const count = signal(0); const doubled = computed(() => count.value * 2);

When writing component tests that involve two-way bindings, context or snippet props, it’s best to create a wrapper component for your specific test and interact with that. @testing-library/svelte contains some examples.

However, to do that, you have to put the $state value in a callback (or getter/setter object) in the parameters of the function. This feels rather clunky and un-Svelte like. Here's an example of what I mean.

let myObj = { ...$get({foo, bar}) }

Explicitly defining the dependency in the reactive declarations

inputValue = updateInputValue(value);

We should instead invalidate the ultimate dependencies of filtered rather than filtered itself:

Svelte currently consistently gives the reactive statement priority over your checked binding.

maybe this just comes down to general footguns with reactivity, and being aware of them as a developer.

Stores are the idiomatic Svelte way when you need to import "reactivity" from your normal JS sources.

if you put the reactive statement $: validate(inputValue); after the function updateInputValue declaration, it's working as expected:

And on any given day, developing with Svelte and its reactive nature is simply a dream to use. You can tell Svelte to track state changes on practically anything using the $: directive. And it’s quite likely that your first reactive changes will produce all the expected UI results. But as you start to rely more on UI updates based on variable or array/object changes, it’s likely that your UI will start skipping a beat and dropping values that you know explicitly to be there.

Personally I think option 1 is the way to go as it doesn't allocate any memory to create a new array but rather modifies the existing array. Then the assignment just lets the compiler know that you modified the array.

Or just watch the value with a reactive statement instead of an event handler on the input like: $: inputValue, onInput() This is a good solution and seems to work in all cases.

What I think is happening is that instantiating the component is immediately running the $: reactive code, which dispatches the event synchronously, before the parent component attaches the listener.

For me, this makes Svelte the most reactive "framework" at the moment

I too have been confused by behavior like this. Perhaps a clearly defined way to isolate atomic units with synchronous reactivity would help those of us still working through the idiosyncrasies of reactivity.

For performance reasons, $: reactive blocks are batched up and run in the next microtask. This is the expected behavior. This is one of the things that we should talk about when we figure out how and where we want to have a section in the docs that goes into more details about reactivity. If you want something that updates synchronously and depends on another value, you can use a derived store:

even though f() reads the reactive variable count in its body, because the variable count isn't present in the reactive statement itself, the statement is not seen as having a dependency on count.

I expected that the param being passed by reference to have the same reactivity of the variable being created in the script tag.

There's an issue #3368 for describing better what triggers reactive updates, as I do think there is some stuff we could be more explicit about

This isn't a bug. Svelte's reactivity is triggered by assignment, so it won't re-render on mutations like array pushing.

this is a common Svelte idiom for manually invalidating something that might have been mutated without there being a visible = sign somewhere

Store is reactive because it should re-evaluate when track is changed.

The $: can also be used to trigger effects.

Any variable referenced on the lines marked with $: is marked as a dependency of the derived variable.

Do these blocks allow for reactive assignments? @lukeed They're read-only, but values update when their dependencies do

$: for watching de­pen­den­cies in ex­pres­sions, a much more concise useEffect, if React is familiar.

As you can see, it uses Javascript label syntax to tell the compiler where recalculation should be carefully managed. And you thought that no one uses labels any more.

You know those useMemo and useCallback hooks? Well, Svelte has something like this. If you declare a variable like this: $: double = counter * 2;, Svelte will recalculate it only if counter changes.

for example, reactive declarations essentially do the work of React's useMemo, useCallback and useEffect without the boilerplate (or indeed the garbage collection overhead of creating inline functions and arrays on each state change).

Since one of Svelte's main principles is reacting only to data changes (as opposed to re-render the whole app with the full state all the time)

Using a reactive statement is a good way to explicitly turn a reactive variable into a store

Non-store reactivity needs to be analyzable at compile time.

Svelte only runs the <script> code once.

$: declares a relationship, where remaining should always be 10 - count. This might seem weird, but it’s conceptually the same as declaring a variable that you know will be re-calculated in every React render loop — except that remaining will get recalculated only when count changes.

Since you often want to do calculations based on state, Svelte also has the “reactive declaration” symbol, $:. It’s like a let declaration, but whenever any variable referenced in the expression — count in this case — is updated, the expression is re-run, the new variable’s value is updated, and the component is re-rendered.

The heart of Svelte’s magic is “reactivity”. Every let declaration sets up a listener, where any time the variable is assigned to, the change triggers a render of the component. So when the increment function calls count++, the component will re-render, which will update the value shown to the user by Count: {count}.

Reactive statements run immediately before the component updates, whenever the values that they depend on have changed.

You're not limited to using $count inside the markup, either — you can use it anywhere in the <script> as well, such as in event handlers or reactive declarations.

$: console.log(`the count is ${$count}`);