You can also use bind:property={get, set}, where get and set are functions, allowing you to perform validation and transformation:

When you open this in two browsers and refresh a few times, one browser after the other, you’ll see the count go up and up (when looking at the page source), proving that the state is shared between both browsers (well, not really, it’s shared on the server, and used by both users). This will have serious consequences if you go this route: if user A is logged in and you’d write the user object to the shared state, and user B is not logged in, they’d still see a flash of user A’s username appear in the navigation bar, until the shared state is overwritten by the undefined user object.

export const state: State = $state({ user: undefined });

But sadly this introduces shared state on the server (when we use SSR), and this is a big problem since we’re now leaking data between different users.

One pattern that I love to use in my SvelteKit projects is returning writable stores from the layout’s load function. This makes it possible to fetch data from the server (for example the user object for the logged in user), and then you make this object available as a writable reactive store throughout the whole application. So when the user updates their username or avatar, you do the PUT request to the server and you get the updated user object back from the server as the response, you can simply update the $user writable store value and every place in your app where you show the user object gets updated immediately.

risk of accidentally exposing one user’s data to another

As with the previous example, this puts one user’s information in a place that is shared by all users.

You just got to be careful to never touch the global store from SSR!

conditionally returned either a readable store (from SSR) or a global writable store (from CSR) to make things like real time updates via WebSockets possible - although that solution does have one big advantage: the global store is always there.

This way when you use client side routing, you will only receive the _data.json generated at build time and use that to populate the page component with required data (kinda like Next.js does it).

If you use a random URL then option b won’t work because you can’t invalidate a random URL using that method.

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}) }

The user variable is shared by everyone who connects to this server.

Conceptually, that is. In reality, servers are often long-lived and shared by multiple users. For that reason it’s important not to store data in shared variables.

Yes, this is intended. You can do this with less boilerplate by doing export const selectedDateCtrl = $state({ selectedDate: new Date() })

If you’re coming from old Svelte: The $effect rune replaces the afterUpdate method (which is deprecated in Svelte 5), and The $effect.pre replaces the beforeUpdate method (which is also deprecated in Svelte 5).

Another shortcut available to diagnose where changes are coming from with the $inspect rune is to use: $inspect(variable).with(console.trace);

When shallow routing, you may want to render another +page.svelte inside the current page.

A universal load function can return an object containing any values, including things like custom classes and component constructors. A server load function must return data that can be serialized with devalue

mdsvex is a markdown preprocessor for Svelte components. Basically MDX for Svelte.

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Use a :global rule to only expose parts of the stylesheet:

you can also replicate the bind:this syntax if you please: Wrapper.svelte <script> let root export { root as this } </script> <div bind:this={root} />

You should mentioned what you listed after the word try_files. Here's what I ended up using that seemed to work: try_files $uri $uri/index.html $uri.html /index.html; The /index.html at the end needs to match the fallback: 'index.html' part of your adapter-static config. Otherwise going directly to a route that doesn't have a matching file at that path -- such as any route with a dynamic param like [id] -- will result in a 404.

In addition, a component that uses Svelte's built in event forwarding system cannot allow event listeners on the "capture" phase of the event lifecycle. It also cannot allow events to be cancelable with the browser's built in preventDefault function.

As a workaround, you can use setters in every affected reactive block instead of direct assignment. let localForm = {}; const setLocalForm = (data) => { localForm = data; }; $: setLocalForm({...$formData});

It's vanilla JS, doesn't bind you to specifc syntax and that's the main reason why I like svelte that it doesn't try to sandbox you into framework constraints.

const unsubscribe = errorStore.subscribe(value => { if (!value) { return } error = value errorStore.set() })

I personally abstract everything away into stores. Stores are amazing. With everything I mean things like fetch, Worker or WebSocket.

Another limitation is that you are forced into the syntax of the {#await} block. What I mean by that is that for example you can't add a loading class to a parent. You can only render stuff for the loading state in the given block. Nowhere else.

export const fibonacci = function (n, initialData) { return readable( { loading: true, error: null, data: initialData, }, (set) => { let controller = new AbortController(); (async () => { try { let result = await fibonacciWorker.calculate(n, { signal: controller.signal }); set({ loading: false, error: null, data: result, }); } catch (err) { // Ignore AbortErrors, they're not unexpected but a feature. // In case of abortion we just keep the loading state because another request is on its way anyway. if (err.name !== 'AbortError') { set({ loading: false, error: err, data: initialData, }); } } })(); return () => { controller.abort(); }; } ); };

<script> import { fibonacci } from './math.js'; $: result = fibonacci(n, 0); </script> <input type=number bind:value={n}> <p>The {n}th Fibonacci number is {$result.data}</p> {#if $result.loading} <p>Show a spinner, add class or whatever you need.</p> <p>You are not limited to the syntax of an #await block. You are free to do whatever you want.</p> {/if}

Yes I love stores.

export const load: Load = async ({ page, session }) => { if (!isPublic(page.path) && !isAuthenticated(session)) { console.log('Unauthorized access to private page'); return { redirect: '/', status: 302 }; } else { console.log('Auth OK'); } return {}; };

In hooks.js I have a handle function that basically does request.locals.jwt = cookies.jwt, and then a getSession function that returns { jwt: locals.jwt }

Instead of render props, we use Svelte's slot props: // React version <Listbox.Button> {({open, disabled} => /* Something using open and disabled */)} </Listbox.Button> <!--- Svelte version ---> <ListboxButton let:open let:disabled> <!--- Something using open and disabled ---> </ListboxButton>

It has many advantages but the main reason for me is that it simplifies your front end code. It's not perfect by any means, but overall its cons are worth it IMO.

SSR is used for pages as well, but prerendering means that rendering happens at build time instead of when a visitor visits the page.

import { goto } from '$app/navigation'; function routeToPage(route: string, replaceState: boolean) { goto(`/${route}`, { replaceState }) } replaceState == true will replace the route instead of adding to the browser history. So, when you click back, you will not go back to the route you came from.

All my stores also have a defaultValue property and reset method

I've said it multiples times and I say it again: in Svelte stores solve all your problems and I love them so much.

It works if you always want b to be the value deriving from a. However in the example above, we want the value of b to be temporarily out of sync of a.

My mental model has always been that a reactive declaration like $: b = a * 2 defines b in it's entirety ("Here's my recipe for b and all the ingredients, please Svelte make b always up to date"). And outside of this declaration b is immutable.

First of all, here is what I meant by updating reactive declared variable

The intention of the issue #2444 was to propagate the changes of the reactive declared variables back to its dependencies. in the context of this example, would meant, updating b should update a as well.

Explicitly defining the dependency in the reactive declarations

inputValue = updateInputValue(value);

I ended up writing a custom store that "buffers" sets for both a small time interval and ensuring only one async action is in flight (and triggering an extra round of async processing if a set was seen after the last async action was launched).

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

You declare a reactive variable and it's dependencies ("recipe").

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

Oh, I just figured out a workaround for my project, in case it helps someone. If you want the source of truth on the prop to come from the child component, then leave it undefined in the parent. Then, you can make the reactive variable have a condition on the presence of that variable. eg: <script> let prop; $: prop && console.log(prop); </script> <Child bind:prop/> Might not work for every use case but maybe that helps someone.

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

The complex typewriting effect is abstracted away in a neat function, out of site and out of mind.

this action is re-usable. Anytime you want a typewriter effect added to an element, you just import the function and apply it. So elegant.

Some call them the lifecycle methods of elements which I think is an apt description.

(you can get pretty far with <svelte:component> and passing component constructors around and spicing up props along the way)

You sure packed lot of good lessons and important concept explanations/illustrations into this little answer/tutorial. Well done.

The consumer component will barely change from our last example. The only difference is the way we'll get a reference to our store (since now the store is exported from the JS module):

In effect, the $ syntax we've seen above will actually setup a subscription to the store. And the subscription will be cancelled when the component is destroyed. If the store is subscribed by multiple components, the store will be disposed only when the last component unsubscribes (it will be reinitialized, if a new subscription is made). This is very handy to manage the lifecycle of disposable things in your code.

lifecycle is also managed automatically by Svelte

(And we've covered 75% of the store topic... They're efficient, to the point... And simple!)

Now your whole WS logic can be encapsulated in your JS module. That makes for good separation of concern.

In order to use this, we need to use a little more advanced readable store. Here's your example updated for this:

In your Svelte component, you can then use your store with the special $ prefix syntax, to access the value of the store ('cause the temperature variable is a reference to the store itself, it's just a mean to our end, the result we need is the value):

// our temperature is now a store with initial value 0 const temperature = writable(0); // now we don't need to change this function, the change will be propaged // by the store itself const getTemperature = () => { return temperature; }

Here's how we would rewrite our callback example with a store instead:

Same as our callback example, except they offer a handful of other very useful tool (like computing derived values from other stores), and also a slick syntax in the Svelte component.

Stores are essentially some kind of simplified streams (or Observable as they're called in ES), that is they represent a value over time.

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

Personally, I prefer an event-based communication, but I don't think it's actually better. The only problem with props is that it can cause problems if badly managed, but normally, both of them are ok. In real life, I would opt for storage.

Finally, I think Actions are great for animations, because you will definitely need to apply the same logic to many different elements. One of my favorite examples are FLIP animations, where a change in DOM position can be animated. For example shuffling a list of items. I will not dive deep into the topic in this article: I've written about some techniques in this article about FLIP animations in React and in this article about how to create spring animations with Web Animation API. Although they are not about Svelte, at the end of the day it all boils down to manipulating the HTML element directly. And Svelte Actions are a great place to do it.

But you can get quite close to directly updating CSS with Actions!

export function autofocus(node) { node.focus(); } That's it. This is a legitimate Svelte Action!

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