Svelte derived store with extended writable functionaility. A quick alternative to https://github.com/PixievoltNo1/svelte-writable-derived.

usage.ts

import { get, writable } from 'svelte/store'
import { writableDerived } from './writable-derived'

const foo = writable('bar')
const hello = writable('world')

const store = writableDerived(
  [foo, hello],
  ([$foo, $hello]) => ({
    fooKey: $foo,
    helloKey: $hello,
  }),
  (v) => {
    foo.set(v.fooKey)
    hello.set(v.helloKey)
  }
)

get(store) // => { fooKey: 'bar', helloKey: 'world' }

store.set({ fooKey: 'baz', helloKey: 'earth' })

get(foo) // => 'baz'

get(hello) // => 'earth'

writable-derived.ts

import { derived, get, Readable, Writable } from 'svelte/store'

/** Callback to update a value. */
type Updater<T> = (value: T) => T

/** One or more `Readable`s. */
type Stores = Readable<any> | [Readable<any>, ...Array<Readable<any>>]

/** One or more values from `Readable` stores. */
type StoresValues<T> = T extends Readable<infer U>
  ? U
  : {
      [K in keyof T]: T[K] extends Readable<infer U> ? U : never
    }

/** Enable `set` and `update` on derived stores */
export function writableDerived<S extends Stores, T>(
  stores: S,
  fn: (stores: StoresValues<S>) => T,
  set: (value: T) => void
): Writable<T> {
  const store = derived(stores, fn)

  return {
    subscribe: store.subscribe,
    set,
    update: (updater: Updater<T>) => set(updater(get(store))),
  }
}