Well, getting the error doesn't work...

doc/help/commands.asciidoc

@@ -20,6 +20,7 @@
 |<<hint,hint>>|Start hinting.
 |<<home,home>>|Open main startpage in current tab.
 |<<inspector,inspector>>|Toggle the web inspector.
+|<<jseval,jseval>>|Evaluate a JavaScript string.
 |<<later,later>>|Execute a command after some time.
 |<<navigate,navigate>>|Open typical prev/next links or navigate using the URL path.
 |<<open,open>>|Open a URL in the current/[count]th tab.
@@ -241,6 +242,19 @@ Open main startpage in current tab.
 === inspector
 Toggle the web inspector.
 
+[[jseval]]
+=== jseval
+Syntax: +:jseval 'js_code'+
+
+Evaluate a JavaScript string.
+
+==== positional arguments
+* +'js_code'+: The string to evaluate.
+
+==== note
+* This command does not split arguments after the last argument and handles quotes literally.
+* With this command, +;;+ is interpreted literally instead of splitting off a second command.
+
 [[later]]
 === later
 Syntax: +:later 'ms' 'command'+

qutebrowser/browser/commands.py

@@ -1513,20 +1513,28 @@ class CommandDispatcher:
 
     @cmdutils.register(instance='command-dispatcher', scope='window',
                        maxsplit=0, no_cmd_split=True)
-    def jseval(self, js):
+    def jseval(self, js_code):
         """Evaluate a JavaScript string.
 
         Args:
-            s: The string to evaluate.
+            js_code: The string to evaluate.
         """
         tabbed_browser = objreg.get('tabbed-browser', scope='window',
                                     window='last-focused')
-        out = tabbed_browser.widget(0).page().mainFrame().evaluateJavaScript(
-            'window.__qute_jseval__ = true;\n' + js)
+        frame = tabbed_browser.widget(0).page().mainFrame()
+        out = frame.evaluateJavaScript(js_code)
 
-        if out is not None:
+        if out is None:
+            # Getting the actual error (if any) seems to be difficult. The
+            # error does end up in BrowserPage.javaScriptConsoleMessage(), but
+            # distinguishing between :jseval errors and errors from the webpage
+            # is not trivial...
+            message.info(self._win_id, 'No output or error')
+        else:
+            # The output can be a string, number, dict, array, etc. But *don't*
+            # output too much data, as this will make qutebrowser hang
+            out = str(out)
+            if len(out) > 5000:
+                message.info(self._win_id, out[:5000] + ' [...trimmed...]')
+            else:
                 message.info(self._win_id, out)
-        elif tabbed_browser.widget(0).page().jseval_error:
-            message.error(self._win_id,
-                          tabbed_browser.widget(0).page().jseval_error)
-            tabbed_browser.widget(0).page().jseval_error = None

qutebrowser/browser/webpage.py

@@ -63,7 +63,6 @@ class BrowserPage(QWebPage):
     def __init__(self, win_id, tab_id, parent=None):
         super().__init__(parent)
         self._win_id = win_id
-        self.jseval_error = None
         self._is_shutting_down = False
         self._extension_handlers = {
             QWebPage.ErrorPageExtension: self._handle_errorpage,
@@ -498,16 +497,6 @@ class BrowserPage(QWebPage):
 
     def javaScriptConsoleMessage(self, msg, line, source):
         """Override javaScriptConsoleMessage to use debug log."""
-        
-        jseval = self.mainFrame().evaluateJavaScript('window.__qute_jseval__')
-        if jseval:
-            self.mainFrame().evaluateJavaScript('window.__qute_jseval__ = undefined;')
-        if source == 'undefined' and jseval:
-            self.mainFrame().evaluateJavaScript('window.__qute_jseval__ = false;')
-            print('jseval errror ->', jseval)
-            self.jseval_error = 'Error on line {}: {}'.format(line, msg)
-        print('other js error ->', msg, line, source)
-
         if config.get('general', 'log-javascript-console'):
             log.js.debug("[{}:{}] {}".format(source, line, msg))