Observables: Simple svelte v3 store wrapper that integrates Immer.

observables.js

import { derive, readable, writable } from 'svelte/store.js'
import { produce } from 'immer'

export const update = produce

export function computed(deps, reactor) {
  const source = derive(deps, reactor)
  return {
    subscribe: source.subscribe,
    get snapshot() { return getSnapshot(source) }
  }
}

export function stream(generator, value) {
  const source = readable(generator, value)
  return {
    subscribe: source.subscribe,
    get snapshot() { return getSnapshot(source) }
  }
}

export function observable(data) {
  const store = writable(data);
  let snapshot = data

  function update(fn) { store.update(state => snapshot = produce(state, fn)) }
  function set(data) { store.set(snapshot = data) }

  return {
    update, set,
    subscribe: store.subscribe,
    get snapshot() { return snapshot },
  };
}

export function getSnapshot(readable) {
  let value
  readable.subscribe((data) => value = data)()
  return value
}