So why the over-complication? What we got now is replicating a chain of && in the former version. This time, however, you will know which condition failed and what went in by using tracing. Look at the trace above - it’s impossible to not understand what was going on.

Now you can easily spot them! The rule is: if you see a Result it means that this function can throw an exception. And you even know its type in advance.

If you teach your users to trust that URL bar is supposed to not change when they click links (e.g. your site uses a big iframe with all the actual content), then the users will not notice anything in the future either in case of actual security vulnerability.

The problem I have with this approach to state and prop variables is that the difference between them is very blurry. In React you can clearly see that a prop is an input to component (because of clear function notation), and that state is something internal. In Svelte they are both just variables, with the exception that props use export keyword.