From what I understand, Svelte 4 calculated dependencies at compile time. Whereas Svelte 5 does it automatically at runtime based on which values are accessed in tracking contexts (effect / computed value callbacks). This means objects marked as reactive have to be wrapped in proxies to intercept gets / sets. And plain values have to be transformed to objects at compile time. The deps are also recalculated after every update, so accessing state in conditionals can cause funkiness.
But after working with Svelte 5 daily for a few months, I don't think I like the implicitness. For one, reactivity doesn't show up in type signatures. So I have to remember whats reactive and what's not. Another is that making plain values (string, numbers, etc) reactive is really a trap. The reactivity doesn't cross file boundaries so you have to do weird things like exporting a getter / setter instead of the variable.
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From what I understand, Svelte 4 calculated dependencies at compile time. Whereas Svelte 5 does it automatically at runtime based on which values are accessed in tracking contexts (effect / computed value callbacks). This means objects marked as reactive have to be wrapped in proxies to intercept gets / sets. And plain values have to be transformed to objects at compile time. The deps are also recalculated after every update, so accessing state in conditionals can cause funkiness.
But after working with Svelte 5 daily for a few months, I don't think I like the implicitness. For one, reactivity doesn't show up in type signatures. So I have to remember whats reactive and what's not. Another is that making plain values (string, numbers, etc) reactive is really a trap. The reactivity doesn't cross file boundaries so you have to do weird things like exporting a getter / setter instead of the variable.