1 files changed, 39 insertions, 0 deletions
diff --git a/browser/components/messagepreview/messagepreview.js b/browser/components/messagepreview/messagepreview.js
new file mode 100644
index 0000000000..48e5fb1ff5
--- /dev/null
+++ b/browser/components/messagepreview/messagepreview.js
@@ -0,0 +1,39 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+/* global MPShowMessage, MPIsEnabled, MPShouldShowHint */
+
+"use strict";
+
+function decodeMessageFromUrl() {
+  const url = new URL(document.location.href);
+
+  if (url.searchParams.has("json")) {
+    const encodedMessage = url.searchParams.get("json");
+
+    return atob(encodedMessage);
+  }
+  return null;
+}
+
+function showHint() {
+  document.body.classList.add("hint-box");
+  document.body.innerHTML = `<div class="hint">Message preview is not enabled. Enable it in about:config by setting <code>browser.newtabpage.activity-stream.asrouter.devtoolsEnabled</code> to true.</div>`;
+}
+
+const message = decodeMessageFromUrl();
+
+if (message) {
+  // If message preview is enabled, show the message.
+  if (MPIsEnabled()) {
+    MPShowMessage(message);
+  } else if (MPShouldShowHint()) {
+    // If running in a local build, show a hint about how to enable preview.
+    if (document.body) {
+      showHint();
+    } else {
+      document.addEventListener("DOMContentLoaded", showHint, { once: true });
+    }
+  }
+}