined') { return require('xhr2'); } else { return XMLHttpRequest; };})()"
+  requireXMLHttpRequestClass :: IO JSXMLHttpRequestClass
+
+foreign import javascript unsafe "new $1()"
+  newXMLHttpRequest :: JSXMLHttpRequestClass -> IO JSXMLHttpRequest
+
+-- * performing requests
+
+-- Performs the xhr and blocks until the response was received
+performXhr :: JSXMLHttpRequest -> Request -> IO ()
+performXhr xhr request = do
+    waiter <- newEmptyMVar
+
+    bracket (acquire waiter) releaseCallback $ \_callback -> do
+        t <- myThreadId
+        s <- newStablePtr t
+
+        openXhr xhr (cs $ method request) (toUrl request) True
+        setHeaders xhr (requestHeaders request)
+        sendXhr xhr (toBody request)
+        takeMVar waiter
+
+        freeStablePtr s
+  where
+    acquire waiter = onReadyStateChange xhr $ do
+      state <- readyState xhr
+      case state of
+        -- onReadyStateChange's callback can fire state 4
+        -- (which means "request finished and response is ready")
+        -- multiple times. By using tryPutMVar, only the first time
+        -- state 4 is fired will cause an MVar to be put. Subsequent
+        -- fires are ignored.
+        4 -> tryPutMVar waiter () >> return ()
+        _ -> return ()
+
+onReadyStateChange :: JSXMLHttpRequest -> IO () -> IO (Callback (IO ()))
+onReadyStateChange xhr action = do
+  callback <- asyncCallback action
+  js_onReadyStateChange xhr callback
+  return callback
+foreign import javascript safe "$1.onreadystatechange = $2;"
+  js_onReadyStateChange :: JSXMLHttpRequest -> Callback (IO ()) -> IO ()
+
+foreign import javascript unsafe "$1.readyState"
+  readyState :: JSXMLHttpRequest -> IO Int
+
+openXhr :: JSXMLHttpRequest -> String -> String -> Bool -> IO ()
+openXhr xhr method url async =
+  js_openXhr xhr (toJSString method) (toJSString url) async
+foreign import javascript unsafe "$1.open($2, $3, $4)"
+  js_openXhr :: JSXMLHttpRequest -> JSVal -> JSVal -> Bool -> IO ()
+
+toUrl :: Request -> String
+toUrl request =
+  let protocol = if secure request then "https" else "http"
+      hostS = cs $ host request
+      portS = show $ port request
+      pathS = cs $ path request
+      queryS = cs $ queryString request
+  in protocol ++ "://" ++ hostS ++ ":" ++ portS ++ pathS ++ queryS
+
+setHeaders :: JSXMLHttpRequest -> RequestHeaders -> IO ()
+setHeaders xhr headers = forM_ headers $ \ (key, value) ->
+  js_setRequestHeader xhr (toJSString $ cs $ original key) (toJSString $ cs value)
+foreign import javascript unsafe "$1.setRequestHeader($2, $3)"
+  js_setRequestHeader :: JSXMLHttpRequest -> JSVal -> JSVal -> IO ()
+
+sendXhr :: JSXMLHttpRequest -> Maybe String -> IO ()
+sendXhr xhr Nothing = js_sendXhr xhr
+sendXhr xhr (Just body) =
+  js_sendXhrWithBody xhr (toJSString body)
+foreign import javascript unsafe "$1.send()"
+  js_sendXhr :: JSXMLHttpRequest -> IO ()
+foreign import javascript unsafe "$1.send($2)"
+  js_sendXhrWithBody :: JSXMLHttpRequest -> JSVal -> IO ()
+
+toBody :: Request -> Maybe String
+toBody request = case requestBody request of
+  RequestBodyLBS "" -> Nothing
+  RequestBodyLBS x -> Just $ cs x
+  _ -> error "servant-client only uses RequestBodyLBS"
+
+-- * inspecting the xhr response
+
+-- This function is only supposed to handle 'ConnectionError's. Other
+-- 'ServantError's are created in Servant.Client.Req.
+toResponse :: JSXMLHttpRequest -> IO (Either ServantError (Response LBS.ByteString))
+toResponse xhr = do
+  status <- getStatus xhr
+  case status of
+    0 -> return $ Left $ ConnectionError $ SomeException $ ErrorCall "connection error"
+    _ -> do
+      statusText <- cs <$> getStatusText xhr
+      headers <- parseHeaders <$> getAllResponseHeaders xhr
+      responseText <- cs <$> getResponseText xhr
+      return $ Right $ Response {
+        HttpClient.responseStatus = mkStatus status statusText,
+        responseVersion = http11, -- this is made up
+        responseHeaders = headers,
+        HttpClient.responseBody = responseText,
+        responseCookieJar = mempty,
+        responseClose' = ResponseClose (return ())
+      }
+
+foreign import javascript unsafe "$1.status"
+  getStatus :: JSXMLHttpRequest -> IO Int
+
+getStatusText :: JSXMLHttpRequest -> IO String
+getStatusText = fmap fromJSString . js_statusText
+foreign import javascript unsafe "$1.statusText"
+  js_statusText :: JSXMLHttpRequest -> IO JSVal
+
+getAllResponseHeaders :: JSXMLHttpRequest -> IO String
+getAllResponseHeaders xhr =
+  fromJSString <$> js_getAllResponseHeaders xhr
+foreign import javascript unsafe "$1.getAllResponseHeaders()"
+  js_getAllResponseHeaders :: JSXMLHttpRequest -> IO JSVal
+
+getResponseText :: JSXMLHttpRequest -> IO String
+getResponseText xhr = fromJSString <$> js_responseText xhr
+foreign import javascript unsafe "$1.responseText"
+  js_responseText :: JSXMLHttpRequest -> IO JSVal
+
+parseHeaders :: String -> ResponseHeaders
+parseHeaders s =
+  fmap (first mk) $
+  fmap (first strip . second strip) $
+  fmap parseHeader $
+  splitOn "\r\n" (cs s)
+  where
+    parseHeader :: BS.ByteString -> (BS.ByteString, BS.ByteString)
+    parseHeader h = case BS.breakSubstring ":" (cs h) of
+      (key, (BS.drop 1 -> value)) -> (key, value)
+
+    splitOn :: BS.ByteString -> BS.ByteString -> [BS.ByteString]
+    splitOn separator input = case BS.breakSubstring separator input of
+      (prefix, "") -> [prefix]
+      (prefix, rest) -> prefix : splitOn separator (BS.drop (BS.length separator) rest)
+
+    strip :: BS.ByteString -> BS.ByteString
+    strip = BS.dropWhile isSpace . BS.reverse . BS.dropWhile isSpace . BS.reverse