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Headless add to cart button

Experiment #1055th April, 2021by Joshua Nussbaum

In the previous experiment, I created a <buy-button> web component. In this experiment, I add an “add to cart” button as well.


The API will be similar to the <buy-button>:

<!-- import js -->
<script src="https://domain.tld/cart.js"></script>

<!-- define a button -->
<add-button sku="t-shirt">Add to Cart</add-button>

Cart Storage

A cart can be stored server side or client side. This example will use the client side approach and persis the data in the browser’s localStorage.

In fact, I built a Svelte store for localStorage called svelte-local-storage-store, which will work perfectly here.

The idea is to have a dictionary with sku as the key and quantity as the value, ie {sku: quantity, ...}. We’ll also define accessor functions, like add(), clear() etc.. And to count the total quantity in the cart, we can use a derived store.

import { derived } from 'svelte/store'
import { writable } from 'svelte-local-storage-store'

// a store backed by local storage
export const cart = writable('cart', {})

// the count of items in the cart is derived from the cart store
export const count = derived(cart, $cart => {
  // use Array.reduce() to sum the quantity.
  return Object
    .reduce((acc, key) => acc + $cart[key], 0)

// add an item to the cart
export function add(sku) {
  // update the store
  cart.update($cart => {
    // check if the item already exists in the cart
    if ($cart[sku]) {
      // there's already one in the cart, so increment quantity
    } else {
      // it doesn't exist, so add it to the store with a quantity of 1
      $cart[sku] = 1

    return $cart

// clear the cart by setting it to an empty dictionary
export function clear() {

Button definition

The <add-button> is very similar the <buy-button> except it uses the cart:

<svelte:options tag="add-button"/>

  import {cart, add} from './cart'

  export let sku

  if (!sku) console.error('<add-button> sku must be set')

  function submit() {

<form on:submit|preventDefault={submit}>
  <button part="button">
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