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()
- Named export:
propertyStore()
- Regarding Subscription-less
svelte-writable-derived
Stores - Examples
- Browser compatibility
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.
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.
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.
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.
import { writable, get } from "svelte/store";
import writableDerived from "svelte-writable-derived";
var jsonStore = writable(`{"I'm a property": true}`);
var objectStore = writableDerived(
jsonStore,
(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}"
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"
valueStore.set("*whinny*");
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(
objectStore,
(object) => object["a horse"],
(reflecting, object) => {
object["a horse"] = reflecting;
return object; // needed to call objectStore.set with the proper value
}
);
// 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]
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"
valueStore.set("treasure");
console.log( get(objectStore) ); // { deeply: { buried: { item: "treasure" } } }
// Using writableDerived directly:
import writableDerived from "svelte-writable-derived";
var valueStore = writableDerived(
objectStore,
(object) => object.deeply.buried.item,
(reflecting, object) => {
object.deeply.buried.item = reflecting;
return object; // needed to call objectStore.set with the proper value
}
);
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"
// 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(
jsonStore,
(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 hoardContents.map( (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 = itemList.map( (itemListEntry) => {
return itemListEntry["item"];
} );
return [hoarder, hoardContents];
}
);
jsonStore.subscribe(console.log);
hoardStore.subscribe(console.log);
hoarderStore.subscribe(console.log);
// skipping hoardContentsStore
itemListStore.subscribe(console.log);
itemListStore.update( (itemList) => {
return itemList.map( (itemListEntry) => {
return {item: itemListEntry.item, owner: "protagonist"};
} );
} );
/*
Upon the update, the console logs:
[{item: "crown", owner: "protagonist"}, {item: "gold", owner: "protagonist"}]
"protagonist"
{owner: "protagonist", possessions: ["crown", "gold"]}
"{\"owner\": \"protagonist\", \"possessions\": [\"crown\", \"gold\"]}"
*/
This package should run anywhere Svelte can run. Use transpilers/polyfills as needed.
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!
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!
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~
Want me to sling some JavaScript for you? Look over my other work and contact me!