1 files changed, 18 insertions, 0 deletions
diff --git a/browser/extensions/screenshots/assertIsBlankDocument.js b/browser/extensions/screenshots/assertIsBlankDocument.js
new file mode 100644
index 0000000000..be2371632a
--- /dev/null
+++ b/browser/extensions/screenshots/assertIsBlankDocument.js
@@ -0,0 +1,18 @@
+/* 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/. */
+
+/* globals browser */
+
+/** For use inside an iframe onload function, throws an Error if iframe src is not blank.html
+
+    Should be applied *inside* catcher.watchFunction
+*/
+this.assertIsBlankDocument = function assertIsBlankDocument(doc) {
+  if (doc.documentURI !== browser.runtime.getURL("blank.html")) {
+    const exc = new Error("iframe URL does not match expected blank.html");
+    exc.foundURL = doc.documentURI;
+    throw exc;
+  }
+};
+null;