message passing by helper functions

src/shared/messages.js

@@ -0,0 +1,19 @@
+const receive = (win, callback) => {
+  win.addEventListener('message', (e) => {
+    let message;
+    try {
+      message = JSON.parse(e.data);
+    } catch (e) {
+      // ignore message posted by author of web page
+      return;
+    }
+
+    callback(message);
+  })
+}
+
+const send = (win, message) => {
+  win.postMessage(JSON.stringify(message), '*');
+}
+
+export { receive, send };