and if I think this is too boilerplatey, I can export a handler from some .js file and pass the error to that: <script> import { onError } from 'svelte' import { genericHandler } from '../my-error-handler.js' onError(genericHandler(e => { // code which is called first to try to handle this locally return true // we've handled it here, don't do anything else. }) </script>

You’re free to test this activity in a separate unit test.

It must be called during the component's initialisation (but doesn't need to live inside the component; it can be called from an external module).

(At the point at which it does make sense to turn this into a separate Tooltip.svelte component, the extraction is a completely mechanical process that could even be automated by tooling.)