What are Custom stores?

Ashutosh Kukreti

Typically in a store (Writable), there are three methods:

  1. subscribe()
  2. set()
  3. update()

As long as we implemented the subscribe() method, the javascript object is a store.

A store provides reactive data that can change over time. What if we want to create stores that restrict updates? It makes sense on large web/mobile applications, where we put restrictions on other components to overwrite the store variables.  In other words, we allow store variables to update within the store only.

To create a custom store:

  1. Create a writable store in a function
  2. Return subscribe() function on an object.

Let's assume we have a shopping cart, and we need to add or remove items from there. In this example, we only increase or decrease the count of the cart. We start with a simple prototype and then move to more complicated in the upcoming articles.

Create a new file shoppingcart.js under the src directory.

import { writable } from 'svelte/store';

function addOrRemoveCartItems() {
    const { subscribe, set, update } = writable(0);

    return {
        add: () => { update(n => n + 1) },
        // Shopping Cart doesnot contain Negative Items
        remove: () => { update(n => n <= 0 ? 0 : n - 1 ) },
        resetCart: () => { set(0) }

export const items = addOrRemoveCartItems();

And in the App.svelte


    import { items } from "./shoppingcart";


    <p> Custom Store Example. </p>

    <h1> Total Cart Items { $items }</h1>

    <button on:click={ items.add }> Add to Cart </button>
    <button on:click={ items.remove }> Remove from Cart </button>
    <button on:click={ items.resetCart }> Remove All Items </button>


	main {
		text-align: center;
		padding: 1em;
		max-width: 240px;
		margin: 0 auto;

	@media (min-width: 640px) {
		main {
			max-width: none;

Refresh the page and it'll look like this

In this article, we learn about custom stores. See you in the upcoming articles. And please don't forget to subscribe to our newsletter to get the latest updates.