Function to get the fallback url for interactive auth

lib/base-apis.js

@@ -290,6 +290,23 @@ MatrixBaseApis.prototype.deactivateAccount = function(auth, callback) {
     );
 };
 
+/**
+ * Get the fallback URL to use for unknown interactive-auth stages.
+ *
+ * @param {string} loginType     the type of stage being attempted
+ * @param {string} authSessionId the auth session ID provided by the homeserver
+ *
+ * @return {string} HS URL to hit to for the fallback interface
+ */
+MatrixBaseApis.prototype.getFallbackAuthUrl = function(loginType, authSessionId) {
+    var path = utils.encodeUri("/auth/$loginType/fallback/web", {
+        $loginType: loginType,
+    });
+
+    return this._http.getUrl(path, {
+        session: authSessionId,
+    }, httpApi.PREFIX_R0);
+};
 
 // Room operations
 // ===============