Browser-side stores and server-side middleware for building realtime Svelte apps

Usage no npm install needed!

<script type="module">
  import matrxSvelteRealtimeStore from 'https://cdn.skypack.dev/@matrx/svelte-realtime-store';



svelte-realtime-store is a drop-in replacement for a Svelte writable store that will synchronize its value across browser windows/tabs in realtime using socket.io.

If you currently have a svelte app that communicates with a node.js server (Sapper or otherwise), you can upgrade your existing application to Meteor or Firebase like realtime literally in a few minutes.

To work, you must have the @matrx/svelte-realtime-server running on your node.js server.


  • Requires only a few additional lines of code
  • Implements the expected interface of a Svelte writable store so Svelte's wonderful syntax just works
  • Caches the value on the server so newly joined and reconnected users catch up to the current state
  • Exposes a Svelte readable store for the connection status so you can decide how it should behave when offline. I recommend that you disable user input when offline unless you implement your own offline mode (including conflict resolution upon reconnect).
  • Uses a socket.io namespace to isolate it from anything else you might be using socket.io for
  • Exposes the socket.io namespaced socket to your application for advanced use cases


To install with npm

npm install --save @matrx/svelte-realtime-store

Head on over to @matrx/svelte-realtime-server to see how to install the server-side component.

Modify your .svelte files to look something like this

    import { getClient } from '@matrx/svelte-realtime-store'
    const realtimeClient = getClient()
    const a = realtimeClient.realtime('a', 1000)
    const connected = realtimeClient.connected
    function handleA(event) {
        $a = $a + 1

<button on:click={handleA} class="button" disabled="{!$connected}">a++</button>

One last step. You'll need to make the socket.io client available to the page. The easiest way to do that is to include this in your index.html file.

<script src="/socket.io/socket.io.js"></script>

This works because the socket.io server serves up its own version-matched client on that endpoint when installed on your server either stand-alone or when included as part of the svelte-realtime-server.

Alternatively, you can install the client as a seperate npm package and include it. See the socket.io documentation for details on how to do that.


  • Notice how I've used the provided realtimeClient.connected store to disable the button when the window is disconnected. This is recommended unless you implement your own offline mode (including conflict resolution upon reconnection).
  • The first parameter of realtimeClient.realtime() can be a simple string which will act as a unique identifier for the store which corresponds exactly to the name of the "room" in socket.io parlance.
  • If you are building a simple single user-base app, then you can do as I've done above and make the store id match the variable name.
  • However, for multi-tenancy or other cases where you want the a variable for one page/user/tenant to be isolated from the a variable for another, you'll need to provide a different id for each seperate instance. This can be done explicitly by proving an object with a storeID field (e.g. realtimeClient.realtime({storeID: 'something'}, 1000)). However, if your object lacks a storeID field it will run JSON.stringify() on it so something like this works using the Sapper-provided page store: realtimeClient.realtime({page, variable: 'a'}, 1000). If you are not using Sapper and don't have to worry about server-side rendering (SSR), you can just do: realtimeClient.realtime({page: window.location.href, variable: 'a'})
  • I'm getting ready to add serialization to a database upon store updates so you may see examples of _entityID rather than storeID. I'll come back later, once I have that working and explain that option.
  • The second parameter of realtimeClient.realtime() is now the default value rather than the initial value. What this means is that if the server has cached a value for this store, it'll start with that value rather than the one you provide.
  • Keep in mind that socket.io is very efficient at cleaning up "rooms" when there are no clients. So, the value will be flushed from the cache when all clients disconnect. The default or last browser update value will repopulate the cache upon reconnection.


realtime(storeConfig, default_value, component = null, start = noop)

storeConfig options

storeConfig can be a string like in the simple example above or it can be an object. If, it's an object and it contains a storeID field, that will be the name of the socket.io room used to identify other stores to synchronize to. If there is no storeID field, it'll fall back first to the _entityID field, if present and failing all that, the results of JSON.stringify(storeConfig) will be used as the storeID.

The debounceWait field if present is the number of milliseconds to wait before sending the changed value to the server for synchronization. Use this for text input fields or any other incrementally altered field to prevent your application from being overly chatty.

svelte-realtime-store allows you to have multiple stores on the same page that connect to the same synchronized room. In these cases, the store will update local copies immdiately ignoring the debounceWait.

Advanced usage

We expose the namespaced socket.io socket if you need it for more advanced uses.

For instance, under the covers, the realtimeClient.connected store is maintaining its state with something like this:

    realtimeClient.socket.on('disconnect', () => {
        console.log('You are no longer connected!')
    realtimeClient.socket.on('connect', () => {
        console.log('You are now connected!')


  • No permanent storage. As of this writing, it just synchronizes the views but the next step is to provide an adapter interface to save your data in the database of your choice. For now, you still need to take care of saving your data when it changes.
  • Beta. As of this writing, this package is under active development and using semver conventions for beta projects. Minor-level upgrades may be backward breaking while patch-level upgrades will be used for changes that are not backward breaking.
  • Svelte 3 only. Unless I'm mistaken, the store API is completely different in Svelte 2.