Svelte · Senior · Question 2 of 75

Why did Svelte 5 move from compiler-instrumented `let` reactivity to runes?

✓Correct answer: Svelte 4 reactivity worked only at the top level of components and was hard to reason about; runes make reactivity explicit and work in .svelte.js files and classes
2Runes are faster to type
3Svelte 4 didn't support TypeScript

Explanation

With $: statements dependencies were determined statically and only at compile time; runes track them at runtime, including through function calls.

Further reading

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