Immutable patterns for updates in Svelte - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does 'immutable pattern' mean in Svelte updates?

It means creating a new copy of data instead of changing the original directly. This helps Svelte detect changes and update the UI efficiently.

beginner

Why should you avoid directly modifying arrays or objects in Svelte stores?

Direct modification does not trigger Svelte's reactivity because the reference stays the same. Using immutable updates by creating new arrays or objects ensures Svelte notices the change.

intermediate

How do you update an array immutably in Svelte?

Use methods like spread syntax [...array, newItem] or array methods that return new arrays (e.g., filter, map) instead of push or splice.

intermediate

Show an example of immutably updating an object in Svelte.

Use spread syntax to create a new object with updated properties: <br> store.update(obj => ({ ...obj, key: newValue }))

beginner

What is the benefit of using immutable patterns in Svelte's reactive updates?

It ensures Svelte can detect changes quickly and update the UI correctly without bugs or missed updates.

In Svelte, why is this update not reactive? <br> array.push(newItem)

A Because newItem is undefined

B Because push is not a valid JavaScript method

C Because Svelte does not support arrays

D Because push modifies the original array without changing its reference

push changes the array in place, so the reference stays the same. Svelte tracks changes by reference, so it won't detect this update.

Which method creates a new array for immutable updates in Svelte?

A push()

B filter()

C splice()

D pop()

filter() returns a new array without modifying the original, making it suitable for immutable updates.

How do you immutably update an object property in Svelte?

A obj.key = newValue

B Object.assign(obj, { key: newValue })

C store.update(obj =&gt; ({ ...obj, key: newValue }))

D delete obj.key

Using spread syntax inside store.update creates a new object with the updated property, triggering reactivity.

What happens if you directly modify a Svelte store's object without creating a new one?

A The UI will not update because the reference is unchanged

B Svelte automatically detects the change

C The app crashes

D The store resets to initial value

Svelte tracks changes by reference. Direct modification keeps the same reference, so no update is triggered.

Which is a correct immutable update for adding an item to a Svelte array store?

A store.update(arr =&gt; [...arr, newItem])

B store.update(arr =&gt; arr.push(newItem))

C store.update(arr =&gt; { arr.push(newItem); return arr; })

D store.update(arr =&gt; arr)

Using spread syntax creates a new array with the new item, triggering Svelte's reactivity.

Explain why immutable updates are important in Svelte and how you perform them for arrays and objects.

Think about how Svelte knows when data changes.

Describe a real-life example where immutable updates prevent bugs in a Svelte app.

Imagine editing a shopping list or to-do list.