TypeScript icon, indicating that this package has built-in type declarations

3.1.0 • Public • Published


npm Bundle size GitHub CI Status License GitHub Repo stars

For users of Svelte v3 and v4, this is a read-write variant of Svelte's derived stores that accepts an extra callback to send values back to the source. It builds on the derived & writable stores provided by Svelte, and emulates their behavior as closely as possible.

This project has a Code of Conduct. By participating in the Git repo or issues tracker, you agree to be as courteous, welcoming, and generally a lovely person as its terms require. 😊

Default & named export: writableDerived()

Parameters: origins (store or array of stores), derive (function), reflect (see documentation), optional initial (any)
Returns a store with writable methods

Create a store that behaves similarly to Svelte's derived, with origins, derive, and initial working like its 1st, 2nd, and 3rd parameters respectively. Values introduced to the store via its set and update methods are passed to the new 3rd parameter, reflect, which can in turn set values for the origin stores.

As long as derived and reflect set stores only by the means provided to them and not via any store's methods, they won't trigger calls to each other.

New parameter: reflect

Function with parameters: reflecting (any), optional old (any)

This function is called when the derived store gets a new value via its set and update methods (not via the derive callback). Its reflecting parameter is this new value, and old is the origin store's current value, or an array of values if origins is an array. It must return a value to set in the origin store, or an array of values to set if origins was an array. If the returned array is sparse or shorter than origins, it will only set the stores it has elements for, and other stores don't necessarily need to be writable.

reflect is called after the derived store's subscriptions are called. If the derived store has its set and/or update methods called again in the process of calling its subscriptions, reflect will be called only once, with the most-recently-set value.

arguments, default parameters, and rest parameters will not receive old unless the function's length is at least 2.

Named export: propertyStore()

Parameters: origin (store), propName (string, number, symbol, or array of strings/numbers/symbols)
Returns a store with writable methods

A utility wrapper for writableDerived. Given a store containing an object, this function returns a store containing the value of the object's property propName. If propName is an array, it's used as a path to navigate nested objects.

Regarding Subscription-less svelte-writable-derived Stores

One of the ways this package emulates the behavior of Svelte's derived is that it does not subscribe to any origin store until the derived store itself has a subscription. However, there's an exception: Calling the set and update methods when the derived store has no subscriptions will subscribe to & then unsubscribe from all its origins.


Making an object store from a JSON string store

import { writable, get } from "svelte/store";
import writableDerived from "svelte-writable-derived";

var jsonStore = writable(`{"I'm a property": true}`);
var objectStore = writableDerived(
	(json) => JSON.parse(json),
	(object) => JSON.stringify(object)
console.log( Object.keys( get(objectStore) ) ); // ["I'm a property"]
objectStore.set({"I'm not a property": false});
console.log( get(jsonStore) ); // "{\"I'm not a property\": false}"

Making a single-value store from an object store

import { writable, get } from "svelte/store";
import { propertyStore } from "svelte-writable-derived";

var objectStore = writable({"a horse": "a horse", "of course": "of course"});
var valueStore = propertyStore(objectStore, "a horse");
console.log( get(valueStore) ); // "a horse"
console.log( get(objectStore) ); // {"a horse": "*whinny*", "of course": "of course"}

// propertyStore is just a wrapper. You could also use writableDerived directly:

import writableDerived from "svelte-writable-derived";

var valueStore = writableDerived(
	(object) => object["a horse"],
	(reflecting, object) => {
		object["a horse"] = reflecting;
		return object; // needed to call objectStore.set with the proper value

... when the object is an array

// An array is an object with numerically-named properties.
// Access them using a number for the propName parameter.

import { writable, get } from "svelte/store";
import { propertyStore } from "svelte-writable-derived";

var treasureCoordinates = writable([7, -2, 31]);
var treasureElevation = propertyStore(treasureCoordinates, 1);
console.log( get(treasureElevation) ); // -2
treasureElevation.set(1); // dig up the treasure
console.log( get(treasureCoordinates) ); // [7, 1, 31]

... when the value is deeply nested in the object

import { writable, get } from "svelte/store";
import { propertyStore } from "svelte-writable-derived";

var objectStore = writable({ deeply: { buried: { item: "trash" } } });
var valueStore = propertyStore(objectStore, ["deeply", "buried", "item"]);
console.log( get(valueStore) ); // "trash"
console.log( get(objectStore) ); // { deeply: { buried: { item: "treasure" } } }

// Using writableDerived directly:

import writableDerived from "svelte-writable-derived";

var valueStore = writableDerived(
	(object) => object.deeply.buried.item,
	(reflecting, object) => {
		object.deeply.buried.item = reflecting;
		return object; // needed to call objectStore.set with the proper value

Making an object store from several single-value stores

import { writable, get } from "svelte/store";
import writableDerived from "svelte-writable-derived";

var valueStore1 = "sparta", valueStore2 = "monty python's flying circus";
var objectStore = writableDerived(
	[valueStore1, valueStore2],
	([value1, value2]) => ( {"this is": value1, "it's": value2} ),
	(object) => [ object["this is"], object["it's"] ]
console.log( get(objectStore) ); // {"this is": "sparta", "it's": "monty python's flying circus"}
objectStore.set( {"this is": "rocket league", "it's": "over 9000"} );
console.log( get(valueStore1), get(valueStore2) ); // "rocket league" "over 9000"

Chaining all of the above together

// What if Rube Goldberg were a JavaScript developer?
import { writable, get } from "svelte/store";
import { writableDerived, propertyStore } from "svelte-writable-derived";

var jsonStore = writable(`{"owner": "dragon", "possessions": ["crown", "gold"]}`);
var hoardStore = writableDerived(
	(json) => JSON.parse(json),
	(object) => JSON.stringify(object)

var hoarderStore = propertyStore(hoardStore, "owner");
var hoardContentsStore = propertyStore(hoardStore, "possessions");

var itemListStore = writableDerived(
	[hoarderStore, hoardContentsStore],
	([hoarder, hoardContents]) => {
		return (item) => {
			return {item, owner: hoarder};
	(itemList) => {
		// This is only for demonstration purposes, so we won't handle differing owners
		var hoarder = itemList[0].owner;
		var hoardContents = (itemListEntry) => {
			return itemListEntry["item"];
		} );
		return [hoarder, hoardContents];

// skipping hoardContentsStore
itemListStore.update( (itemList) => {
	return (itemListEntry) => {
		return {item: itemListEntry.item, owner: "protagonist"};
	} );
} );
	Upon the update, the console logs:
	[{item: "crown", owner: "protagonist"}, {item: "gold", owner: "protagonist"}]
	{owner: "protagonist", possessions: ["crown", "gold"]}
	"{\"owner\": \"protagonist\", \"possessions\": [\"crown\", \"gold\"]}"

Browser compatibility

This package should run anywhere Svelte can run. Use transpilers/polyfills as needed.

💖 Support the developer

I muchly appreciate any way you'd like to show your thanks - knowing people are helped gives me warm fuzzies and makes it all worthwhile!

💸 ... with money

At my Ko-Fi page, you can make a one-time or monthly donation, or commission work on an issue.

You can also support all your dependencies at once using StackAid!

💌 ... with kind words

Current contact info is on this page - or you can create an "issue" on this repo just to say thanks! Thank-you "issues" will be closed right away, but are treasured regardless~

🤝 ... with a job

Want me to sling some JavaScript for you? Look over my other work and contact me!

Package Sidebar


npm i svelte-writable-derived

Weekly Downloads






Unpacked Size

21.9 kB

Total Files


Last publish


  • pixievoltno1