svelte3 actions and getters using nested object store

colors.js

import {readable, writable} from 'svelte/store';

// could be its own module
const util = {
  _root: null,

  // similar to createSelector but instead returns a reactive variable
  createGetter(store, fn, initialData) {
    return readable(initialData, set => {
      store.subscribe(state => set(fn(state, this._root)));
    });
  },

  // set the root store
  setRoot(root) {
    this._root = root;
  },
};

//----------------------------------------------------------------------------
// State
//----------------------------------------------------------------------------

const initialState = {
  selected: 'green',
  colors: {
    red: '#f00',
    green: '#0f0',
    blue: '#00f',
  },
};

const store = writable(initialState);

//----------------------------------------------------------------------------
// Actions
//----------------------------------------------------------------------------

// writable publishes updates so no need to be immutable

export const add = (k, v) =>
  store.update(state => {
    state.colors[k] = v;
    return state;
  });

export const setSelected = k =>
  store.update(state => {
    state.selected = k;
    return state;
  });

//----------------------------------------------------------------------------
// Getters
//----------------------------------------------------------------------------

// usage: {#each $colorsGetter as [k, v]} ... {/each}
export const colorsGetter = util.createGetter(store, state => {
  const result = [];
  for (const k in state.colors) {
    // skip red to show filtering
    if (k === 'red') continue;
    result.push([k, state.colors[k]]);
  }
  return result;
});

export default store;

I don't think updates need to be immutable since writable publishes new value to subscribers.