Svelte: derived stores based on an arbitrary array of stores

Say you have an array of stores, the length of which is not necessarily known. You want to create a derived store that updates its value when any of these array values changes. The derived store syntax is a bit obtuse on how this can be done, but it is possible. Here's an example:

import { writable } from "svelte/store"

const storeArray = [writable(1), writable(1), writable(1)]

const derivedStore = derived(storeArray, ([...arr]) => {
  // Do stuff with your array of stores
  // e.g., this code will set the derived store's value to true whenever all the stores are greater than 10:
  return arr.every(item => item > 10);
});

The ... spread syntax here allows any number of stores to be passed into the derived constructor's callback function.

A caveat - the derived store won't be notified if new items are added to the store array, or the array itself is redeclared. For that, you could consider making the array itself a store, and use set and update whenever you want to change it.

storeArray = writable(storeArray)
const derivedStore = derived(
  [storeArray, ...storeArray], 
  ([...arr]) => {
    // arr[0] is the array itself, and arr.slice(1) contains the contents
  }
);