Getting Started with Readable & Writable Stores in Svelte

Let’s go ahead and create a global state management file in our Svelte project - let’s call it store.js and import the writable function.

import { writable } from "svelte/store";

let counter = writable(1);

export {counter}

We’ve created a variable called counter, which is a writable store. counter now has the following self-explanatory methods:

• set
• update

Let’s create a custom component called Nested.svelte and use the counter store we just created.

<script>
import { counter } from "./store.js";
</script>

<div>
 counter value: {$counter}
</div>

Notice that during usage, we prefix the the variable with $, since this is a named import.

Let’s wrap it up by importing the component in the App.svelte file and create a method to write the counter variable to observe reactivity across nested components.

<script>
import Nested from "./Nested.svelte";
import { counter } from "./store.js";

function incrementCounter() {
  counter.update(n => n + 1);
}
</script>

<div>
<button on:click={incrementCounter}>Update</button>
<Nested />
</div>

the counter uses an update method that takes a function whose parameter is the current value of the writable store and returns the modified value. If we run this app, we should be able to see the value inside the Nested component getting updated as we click on the button.

While we’re at it, let’s go ahead and add a reset button to App.svelte.

function resetCounter() {
  counter.set(1);
}

<button on:click={resetCounter}>Reset</button>

The resetCounter uses the set method of our writable store.

Now, the writable function also supports a second argument which is also a function. Here’s the signature for that function:

writable(value: any, (set: (value: any) => void) => () => void)

This function is fired when the first subscriber is created and it returns another function that is fired when the last subscription to the variable is destroyed. Let’s see that in action.

In our store.js, add the second argument to the writable function:

let counter = writable(1, () => {
  console.log("First subscriber added!");
  return () => {
    console.log("Last subscriber deleted :(");
  };
});

To test this, we’ll mount and unmount the Nested component to observe the behavior, in App.svelte:

<script>
// ...
let flag = false;
function toggleMount() {
    flag = !flag;
}
</script>

  <!-- ... -->

  <button on:click={toggleMount}>Mount/Unmount</button>

  {#if flag}
    <Nested />
  {/if}
</div>

Svelte also offers the readable function, which allows for creating readable stores whose values cannot be updated from other components. The value has to set from within the store. Let’s try this out, modify the store.js -

import { readable } from "svelte/store";

let initialVal = Math.floor(Math.random()*100);

let counter = readable(initialVal, (set) => {
  let incrementCounter = setInterval( () => {
    let newVal = Math.floor(Math.random()*100);
    set(newVal);
  }, 1000);
  return () => {
    clearInterval(incrementCounter);
  };
});

export {counter}

Here the readable counter is set with the initialVal, which is being passed as the first argument. The second argument is the same as with writable stores, but this time it’s a required parameter since without it there would be no other way to access the counter value to reset it.

In this example, we generate random numbers between 0 to 100 and assign this to counter by using the set method. update is not available. This is a simple demo, but in real apps readable stores can use the second argument to make API calls and, based on some logic, set the value. This will render the components that are subscribed to this store.

As you saw, by using writable and readable stores in Svelte, we can achieve a basic form of global state management pretty easily! ✨