diff --git a/test/de/inetsoftware/jwebassembly/SpiderMonkey.java b/test/de/inetsoftware/jwebassembly/SpiderMonkey.java
new file mode 100644
index 0000000..1568b07
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+++ b/test/de/inetsoftware/jwebassembly/SpiderMonkey.java
@@ -0,0 +1,96 @@
+/*
+ * Copyright 2017 Volker Berlin (i-net software)
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package de.inetsoftware.jwebassembly;
+
+import java.io.File;
+import java.io.IOException;
+import java.io.InputStream;
+import java.net.HttpURLConnection;
+import java.net.URL;
+import java.nio.file.Files;
+import java.nio.file.StandardCopyOption;
+import java.util.zip.ZipEntry;
+import java.util.zip.ZipInputStream;
+
+/**
+ * Download the JavaScript engine SpiderMonkey.
+ * 
+ * @author Volker Berlin
+ */
+public class SpiderMonkey {
+
+    private String command;
+
+    /**
+     * Check if there is a new version of the script engine
+     * 
+     * @throws IOException
+     *             if any error occur
+     */
+    private void download() throws IOException {
+        boolean is32 = "32".equals( System.getProperty( "sun.arch.data.model" ) );
+        String fileName;
+        final String os = System.getProperty( "os.name", "" ).toLowerCase();
+        if( os.contains( "windows" ) ) {
+            fileName = is32 ? "win32" : "win64";
+        } else if( os.contains( "mac" ) ) {
+            fileName = is32 ? "mac" : "mac64";
+        } else if( os.contains( "linux" ) ) {
+            fileName = is32 ? "linux-i686" : "linux-x86_64";
+        } else {
+            throw new IllegalStateException( "Unknown OS: " + os );
+        }
+        File target = new File( System.getProperty( "java.io.tmpdir" ) + "/SpiderMonkey" );
+        URL url = new URL( "https://archive.mozilla.org/pub/firefox/nightly/latest-mozilla-central/jsshell-" + fileName
+                        + ".zip" );
+        System.out.println( "\tDownload: " + url );
+        HttpURLConnection conn = (HttpURLConnection)url.openConnection();
+        if( target.exists() ) {
+            System.out.println( "\tUP-TP-DATE" );
+            conn.setIfModifiedSince( target.lastModified() );
+        }
+        InputStream input = conn.getInputStream();
+        command = target.getAbsolutePath() + "/js";
+        if( conn.getResponseCode() == HttpURLConnection.HTTP_NOT_MODIFIED ) {
+            return;
+        }
+        ZipInputStream zip = new ZipInputStream( input );
+        long lastModfied = conn.getLastModified();
+        do {
+            ZipEntry entry = zip.getNextEntry();
+            if( entry == null ) {
+                break;
+            }
+            if( entry.isDirectory() ) {
+                continue;
+            }
+            File file = new File( target, entry.getName() );
+            file.getParentFile().mkdirs();
+
+            Files.copy( zip, file.toPath(), StandardCopyOption.REPLACE_EXISTING );
+            file.setLastModified( entry.getTime() );
+        } while( true );
+        target.setLastModified( lastModfied );
+    }
+
+    public String getCommand() throws IOException {
+        if( command == null ) {
+            download();
+        }
+        return command;
+    }
+
+}