If you are migrating from Svelte 4 to Svelte 5, you have likely encountered the "Silent Failure." You update a value, your event handler logs the correct new data to the console, but the DOM remains stubbornly frozen. The $derived rune—the successor to the reactive label $: —is not recalculating. This usually stems from two fundamental misunderstandings of the new Signal-based architecture: Broken Reactivity Chains: Passing raw JavaScript objects (references) into components where Svelte expects Proxies. The Side-Effect Fallacy: Treating $derived as an event hook rather than a pure mathematical derivation. Here is how to diagnose and fix these "broken" signals. The Hook: Why $derived Stops Listening In Svelte 4, reactivity was compiled. Svelte looked at your code, saw foo = bar + 1 , and injected code to update foo whenever bar was invalidated. In Svelte 5, reactivity is runtime-based usi...
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