diff --git a/src/client/components/components-web-view.vala b/src/client/components/components-web-view.vala
index 4bda1c11..368b6a8d 100644
--- a/src/client/components/components-web-view.vala
+++ b/src/client/components/components-web-view.vala
@@ -370,9 +370,7 @@ public abstract class Components.WebView : WebKit.WebView, Geary.BaseInterface {
      * Returns the view's content as an HTML string.
      */
     public async string? get_html() throws Error {
-        return Util.JS.to_string(
-            yield call(Util.JS.callable("geary.getHtml"), null)
-        );
+        return yield call_returning<string?>(Util.JS.callable("getHtml"), null);
     }
 
     /**
@@ -410,7 +408,7 @@ public abstract class Components.WebView : WebKit.WebView, Geary.BaseInterface {
      * Load any remote images previously that were blocked.
      */
     public void load_remote_images() {
-        this.call.begin(Util.JS.callable("geary.loadRemoteImages"), null);
+        this.call_void.begin(Util.JS.callable("loadRemoteImages"), null);
     }
 
     /**
@@ -455,21 +453,100 @@ public abstract class Components.WebView : WebKit.WebView, Geary.BaseInterface {
     public new async void set_editable(bool enabled,
                                        Cancellable? cancellable)
         throws Error {
-        yield call(
-            Util.JS.callable("geary.setEditable").bool(enabled), cancellable
+        yield call_void(
+            Util.JS.callable("setEditable").bool(enabled), cancellable
         );
     }
 
     /**
      * Invokes a {@link Util.JS.Callable} on this web view.
+     *
+     * This calls the given callable on the `geary` object for the
+     * current view, any returned value are ignored.
      */
-    protected async JSC.Value call(Util.JS.Callable target,
+    protected async void call_void(Util.JS.Callable target,
                                    GLib.Cancellable? cancellable)
         throws GLib.Error {
-        WebKit.JavascriptResult result = yield run_javascript(
-            target.to_string(), cancellable
+        yield send_message_to_page(
+            target.to_message(), cancellable
         );
-        return result.get_js_value();
+    }
+
+    /**
+     * Invokes a {@link Util.JS.Callable} on this web view.
+     *
+     * This calls the given callable on the `geary` object for the
+     * current view. The value returned by the call is returned by
+     * this method.
+     *
+     * The type parameter `T` must match the type returned by the
+     * call, else an error is thrown. Only simple nullable value types
+     * are supported for T, for more complex return types (arrays,
+     * dictionaries, etc) specify {@link GLib.Variant} for `T` and
+     * manually parse that.
+     */
+    protected async T call_returning<T>(Util.JS.Callable target,
+                                        GLib.Cancellable? cancellable)
+        throws GLib.Error {
+        WebKit.UserMessage? response = yield send_message_to_page(
+            target.to_message(), cancellable
+        );
+        if (response == null) {
+            throw new Util.JS.Error.TYPE(
+                "Method call did not return a value: %s", target.to_string()
+            );
+        }
+        GLib.Variant? param = response.parameters;
+        T ret_value = null;
+        var ret_type = typeof(T);
+        if (ret_type == typeof(GLib.Variant)) {
+            ret_value = param;
+        } else {
+            if (param != null && param.get_type().is_maybe()) {
+                param = param.get_maybe();
+            }
+            if (param != null) {
+                // Since these replies are coming from JS via
+                // Util.JS.value_to_variant, they will only be one of
+                // string, double, bool, array or dict
+                var param_type = param.classify();
+                if (ret_type == typeof(string) && param_type == STRING) {
+                    ret_value = param.get_string();
+                } else if (ret_type == typeof(bool) && param_type == BOOLEAN) {
+                    ret_value = (bool?) param.get_boolean();
+                } else if (ret_type == typeof(int) && param_type == DOUBLE) {
+                    ret_value = (int?) ((int) param.get_double());
+                } else if (ret_type == typeof(short) && param_type == DOUBLE) {
+                    ret_value = (short?) ((short) param.get_double());
+                } else if (ret_type == typeof(char) && param_type == DOUBLE) {
+                    ret_value = (char?) ((char) param.get_double());
+                } else if (ret_type == typeof(long) && param_type == DOUBLE) {
+                    ret_value = (long?) ((long) param.get_double());
+                } else if (ret_type == typeof(int64) && param_type == DOUBLE) {
+                    ret_value = (int64?) ((int64) param.get_double());
+                } else if (ret_type == typeof(uint) && param_type == DOUBLE) {
+                    ret_value = (uint?) ((uint) param.get_double());
+                } else if (ret_type == typeof(uchar) && param_type == DOUBLE) {
+                    ret_value = (uchar?) ((uchar) param.get_double());
+                } else if (ret_type == typeof(ushort) && param_type == DOUBLE) {
+                    ret_value = (ushort?) ((ushort) param.get_double());
+                } else if (ret_type == typeof(ulong) && param_type == DOUBLE) {
+                    ret_value = (ulong?) ((ulong) param.get_double());
+                } else if (ret_type == typeof(uint64) && param_type == DOUBLE) {
+                    ret_value = (uint64?) ((uint64) param.get_double());
+                } else if (ret_type == typeof(double) && param_type == DOUBLE) {
+                    ret_value = (double?) param.get_double();
+                } else if (ret_type == typeof(float) && param_type == DOUBLE) {
+                    ret_value = (float?) ((float) param.get_double());
+                } else {
+                    throw new Util.JS.Error.TYPE(
+                        "%s is not a supported type for %s",
+                        ret_type.name(), param_type.to_string()
+                    );
+                }
+            }
+        }
+        return ret_value;
     }
 
     /**
diff --git a/src/client/composer/composer-web-view.vala b/src/client/composer/composer-web-view.vala
index f8ecccf6..24a2740c 100644
--- a/src/client/composer/composer-web-view.vala
+++ b/src/client/composer/composer-web-view.vala
@@ -202,8 +202,8 @@ public class Composer.WebView : Components.WebView {
      * Returns the view's content as HTML without being cleaned.
      */
     public async string? get_html_for_draft() throws Error {
-        return Util.JS.to_string(
-            yield call(Util.JS.callable("geary.getHtml").bool(false), null)
+        return yield call_returning<string?>(
+            Util.JS.callable("getHtml").bool(false), null
         );
     }
 
@@ -213,8 +213,8 @@ public class Composer.WebView : Components.WebView {
     public void set_rich_text(bool enabled) {
         this.is_rich_text = enabled;
         if (this.is_content_loaded) {
-            this.call.begin(
-                Util.JS.callable("geary.setRichText").bool(enabled), null
+            this.call_void.begin(
+                Util.JS.callable("setRichText").bool(enabled), null
             );
         }
     }
@@ -223,14 +223,14 @@ public class Composer.WebView : Components.WebView {
      * Undoes the last edit operation.
      */
     public void undo() {
-        this.call.begin(Util.JS.callable("geary.undo"), null);
+        this.call_void.begin(Util.JS.callable("undo"), null);
     }
 
     /**
      * Redoes the last undone edit operation.
      */
     public void redo() {
-        this.call.begin(Util.JS.callable("geary.redo"), null);
+        this.call_void.begin(Util.JS.callable("redo"), null);
     }
 
     /**
@@ -239,9 +239,9 @@ public class Composer.WebView : Components.WebView {
      * Returns an id to be used to refer to the selection in
      * subsequent calls.
      */
-    public async string save_selection() throws Error {
-        return Util.JS.to_string(
-            yield call(Util.JS.callable("geary.saveSelection"), null)
+    public async string? save_selection() throws Error {
+        return yield call_returning<string?>(
+            Util.JS.callable("saveSelection"), null
         );
     }
 
@@ -249,9 +249,7 @@ public class Composer.WebView : Components.WebView {
      * Removes a saved selection.
      */
     public void free_selection(string id) {
-        this.call.begin(
-            Util.JS.callable("geary.freeSelection").string(id), null
-        );
+        this.call_void.begin(Util.JS.callable("freeSelection").string(id), null);
     }
 
     /**
@@ -357,9 +355,9 @@ public class Composer.WebView : Components.WebView {
      * will be inserted wrapping the selection.
      */
     public void insert_link(string href, string selection_id) {
-        this.call.begin(
+        this.call_void.begin(
             Util.JS.callable(
-                "geary.insertLink"
+                "insertLink"
             ).string(href).string(selection_id),
             null
         );
@@ -373,8 +371,8 @@ public class Composer.WebView : Components.WebView {
      * unlinked section.
      */
     public void delete_link(string selection_id) {
-        this.call.begin(
-            Util.JS.callable("geary.deleteLink").string(selection_id),
+        this.call_void.begin(
+            Util.JS.callable("deleteLink").string(selection_id),
             null
         );
     }
@@ -396,23 +394,23 @@ public class Composer.WebView : Components.WebView {
      * Indents the line at the current text cursor location.
      */
     public void indent_line() {
-        this.call.begin(Util.JS.callable("geary.indentLine"), null);
+        this.call_void.begin(Util.JS.callable("indentLine"), null);
     }
 
     public void insert_olist() {
-        this.call.begin(Util.JS.callable("geary.insertOrderedList"), null);
+        this.call_void.begin(Util.JS.callable("insertOrderedList"), null);
     }
 
     public void insert_ulist() {
-        this.call.begin(Util.JS.callable("geary.insertUnorderedList"), null);
+        this.call_void.begin(Util.JS.callable("insertUnorderedList"), null);
     }
 
     /**
      * Updates the signature block if it has not been deleted.
      */
     public new void update_signature(string signature) {
-        this.call.begin(
-            Util.JS.callable("geary.updateSignature").string(signature), null
+        this.call_void.begin(
+            Util.JS.callable("updateSignature").string(signature), null
         );
     }
 
@@ -420,22 +418,21 @@ public class Composer.WebView : Components.WebView {
      * Removes the quoted message (if any) from the composer.
      */
     public void delete_quoted_message() {
-        this.call.begin(Util.JS.callable("geary.deleteQuotedMessage"), null);
+        this.call_void.begin(Util.JS.callable("deleteQuotedMessage"), null);
     }
 
     /**
      * Determines if the editor content contains an attachment keyword.
      */
-    public async bool contains_attachment_keywords(string keyword_spec,
-                                                   string subject) {
+    public async bool? contains_attachment_keywords(string keyword_spec,
+                                                    string subject) {
         try {
-            return Util.JS.to_bool(
-                yield call(
-                    Util.JS.callable("geary.containsAttachmentKeyword")
-                    .string(keyword_spec)
-                    .string(subject),
-                    null)
-                );
+            return yield call_returning<bool?>(
+                Util.JS.callable("containsAttachmentKeyword")
+                .string(keyword_spec)
+                .string(subject),
+                null
+            );
         } catch (Error err) {
             debug("Error checking or attachment keywords: %s", err.message);
             return false;
@@ -449,7 +446,7 @@ public class Composer.WebView : Components.WebView {
      * this.
      */
     public async void clean_content() throws Error {
-        this.call.begin(Util.JS.callable("geary.cleanContent"), null);
+        this.call_void.begin(Util.JS.callable("cleanContent"), null);
     }
 
     /**
@@ -459,10 +456,10 @@ public class Composer.WebView : Components.WebView {
         const int MAX_BREAKABLE_LEN = 72; // F=F recommended line limit
         const int MAX_UNBREAKABLE_LEN = 998; // SMTP line limit
 
-        string body_text = Util.JS.to_string(
-            yield call(Util.JS.callable("geary.getText"), null)
+        string? body_text = yield call_returning<string?>(
+            Util.JS.callable("getText"), null
         );
-        string[] lines = body_text.split("\n");
+        string[] lines = (body_text ?? "").split("\n");
         GLib.StringBuilder flowed = new GLib.StringBuilder.sized(body_text.length);
         foreach (string line in lines) {
             // Strip trailing whitespace, so it doesn't look like a
diff --git a/src/client/composer/composer-widget.vala b/src/client/composer/composer-widget.vala
index 4c4d0caf..9148a88e 100644
--- a/src/client/composer/composer-widget.vala
+++ b/src/client/composer/composer-widget.vala
@@ -1450,15 +1450,16 @@ public class Composer.Widget : Gtk.EventBox, Geary.BaseInterface {
             confirmation = _("Send message with an empty subject?");
         } else if (!has_body && !has_attachment) {
             confirmation = _("Send message with an empty body?");
-        } else if (!has_attachment &&
-                   yield this.editor.body.contains_attachment_keywords(
-                       string.join(
-                           "|",
-                           ATTACHMENT_KEYWORDS,
-                           ATTACHMENT_KEYWORDS_LOCALISED
-                       ),
-                       this.subject)) {
-            confirmation = _("Send message without an attachment?");
+        } else if (!has_attachment) {
+            var keywords = string.join(
+                "|", ATTACHMENT_KEYWORDS, ATTACHMENT_KEYWORDS_LOCALISED
+            );
+            var contains = yield this.editor.body.contains_attachment_keywords(
+                keywords, this.subject
+            );
+            if (contains != null && contains) {
+                confirmation = _("Send message without an attachment?");
+            }
         }
         if (confirmation != null) {
             ConfirmationDialog dialog = new ConfirmationDialog(container.top_window,
diff --git a/src/client/conversation-viewer/conversation-web-view.vala b/src/client/conversation-viewer/conversation-web-view.vala
index ffa36394..d77af642 100644
--- a/src/client/conversation-viewer/conversation-web-view.vala
+++ b/src/client/conversation-viewer/conversation-web-view.vala
@@ -89,20 +89,18 @@ public class ConversationWebView : Components.WebView {
      * Returns the current selection, for prefill as find text.
      */
     public async string? get_selection_for_find() throws Error{
-        JSC.Value result = yield call(
-            Util.JS.callable("geary.getSelectionForFind"), null
+        return yield call_returning<string?>(
+            Util.JS.callable("getSelectionForFind"), null
         );
-        return Util.JS.to_string(result);
     }
 
     /**
      * Returns the current selection, for quoting in a message.
      */
     public async string? get_selection_for_quoting() throws Error {
-        JSC.Value result = yield call(
-            Util.JS.callable("geary.getSelectionForQuoting"), null
+        return yield call_returning<string?>(
+            Util.JS.callable("getSelectionForQuoting"), null
         );
-        return Util.JS.to_string(result);
     }
 
     /**
@@ -110,10 +108,9 @@ public class ConversationWebView : Components.WebView {
      */
     public async int? get_anchor_target_y(string anchor_body)
         throws GLib.Error {
-        JSC.Value result = yield call(
-            Util.JS.callable("geary.getAnchorTargetY").string(anchor_body), null
+        return yield call_returning<int?>(
+            Util.JS.callable("getAnchorTargetY").string(anchor_body), null
         );
-        return (int) Util.JS.to_int32(result);
     }
 
     /**
diff --git a/src/client/util/util-js.vala b/src/client/util/util-js.vala
index 2f05a3e2..d2ce9f2e 100644
--- a/src/client/util/util-js.vala
+++ b/src/client/util/util-js.vala
@@ -348,40 +348,54 @@ namespace Util.JS {
      */
     public class Callable {
 
-        private string base_name;
-        private string[] safe_args = new string[0];
+        private string name;
+        private GLib.Variant[] args = {};
 
 
-        public Callable(string base_name) {
-            this.base_name = base_name;
+        public Callable(string name) {
+            this.name = name;
+        }
+
+        public WebKit.UserMessage to_message() {
+            GLib.Variant? args = null;
+            if (this.args.length == 1) {
+                args = this.args[0];
+            } else if (this.args.length > 1) {
+                args = new GLib.Variant.tuple(this.args);
+            }
+            return new WebKit.UserMessage(this.name, args);
         }
 
         public string to_string() {
-            return base_name + "(" + global::string.joinv(",", safe_args) + ");";
+            string[] args = new string[this.args.length];
+            for (int i = 0; i < args.length; i++) {
+                args[i] = this.args[i].print(true);
+            }
+            return this.name + "(" + global::string.joinv(",", args) + ")";
         }
 
         public Callable string(string value) {
-            add_param("\"" + escape_string(value) + "\"");
+            add_param(new GLib.Variant.string(value));
             return this;
         }
 
         public Callable double(double value) {
-            add_param(value.to_string());
+            add_param(new GLib.Variant.double(value));
             return this;
         }
 
         public Callable int(int value) {
-            add_param(value.to_string());
+            add_param(new GLib.Variant.int32(value));
             return this;
         }
 
         public Callable bool(bool value) {
-            add_param(value ? "true" : "false");
+            add_param(new GLib.Variant.boolean(value));
             return this;
         }
 
-        private inline void add_param(string value) {
-            this.safe_args += value;
+        private inline void add_param(GLib.Variant value) {
+            this.args += value;
         }
 
     }
diff --git a/src/client/web-process/web-process-extension.vala b/src/client/web-process/web-process-extension.vala
index 4bba5154..86f7f44c 100644
--- a/src/client/web-process/web-process-extension.vala
+++ b/src/client/web-process/web-process-extension.vala
@@ -30,6 +30,10 @@ public void webkit_web_extension_initialize_with_user_data(WebKit.WebExtension e
  */
 public class GearyWebExtension : Object {
 
+    private const string PAGE_STATE_OBJECT_NAME = "geary";
+    private const string MESSAGE_RETURN_VALUE_NAME = "__return__";
+    private const string MESSAGE_EXCEPTION_NAME = "__exception__";
+
     private const string[] ALLOWED_SCHEMES = { "cid", "geary", "data", "blob" };
 
     private const string REMOTE_LOAD_VAR = "_gearyAllowRemoteResourceLoads";
@@ -157,6 +161,55 @@ public class GearyWebExtension : Object {
         page.get_editor().selection_changed.connect(() => {
                 selection_changed(page);
             });
+        page.user_message_received.connect(on_page_message_received);
+    }
+
+    private bool on_page_message_received(WebKit.WebPage page,
+                                          WebKit.UserMessage message) {
+        WebKit.Frame frame = page.get_main_frame();
+        JSC.Context context = frame.get_js_context();
+        JSC.Value page_state = context.get_value(PAGE_STATE_OBJECT_NAME);
+
+        try {
+            JSC.Value[]? call_param = null;
+            GLib.Variant? message_param = message.parameters;
+            if (message_param != null) {
+                if (message_param.is_container()) {
+                    size_t len = message_param.n_children();
+                    call_param = new JSC.Value[len];
+                    for (size_t i = 0; i < len; i++) {
+                        call_param[i] = Util.JS.variant_to_value(
+                            context,
+                            message_param.get_child_value(i)
+                        );
+                    }
+                } else {
+                    call_param = {
+                        Util.JS.variant_to_value(context, message_param)
+                    };
+                }
+            }
+
+            JSC.Value ret = page_state.object_invoke_methodv(
+                message.name, call_param
+            );
+
+            // Must send a reply, even for void calls, otherwise
+            // WebKitGTK will complain. So return a message return
+            // rain hail or shine.
+            // https://bugs.webkit.org/show_bug.cgi?id=215880
+
+            message.send_reply(
+                new WebKit.UserMessage(
+                    MESSAGE_RETURN_VALUE_NAME,
+                    Util.JS.value_to_variant(ret)
+                )
+            );
+        } catch (GLib.Error err) {
+            debug("Failed to handle message: %s", err.message);
+        }
+
+        return true;
     }
 
 }
diff --git a/test/js/components-page-state-test.vala b/test/js/components-page-state-test.vala
index 5ec75746..562c6cda 100644
--- a/test/js/components-page-state-test.vala
+++ b/test/js/components-page-state-test.vala
@@ -14,12 +14,24 @@ class Components.PageStateTest : WebViewTestCase<WebView> {
             base(config);
         }
 
+        public new async void call_void(Util.JS.Callable callable)
+            throws GLib.Error {
+            yield base.call_void(callable, null);
+        }
+
+        public new async string call_returning(Util.JS.Callable callable)
+            throws GLib.Error {
+            return yield base.call_returning<string>(callable, null);
+        }
+
     }
 
 
     public PageStateTest() {
         base("Components.PageStateTest");
         add_test("content_loaded", content_loaded);
+        add_test("call_void", call_void);
+        add_test("call_returning", call_returning);
 
         try {
             WebView.load_resources(GLib.File.new_for_path("/tmp"));
@@ -45,6 +57,30 @@ class Components.PageStateTest : WebViewTestCase<WebView> {
         assert(content_loaded_triggered);
     }
 
+    public void call_void() throws GLib.Error {
+        load_body_fixture("OHHAI");
+        var test_article = this.test_view as TestWebView;
+
+        test_article.call_void.begin(
+            new Util.JS.Callable("testVoid"), this.async_completion
+        );
+        test_article.call_void.end(this.async_result());
+        assert_test_result("void");
+    }
+
+    public void call_returning() throws GLib.Error {
+        load_body_fixture("OHHAI");
+        var test_article = this.test_view as TestWebView;
+
+        test_article.call_returning.begin(
+            new Util.JS.Callable("testReturn").string("check 1-2"),
+            this.async_completion
+        );
+        string ret = test_article.call_returning.end(this.async_result());
+        assert_equal(ret, "check 1-2");
+        assert_test_result("check 1-2");
+    }
+
     protected override WebView set_up_test_view() {
         WebKit.UserScript test_script;
         test_script = new WebKit.UserScript(
@@ -60,4 +96,13 @@ class Components.PageStateTest : WebViewTestCase<WebView> {
         return view;
     }
 
+    private void assert_test_result(string expected)
+        throws GLib.Error {
+        string? result = Util.JS.to_string(
+            run_javascript("geary.testResult")
+            .get_js_value()
+        );
+        assert_equal(result, expected);
+    }
+
 }
diff --git a/ui/components-web-view.js b/ui/components-web-view.js
index 80e86d7c..289abca0 100644
--- a/ui/components-web-view.js
+++ b/ui/components-web-view.js
@@ -87,6 +87,8 @@ PageState.prototype = {
         window.addEventListener("transitionend", function(e) {
             queuePreferredHeightUpdate();
         }, false); // load does not bubble
+
+        this.testResult = null;
     },
     getPreferredHeight: function() {
         // Return the scroll height of the HTML element since the BODY
@@ -184,5 +186,13 @@ PageState.prototype = {
             this.hasSelection = hasSelection;
             window.webkit.messageHandlers.selectionChanged.postMessage(hasSelection);
         }
+    },
+    // Methods below are for unit tests.
+    testVoid: function() {
+        this.testResult = "void";
+    },
+    testReturn: function(value) {
+        this.testResult = value;
+        return value;
     }
 };