diff --git a/src/client/conversation-viewer/conversation-web-view.vala b/src/client/conversation-viewer/conversation-web-view.vala
index 0df3a21d..642eb7fd 100644
--- a/src/client/conversation-viewer/conversation-web-view.vala
+++ b/src/client/conversation-viewer/conversation-web-view.vala
@@ -183,7 +183,7 @@ public class ConversationWebView : ClientWebView {
 
     private void on_deceptive_link_clicked(WebKit.JavascriptResult result) {
         try {
-            JS.GlobalContext context = result.get_global_context();
+            unowned JS.GlobalContext context = result.get_global_context();
             JS.Object details = WebKitUtil.to_object(result);
 
             uint reason = (uint) Geary.JS.to_number(
diff --git a/src/client/util/util-webkit.vala b/src/client/util/util-webkit.vala
index 319e28ad..cba9eaf2 100644
--- a/src/client/util/util-webkit.vala
+++ b/src/client/util/util-webkit.vala
@@ -18,8 +18,8 @@ namespace WebKitUtil {
      */
     public bool to_bool(WebKit.JavascriptResult result)
         throws Geary.JS.Error {
-        JS.GlobalContext context = result.get_global_context();
-        JS.Value value = result.get_value();
+        unowned JS.GlobalContext context = result.get_global_context();
+        unowned JS.Value value = result.get_value();
         if (!value.is_boolean(context)) {
             throw new Geary.JS.Error.TYPE("Result is not a JS Boolean object");
         }
@@ -59,12 +59,12 @@ namespace WebKitUtil {
      */
     public string as_string(WebKit.JavascriptResult result)
         throws Geary.JS.Error {
-        JS.GlobalContext context = result.get_global_context();
-        JS.Value js_str_value = result.get_value();
+        unowned JS.GlobalContext context = result.get_global_context();
+        unowned JS.Value js_str_value = result.get_value();
         JS.Value? err = null;
         JS.String js_str = js_str_value.to_string_copy(context, out err);
         Geary.JS.check_exception(context, err);
-        return Geary.JS.to_string_released(js_str);
+        return Geary.JS.to_string_released((owned) js_str);
     }
 
     /**
diff --git a/src/client/web-process/web-process-extension.vala b/src/client/web-process/web-process-extension.vala
index abd9ae42..1f478a6c 100644
--- a/src/client/web-process/web-process-extension.vala
+++ b/src/client/web-process/web-process-extension.vala
@@ -87,10 +87,9 @@ public class GearyWebExtension : Object {
         bool should_load = false;
         WebKit.Frame frame = page.get_main_frame();
         // Explicit cast fixes build on s390x/ppc64. Bug 783882
-        JS.GlobalContext context = (JS.GlobalContext)
-            frame.get_javascript_global_context();
+        unowned JS.GlobalContext context = frame.get_javascript_global_context();
         try {
-            JS.Value ret = execute_script(
+            unowned JS.Value ret = execute_script(
                 context, "geary.allowRemoteImages", int.parse("__LINE__")
             );
             should_load = ret.to_boolean(context);
@@ -106,8 +105,7 @@ public class GearyWebExtension : Object {
     private void remote_image_load_blocked(WebKit.WebPage page) {
         WebKit.Frame frame = page.get_main_frame();
         // Explicit cast fixes build on s390x/ppc64. Bug 783882
-        JS.GlobalContext context = (JS.GlobalContext)
-            frame.get_javascript_global_context();
+        unowned JS.GlobalContext context = frame.get_javascript_global_context();
         try {
             execute_script(
                 context, "geary.remoteImageLoadBlocked();", int.parse("__LINE__")
@@ -123,8 +121,7 @@ public class GearyWebExtension : Object {
     private void selection_changed(WebKit.WebPage page) {
         WebKit.Frame frame = page.get_main_frame();
         // Explicit cast fixes build on s390x/ppc64. Bug 783882
-        JS.GlobalContext context = (JS.GlobalContext)
-            frame.get_javascript_global_context();
+        unowned JS.GlobalContext context = frame.get_javascript_global_context();
         try {
             execute_script(
                 context, "geary.selectionChanged();", int.parse("__LINE__")
@@ -136,20 +133,18 @@ public class GearyWebExtension : Object {
 
     // Return type is nullable as a workaround for Bug 778046, it will
     // never actually be null.
-    private JS.Value? execute_script(JS.Context context, string script, int line)
+    private unowned JS.Value? execute_script(JS.Context context, string script, int line)
     throws Geary.JS.Error {
-        JS.String js_script = JS.String.create_with_utf8_cstring(script);
-        JS.String js_source = JS.String.create_with_utf8_cstring("__FILE__");
+        JS.String js_script = new JS.String.create_with_utf8_cstring(script);
+        JS.String js_source = new JS.String.create_with_utf8_cstring("__FILE__");
         JS.Value? err = null;
         try {
-            JS.Value ret = context.evaluate_script(
+            unowned JS.Value ret = context.evaluate_script(
                 js_script, null, js_source, line, out err
             );
             Geary.JS.check_exception(context, err);
             return ret;
         } finally {
-            js_script.release();
-            js_source.release();
         }
     }
 
diff --git a/src/engine/util/util-js.vala b/src/engine/util/util-js.vala
index e53ff747..dcfac4b1 100644
--- a/src/engine/util/util-js.vala
+++ b/src/engine/util/util-js.vala
@@ -72,7 +72,7 @@ namespace Geary.JS {
         global::JS.String js_str = value.to_string_copy(context, out err);
         Geary.JS.check_exception(context, err);
 
-        return Geary.JS.to_string_released(js_str);
+        return Geary.JS.to_string_released((owned) js_str);
     }
 
     /**
@@ -101,12 +101,11 @@ namespace Geary.JS {
     /**
      * Returns a JSC {@link JS.String} as a Vala {@link string}.
      */
-    public inline string to_string_released(global::JS.String js) {
-        int len = js.get_maximum_utf8_cstring_size();
-        string str = string.nfill(len, 0);
-        js.get_utf8_cstring(str, len);
-        js.release();
-        return str;
+    public inline string to_string_released(owned global::JS.String js) {
+        size_t len = js.get_maximum_utf8_cstring_size();
+        uint8[] str = new uint8[len];
+        js.get_utf8_cstring(ref str);
+        return (string) str;
     }
 
     /**
@@ -122,13 +121,12 @@ namespace Geary.JS {
                                                 global::JS.Object object,
                                                 string name)
         throws Geary.JS.Error {
-        global::JS.String js_name = global::JS.String.create_with_utf8_cstring(name);
+        global::JS.String js_name = new global::JS.String.create_with_utf8_cstring(name);
         global::JS.Value? err = null;
         global::JS.Value prop = object.get_property(context, js_name, out err);
         try {
             Geary.JS.check_exception(context, err);
         } finally {
-            js_name.release();
         }
         return prop;
     }
@@ -157,7 +155,7 @@ namespace Geary.JS {
 
             throw new Error.EXCEPTION(
                 "JS exception thrown [%s]: %s"
-                .printf(err_type.to_string(), to_string_released(err_str))
+                .printf(err_type.to_string(), to_string_released((owned) err_str))
             );
         }
     }