How do you update Svelte arrays and objects to trigger reactivity?
This tests knowledge of Svelte 5 deep reactivity: $state wraps arrays in proxies so push and property mutations trigger updates. Mention $state first, then note legacy syntax needs reassignment. A red flag is claiming push never works in Svelte.
What's really being asked
This question checks whether you understand the difference between shallow and deep reactivity in modern Svelte, specifically the Svelte 5 runes model. Interviewers want to see if you know that the $state rune creates a reactive proxy that intercepts mutations, or if you are still thinking in terms of older Svelte patterns where only assignment triggered updates.
The full answer
First, declare the array or object with the $state rune so Svelte wraps it in a proxy. Second, explain that once wrapped, native mutations such as push, pop, splice, or direct property assignment automatically trigger reactivity because the proxy intercepts the operation. Third, note a critical caveat from the documentation: mutations to the proxy do not mutate the original object, so the proxy and the original are decoupled. Fourth, acknowledge that in legacy Svelte without runes, you must use reassignment such as assigning the variable to itself or using spread syntax to force an update.
The mistakes people make
Claiming that push never triggers reactivity in any version of Svelte is the biggest red flag. Another mistake is suggesting self-assignment or spread reassignment as the primary or only solution in Svelte 5, which reveals unfamiliarity with runes and deep reactivity. Confusing Vue's reactivity system with Svelte's proxy implementation is also a signal of shallow knowledge.
What usually comes next
An interviewer might ask what happens if you pass a non-proxied array into a $state declaration, or how deep reactivity handles nested objects and arrays. They may also ask about performance implications of proxy-based observation versus explicit reassignment, or how to opt out of deep reactivity if you need fine-grained control.
A concrete example
Suppose you initialize a reactive array with let numbers equal $state and an array containing one, two, and three. In your addNumber function, you can write numbers dot push with numbers dot length plus one, and the user interface updates immediately. If you had instead declared numbers as a plain array without $state, the same push call would not update the view, and you would need to reassign the variable to itself or use spread syntax to trigger reactivity.
Interview question
In Svelte 5, you declare an array with let items = $state([1, 2]) and later call items.push(3). What is the result?
- a.You must wrap the push call in an $effect rune to trigger the update
- b.The compiler converts the push call into an immutable spread operation
- c.The UI updates automatically because the array is wrapped in a reactive proxyCorrect
- d.Nothing updates until you reassign the items variable to itself
Why? this is the answer
In Svelte 5, the $state rune wraps arrays in proxies that intercept mutations like push, so the UI updates immediately without reassignment. Reassigning the variable to itself is a legacy pattern that is unnecessary when using runes.
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