diff --git a/pri/install.pri b/pri/install.pri
index 1948f8d36..ed4c9d604 100644
--- a/pri/install.pri
+++ b/pri/install.pri
@@ -175,7 +175,10 @@ win32 {
   lib.path = $$PREFIX
   lib.CONFIG = no_check_exist
 
-  INSTALLS += target lib
+  clng.files = ../../resources/scripts/clang-format/clang-format.exe
+  clng.path = $$PREFIX
+
+  INSTALLS += target lib clng
 
   INSTALL_HEADERS_PREFIX = $$quote($$PREFIX/include/librssguard)
 }
diff --git a/resources/scripts/clang-format/clang-format.exe b/resources/scripts/clang-format/clang-format.exe
new file mode 100755
index 000000000..cc26d5b09
Binary files /dev/null and b/resources/scripts/clang-format/clang-format.exe differ
diff --git a/src/librssguard/core/feeddownloader.cpp b/src/librssguard/core/feeddownloader.cpp
index 25a8d2dc8..6f7d659ad 100644
--- a/src/librssguard/core/feeddownloader.cpp
+++ b/src/librssguard/core/feeddownloader.cpp
@@ -4,6 +4,7 @@
 
 #include "core/messagefilter.h"
 #include "definitions/definitions.h"
+#include "exceptions/filteringexception.h"
 #include "miscellaneous/application.h"
 #include "services/abstract/cacheforserviceroot.h"
 #include "services/abstract/feed.h"
@@ -147,23 +148,31 @@ void FeedDownloader::updateOneFeed(Feed* feed) {
 
         tmr.restart();
 
-        FilteringAction decision = msg_filter->filterMessage(&filter_engine);
+        try {
+          FilteringAction decision = msg_filter->filterMessage(&filter_engine);
 
-        qDebug().nospace() << "Running filter script, it took " << tmr.nsecsElapsed() / 1000 << " microseconds.";
+          qDebug().nospace() << "Running filter script, it took " << tmr.nsecsElapsed() / 1000 << " microseconds.";
 
-        switch (decision) {
-          case FilteringAction::Accept:
+          switch (decision) {
+            case FilteringAction::Accept:
 
-            // Message is normally accepted, it could be tweaked by the filter.
-            continue;
+              // Message is normally accepted, it could be tweaked by the filter.
+              continue;
 
-          case FilteringAction::Ignore:
+            case FilteringAction::Ignore:
 
-            // Remove the message, we do not want it.
-            msgs.removeAt(i--);
-            break;
+              // Remove the message, we do not want it.
+              msgs.removeAt(i--);
+              break;
+          }
+        }
+        catch (const FilteringException& ex) {
+          qCritical("Error when evaluating filtering JS function: '%s'. Accepting message.", qPrintable(ex.message()));
+          continue;
         }
 
+        // If we reach this point. Then we ignore the message which is by now
+        // already removed, go to next message.
         break;
       }
     }
diff --git a/src/librssguard/core/messagefilter.cpp b/src/librssguard/core/messagefilter.cpp
index 966e43f08..8b01d0e0b 100755
--- a/src/librssguard/core/messagefilter.cpp
+++ b/src/librssguard/core/messagefilter.cpp
@@ -3,6 +3,7 @@
 #include "core/messagefilter.h"
 
 #include "core/message.h"
+#include "exceptions/filteringexception.h"
 
 #include <QJSEngine>
 
@@ -43,15 +44,19 @@ FilteringAction MessageFilter::filterMessage(QJSEngine* engine) {
   QJSValue filter_func = engine->evaluate(m_script);
 
   if (filter_func.isError()) {
-    qCritical("Error when evaluating script from filter '%d'. Error is: '%s'", id(), qPrintable(filter_func.toString()));
-    return FilteringAction::Accept;
+    QJSValue::ErrorType error = filter_func.errorType();
+    QString message = filter_func.toString();
+
+    throw FilteringException(error, message);
   }
 
   auto filter_output = engine->evaluate(QSL("filterMessage()"));
 
   if (filter_output.isError()) {
-    qCritical("Error when calling filtering function '%d'. Error is: '%s'", id(), qPrintable(filter_output.toString()));
-    return FilteringAction::Accept;
+    QJSValue::ErrorType error = filter_output.errorType();
+    QString message = filter_output.toString();
+
+    throw FilteringException(error, message);
   }
 
   return FilteringAction(filter_output.toInt());
diff --git a/src/librssguard/exceptions/applicationexception.h b/src/librssguard/exceptions/applicationexception.h
index 982f606fa..f3e89a199 100644
--- a/src/librssguard/exceptions/applicationexception.h
+++ b/src/librssguard/exceptions/applicationexception.h
@@ -8,7 +8,6 @@
 class ApplicationException {
   public:
     explicit ApplicationException(QString message = QString());
-    virtual ~ApplicationException() = default;
 
     QString message() const;
 
diff --git a/src/librssguard/exceptions/filteringexception.cpp b/src/librssguard/exceptions/filteringexception.cpp
new file mode 100755
index 000000000..d061e1d51
--- /dev/null
+++ b/src/librssguard/exceptions/filteringexception.cpp
@@ -0,0 +1,6 @@
+// For license of this file, see <project-root-folder>/LICENSE.md.
+
+#include "exceptions/filteringexception.h"
+
+FilteringException::FilteringException(QJSValue::ErrorType js_error, QString message)
+  : ApplicationException(message), m_errorType(js_error) {}
diff --git a/src/librssguard/exceptions/filteringexception.h b/src/librssguard/exceptions/filteringexception.h
new file mode 100755
index 000000000..493220da5
--- /dev/null
+++ b/src/librssguard/exceptions/filteringexception.h
@@ -0,0 +1,18 @@
+// For license of this file, see <project-root-folder>/LICENSE.md.
+
+#ifndef FILTERINGEXCEPTION_H
+#define FILTERINGEXCEPTION_H
+
+#include "exceptions/applicationexception.h"
+
+#include <QJSValue>
+
+class FilteringException : public ApplicationException {
+  public:
+    explicit FilteringException(QJSValue::ErrorType js_error, QString message = QString());
+
+  private:
+    QJSValue::ErrorType m_errorType;
+};
+
+#endif // FILTERINGEXCEPTION_H
diff --git a/src/librssguard/exceptions/ioexception.h b/src/librssguard/exceptions/ioexception.h
index 4d3bf1a2a..42fedde6e 100644
--- a/src/librssguard/exceptions/ioexception.h
+++ b/src/librssguard/exceptions/ioexception.h
@@ -8,7 +8,6 @@
 class IOException : public ApplicationException {
   public:
     explicit IOException(const QString& message = QString());
-    virtual ~IOException() = default;
 };
 
 #endif // IOEXCEPTION_H
diff --git a/src/librssguard/gui/dialogs/formmessagefiltersmanager.cpp b/src/librssguard/gui/dialogs/formmessagefiltersmanager.cpp
index 8ed317fa4..a8baaaa0b 100644
--- a/src/librssguard/gui/dialogs/formmessagefiltersmanager.cpp
+++ b/src/librssguard/gui/dialogs/formmessagefiltersmanager.cpp
@@ -6,6 +6,7 @@
 #include "gui/dialogs/formmessagefiltersmanager.h"
 
 #include "core/messagefilter.h"
+#include "exceptions/filteringexception.h"
 #include "gui/guiutilities.h"
 #include "miscellaneous/application.h"
 #include "miscellaneous/feedreader.h"
@@ -38,6 +39,7 @@ FormMessageFiltersManager::FormMessageFiltersManager(FeedReader* reader, const Q
   connect(m_ui.m_btnTest, &QPushButton::clicked, this, &FormMessageFiltersManager::testFilter);
 
   initializeTestingMessage();
+  loadFilter();
 }
 
 FormMessageFiltersManager::~FormMessageFiltersManager() {
@@ -95,6 +97,12 @@ void FormMessageFiltersManager::loadFilter() {
 }
 
 void FormMessageFiltersManager::testFilter() {
+  // TODO: Add button to beautify JavaScript code, call clang-format and distribute
+  // it under windows. On other platforms, just try to call and raise messagebox
+  // error with "install clang-format" if not found.
+  // then call like this with qt process api.
+  // echo "script-code" | ./clang-format.exe --assume-filename="script.js" --style="Chromium"
+
   // Perform per-message filtering.
   QJSEngine filter_engine;
   QSqlDatabase database = qApp->database()->connection(metaObject()->className());
@@ -113,8 +121,37 @@ void FormMessageFiltersManager::testFilter() {
   msg_obj.setMessage(&msg);
 
   auto* fltr = selectedFilter();
-  FilteringAction decision = fltr->filterMessage(&filter_engine);
-  int aa = 5;
+
+  try {
+    FilteringAction decision = fltr->filterMessage(&filter_engine);
+
+    m_ui.m_txtErrors->setTextColor(decision == FilteringAction::Accept ? Qt::GlobalColor::darkGreen : Qt::GlobalColor::red);
+
+    QString answer = tr("Message will be %1.\n").arg(decision == FilteringAction::Accept
+                                                   ? tr("accepted")
+                                                   : tr("rejected"));
+
+    answer += tr("Output (modified) message is:\n"
+                 "  Title = '%1'\n"
+                 "  URL = '%2'\n"
+                 "  Author = '%3'\n"
+                 "  Is read/important = '%4/%5'\n"
+                 "  Created on = '%6'\n"
+                 "  Contents = '%7'").arg(msg.m_title, msg.m_url, msg.m_author,
+                                          msg.m_isRead ? tr("yes") : tr("no"),
+                                          msg.m_isImportant ? tr("yes") : tr("no"),
+                                          QString::number(msg.m_created.toMSecsSinceEpoch()),
+                                          msg.m_contents);
+
+    m_ui.m_txtErrors->setPlainText(answer);
+  }
+  catch (const FilteringException& ex) {
+    m_ui.m_txtErrors->setTextColor(Qt::GlobalColor::red);
+    m_ui.m_txtErrors->setPlainText(tr("JavaScript-based filter contains errors: '%1'.").arg(ex.message()));
+  }
+
+  // See output.
+  m_ui.m_tcMessage->setCurrentIndex(1);
 }
 
 void FormMessageFiltersManager::showFilter(MessageFilter* filter) {
@@ -131,6 +168,8 @@ void FormMessageFiltersManager::showFilter(MessageFilter* filter) {
     m_ui.m_gbDetails->setEnabled(true);
   }
 
+  // See message.
+  m_ui.m_tcMessage->setCurrentIndex(0);
   m_loadingFilter = false;
 }
 
diff --git a/src/librssguard/gui/dialogs/formmessagefiltersmanager.ui b/src/librssguard/gui/dialogs/formmessagefiltersmanager.ui
index f185faf8b..ef4036be7 100644
--- a/src/librssguard/gui/dialogs/formmessagefiltersmanager.ui
+++ b/src/librssguard/gui/dialogs/formmessagefiltersmanager.ui
@@ -50,19 +50,6 @@
    </item>
    <item row="0" column="1">
     <layout class="QFormLayout" name="formLayout_3">
-     <item row="1" column="0" colspan="2">
-      <widget class="QTreeView" name="m_treeFeeds">
-       <property name="minimumSize">
-        <size>
-         <width>0</width>
-         <height>150</height>
-        </size>
-       </property>
-      </widget>
-     </item>
-     <item row="0" column="1">
-      <widget class="QComboBox" name="comboBox"/>
-     </item>
      <item row="0" column="0">
       <widget class="QLabel" name="label_9">
        <property name="text">
@@ -73,6 +60,54 @@
        </property>
       </widget>
      </item>
+     <item row="0" column="1">
+      <widget class="QComboBox" name="comboBox"/>
+     </item>
+     <item row="1" column="0" colspan="2">
+      <layout class="QHBoxLayout" name="horizontalLayout_5">
+       <item>
+        <widget class="QTreeView" name="m_treeFeeds">
+         <property name="minimumSize">
+          <size>
+           <width>0</width>
+           <height>150</height>
+          </size>
+         </property>
+        </widget>
+       </item>
+       <item>
+        <layout class="QVBoxLayout" name="verticalLayout">
+         <item>
+          <widget class="QPushButton" name="pushButton">
+           <property name="text">
+            <string>&amp;Check all</string>
+           </property>
+          </widget>
+         </item>
+         <item>
+          <widget class="QPushButton" name="pushButton_2">
+           <property name="text">
+            <string>&amp;Uncheck all</string>
+           </property>
+          </widget>
+         </item>
+         <item>
+          <spacer name="verticalSpacer">
+           <property name="orientation">
+            <enum>Qt::Vertical</enum>
+           </property>
+           <property name="sizeHint" stdset="0">
+            <size>
+             <width>20</width>
+             <height>40</height>
+            </size>
+           </property>
+          </spacer>
+         </item>
+        </layout>
+       </item>
+      </layout>
+     </item>
     </layout>
    </item>
    <item row="1" column="0" colspan="2">
@@ -147,108 +182,154 @@
          </widget>
         </item>
         <item>
-         <widget class="QGroupBox" name="groupBox_2">
-          <property name="title">
-           <string>Sample message</string>
+         <widget class="QTabWidget" name="m_tcMessage">
+          <property name="currentIndex">
+           <number>0</number>
           </property>
-          <layout class="QFormLayout" name="formLayout_2">
-           <item row="0" column="0" colspan="2">
-            <layout class="QHBoxLayout" name="horizontalLayout_4">
-             <item>
-              <widget class="QCheckBox" name="m_cbSampleRead">
-               <property name="text">
-                <string>Read</string>
-               </property>
-              </widget>
-             </item>
-             <item>
-              <widget class="QCheckBox" name="m_cbSampleImportant">
-               <property name="text">
-                <string>Important</string>
-               </property>
-              </widget>
-             </item>
-             <item>
-              <spacer name="horizontalSpacer_4">
-               <property name="orientation">
-                <enum>Qt::Horizontal</enum>
-               </property>
-               <property name="sizeHint" stdset="0">
-                <size>
-                 <width>40</width>
-                 <height>20</height>
-                </size>
-               </property>
-              </spacer>
-             </item>
-            </layout>
-           </item>
-           <item row="1" column="0">
-            <widget class="QLabel" name="label_4">
-             <property name="text">
-              <string>Title</string>
-             </property>
-             <property name="buddy">
-              <cstring>m_txtSampleTitle</cstring>
-             </property>
-            </widget>
-           </item>
-           <item row="1" column="1">
-            <widget class="QLineEdit" name="m_txtSampleTitle"/>
-           </item>
-           <item row="2" column="0">
-            <widget class="QLabel" name="label_6">
-             <property name="text">
-              <string>URL</string>
-             </property>
-             <property name="buddy">
-              <cstring>m_txtSampleUrl</cstring>
-             </property>
-            </widget>
-           </item>
-           <item row="2" column="1">
-            <widget class="QLineEdit" name="m_txtSampleUrl"/>
-           </item>
-           <item row="3" column="0">
-            <widget class="QLabel" name="label_7">
-             <property name="text">
-              <string>Author</string>
-             </property>
-             <property name="buddy">
-              <cstring>m_txtSampleAuthor</cstring>
-             </property>
-            </widget>
-           </item>
-           <item row="3" column="1">
-            <widget class="QLineEdit" name="m_txtSampleAuthor"/>
-           </item>
-           <item row="4" column="0">
-            <widget class="QLabel" name="label_8">
-             <property name="text">
-              <string>Created on</string>
-             </property>
-             <property name="buddy">
-              <cstring>m_txtSampleCreatedOn</cstring>
-             </property>
-            </widget>
-           </item>
-           <item row="4" column="1">
-            <widget class="QLineEdit" name="m_txtSampleCreatedOn"/>
-           </item>
-           <item row="5" column="0">
-            <widget class="QLabel" name="label_5">
-             <property name="text">
-              <string>Contents</string>
-             </property>
-             <property name="buddy">
-              <cstring>m_txtSampleContents</cstring>
-             </property>
-            </widget>
-           </item>
-           <item row="5" column="1">
-            <widget class="QPlainTextEdit" name="m_txtSampleContents"/>
-           </item>
-          </layout>
+          <widget class="QWidget" name="m_tabSample">
+           <attribute name="title">
+            <string>Sample message</string>
+           </attribute>
+           <layout class="QFormLayout" name="formLayout_2">
+            <property name="leftMargin">
+             <number>3</number>
+            </property>
+            <property name="topMargin">
+             <number>3</number>
+            </property>
+            <property name="rightMargin">
+             <number>3</number>
+            </property>
+            <property name="bottomMargin">
+             <number>3</number>
+            </property>
+            <item row="0" column="0" colspan="2">
+             <layout class="QHBoxLayout" name="horizontalLayout_4">
+              <item>
+               <widget class="QCheckBox" name="m_cbSampleRead">
+                <property name="text">
+                 <string>Read</string>
+                </property>
+               </widget>
+              </item>
+              <item>
+               <widget class="QCheckBox" name="m_cbSampleImportant">
+                <property name="text">
+                 <string>Important</string>
+                </property>
+               </widget>
+              </item>
+              <item>
+               <spacer name="horizontalSpacer_4">
+                <property name="orientation">
+                 <enum>Qt::Horizontal</enum>
+                </property>
+                <property name="sizeHint" stdset="0">
+                 <size>
+                  <width>40</width>
+                  <height>20</height>
+                 </size>
+                </property>
+               </spacer>
+              </item>
+             </layout>
+            </item>
+            <item row="1" column="0">
+             <widget class="QLabel" name="label_4">
+              <property name="text">
+               <string>Title</string>
+              </property>
+              <property name="buddy">
+               <cstring>m_txtSampleTitle</cstring>
+              </property>
+             </widget>
+            </item>
+            <item row="1" column="1">
+             <widget class="QLineEdit" name="m_txtSampleTitle"/>
+            </item>
+            <item row="2" column="0">
+             <widget class="QLabel" name="label_6">
+              <property name="text">
+               <string>URL</string>
+              </property>
+              <property name="buddy">
+               <cstring>m_txtSampleUrl</cstring>
+              </property>
+             </widget>
+            </item>
+            <item row="2" column="1">
+             <widget class="QLineEdit" name="m_txtSampleUrl"/>
+            </item>
+            <item row="3" column="0">
+             <widget class="QLabel" name="label_7">
+              <property name="text">
+               <string>Author</string>
+              </property>
+              <property name="buddy">
+               <cstring>m_txtSampleAuthor</cstring>
+              </property>
+             </widget>
+            </item>
+            <item row="3" column="1">
+             <widget class="QLineEdit" name="m_txtSampleAuthor"/>
+            </item>
+            <item row="4" column="0">
+             <widget class="QLabel" name="label_8">
+              <property name="text">
+               <string>Created on</string>
+              </property>
+              <property name="buddy">
+               <cstring>m_txtSampleCreatedOn</cstring>
+              </property>
+             </widget>
+            </item>
+            <item row="4" column="1">
+             <widget class="QLineEdit" name="m_txtSampleCreatedOn"/>
+            </item>
+            <item row="5" column="0">
+             <widget class="QLabel" name="label_5">
+              <property name="text">
+               <string>Contents</string>
+              </property>
+              <property name="buddy">
+               <cstring>m_txtSampleContents</cstring>
+              </property>
+             </widget>
+            </item>
+            <item row="5" column="1">
+             <widget class="QPlainTextEdit" name="m_txtSampleContents"/>
+            </item>
+           </layout>
+          </widget>
+          <widget class="QWidget" name="m_tabScriptOutput">
+           <attribute name="title">
+            <string>Script output</string>
+           </attribute>
+           <layout class="QHBoxLayout" name="horizontalLayout_6">
+            <property name="leftMargin">
+             <number>3</number>
+            </property>
+            <property name="topMargin">
+             <number>3</number>
+            </property>
+            <property name="rightMargin">
+             <number>3</number>
+            </property>
+            <property name="bottomMargin">
+             <number>3</number>
+            </property>
+            <item>
+             <widget class="QTextEdit" name="m_txtErrors">
+              <property name="sizePolicy">
+               <sizepolicy hsizetype="Expanding" vsizetype="Expanding">
+                <horstretch>0</horstretch>
+                <verstretch>1</verstretch>
+               </sizepolicy>
+              </property>
+             </widget>
+            </item>
+           </layout>
+          </widget>
          </widget>
         </item>
        </layout>
@@ -265,7 +346,7 @@
         <item>
          <widget class="QPushButton" name="m_btnDetailedHelp">
           <property name="text">
-           <string>&amp;Detailed &amp;help</string>
+           <string>Detailed &amp;help</string>
           </property>
          </widget>
         </item>
@@ -284,32 +365,6 @@
         </item>
        </layout>
       </item>
-      <item row="4" column="0">
-       <widget class="QLabel" name="label_3">
-        <property name="text">
-         <string>Test output</string>
-        </property>
-        <property name="buddy">
-         <cstring>m_txtErrors</cstring>
-        </property>
-       </widget>
-      </item>
-      <item row="4" column="1">
-       <widget class="QPlainTextEdit" name="m_txtErrors">
-        <property name="enabled">
-         <bool>false</bool>
-        </property>
-        <property name="sizePolicy">
-         <sizepolicy hsizetype="Expanding" vsizetype="Expanding">
-          <horstretch>0</horstretch>
-          <verstretch>1</verstretch>
-         </sizepolicy>
-        </property>
-        <property name="readOnly">
-         <bool>false</bool>
-        </property>
-       </widget>
-      </item>
      </layout>
     </widget>
    </item>
@@ -334,10 +389,13 @@
   <tabstop>m_btnRemoveSelected</tabstop>
   <tabstop>comboBox</tabstop>
   <tabstop>m_treeFeeds</tabstop>
+  <tabstop>pushButton</tabstop>
+  <tabstop>pushButton_2</tabstop>
   <tabstop>m_txtTitle</tabstop>
   <tabstop>m_txtScript</tabstop>
   <tabstop>m_btnTest</tabstop>
   <tabstop>m_btnDetailedHelp</tabstop>
+  <tabstop>m_tcMessage</tabstop>
   <tabstop>m_cbSampleRead</tabstop>
   <tabstop>m_cbSampleImportant</tabstop>
   <tabstop>m_txtSampleTitle</tabstop>
diff --git a/src/librssguard/librssguard.pro b/src/librssguard/librssguard.pro
index b4d76d751..3ba224694 100644
--- a/src/librssguard/librssguard.pro
+++ b/src/librssguard/librssguard.pro
@@ -44,6 +44,7 @@ HEADERS += core/feeddownloader.h \
            dynamic-shortcuts/shortcutbutton.h \
            dynamic-shortcuts/shortcutcatcher.h \
            exceptions/applicationexception.h \
+           exceptions/filteringexception.h \
            exceptions/ioexception.h \
            gui/baselineedit.h \
            gui/basetoolbar.h \
@@ -189,6 +190,7 @@ SOURCES += core/feeddownloader.cpp \
            dynamic-shortcuts/shortcutbutton.cpp \
            dynamic-shortcuts/shortcutcatcher.cpp \
            exceptions/applicationexception.cpp \
+           exceptions/filteringexception.cpp \
            exceptions/ioexception.cpp \
            gui/baselineedit.cpp \
            gui/basetoolbar.cpp \