catch does not work with fetch statement with if

[shaozi]

The if in fetch caused the catch not working.

What is wrong

Setup

client:

 <button
            class="btn"
            _="
               on click fetch /api/toggle/1
                        if it is 'on' then add .on to me else remove .on from me end
                        catch e log e
                        "
          >
           Toggle
          </button>

server: returnss a 404 page not found error.

Result

However, the catch e log e is never triggered.

What is working

But if I replace the if with a log, it works.

 <button
            class="btn"
            _="
               on click fetch /api/toggle/1
                        log it
                        catch e log e
                        "
          >
          Toggle
          </button>

[1cg]

believe it or not fetch() doesn't throw on a 404:

https://stackoverflow.com/questions/38235715/fetch-reject-promise-and-catch-the-error-if-status-is-not-ok

so what's happening here is that no exception is happening. In the second example it's the log it that is printing the response.

[shaozi]

aha!
So in fetch how to check the return code?