Even though it often isn't spelled out, this is mostly about content-controlled JS loaded from the web, though the language itself also has its problems. And se

Even though it often isn't spelled out, this is mostly about content-controlled JS loaded from the web, though the language itself also has its problems. And security-wise that app would not be better than one that stored the messages on a server, which only allows you to read them back when you provide the password.