give-svelte-store-previous-behaviour

A wrapper for any Svelte Store instance, giving access to the previously set value in a style that follows familiar Svelte Store semantics.

It's a wrap

give-svelte-store-previous-behaviour does not instantiate a Store. Rather, it wraps an existing Store, leaving you with full control over how your Store is instantiated. This is important because:

• You are not prevented from further adding your own augmentations to the Store, either before it has been given Previous Behaviour or after.

• It allows you to apply other wrappers, too, such as give-svelte-store-persistence-behaviour.

This philosophy allows for a flexible, compositional approach, as is used in the core Svelte Store code that creates readable and derived Stores.

Examples

Once wrapped with Previous Behaviour, a Store gains three extra pieces of functionality. Here is an example of each:

Using getPrevious

Use of getPrevious follows the same pattern as Svelte's native Store get function:

import { writable, set, get } from "svelte/store";

import {
    giveSvelteStorePreviousBehaviour,
    getPrevious
} from "@drtrt/give-svelte-store-previous-behaviour";

// Initialise a `writable` store and then wrap it:
const storeWithPrevious =
    giveSvelteStorePreviousBehaviour(
        writable("secondValue")
    );

// Set the store to a new value:
storeWithPrevious.set("firstValue");

// Use `get` to get the current value:
console.log(get(storeWithPrevious));
// Output: firstValue

// Use `getPrevious` to get the previous value:
console.log(getPrevious(storeWithPrevious));
// Output: secondValue

Additional points to consider for getPrevious:

• Until the Store's value has changed at least once after initialisation, getPrevious will return undefined.
• As per the guidance for using get, one would usually read the Previous Value by subscribing to the Store rather than using getPrevious.

Using subscribe

The Store's subscribe function will, in addition to its existing value parameter, gain an extra previousValue param:

import { writable } from "svelte/store";

import {
    giveSvelteStorePreviousBehaviour
} from "@drtrt/give-svelte-store-previous-behaviour";

// Function imported from an example logging utility
import { logStateChange } from "./logging";

// Initialise a `writable` store and then wrap it:
const storeWithPrevious =
    giveSvelteStorePreviousBehaviour(
        writable("firstValue")
    );

// Use `subscribe` to, for example, log changes to state:
storeWithPrevious.subscribe((value, previousValue) => {
    logStateChange({
        storeName: "storeWithPrevious",
        from: value,
        to: previousValue,
    });
});

// `subscribe` can still be used as normal:
storeWithPrevious.subscribe((value) => {
    console.info(`Store value changed to ${value}`)
});

Considerations for using subscribe:

• Until the Store's value has changed at least once after initialisation, the previousValue parameter passed to the Subscribe Function will be undefined.
• subscribe will also continue to function as normal, in that you can still give it a function that has only the one value parameter.

Using previousValueStore

The Store will have an additional previousValueStore property that yields a Readable store containing the previous value. This is so you can use Reactive Bindings for the Previous Value, too:

<script>
    import { writable } from "svelte/store";

    import {
        giveSvelteStorePreviousBehaviour
    } from "@drtrt/give-svelte-store-previous-behaviour";

    // Initialise a `writable` store and then wrap it:
    const booleanStore =
        giveSvelteStorePreviousBehaviour(
            writable(true)
        );

    function flipBoolean() {
        booleanStore.update(x => !x);
    }

    // Get `previousValueStore`
    const { previousValueStore } = booleanStore;
</script>

<div>
    Current Boolean Value is: {$booleanStore}
</div>

<div>
    Previous Boolean Value was: {$previousValueStore}
</div>

<button on:click={flipBoolean}>
    Flip Boolean
</button>

Further considerations for using previousValueStore:

• Until the Store's value has changed at least once after initialisation, the value of previousValueStore will be undefined.
• previousValueStore instantiation is optimized such that it is created when accessed, and not before.
• previousValueStore can be retrieved immediately or at any time; it will still hold the correct Previous Value at the time it is accessed.

Installation

NPM

npm install @drtrt/give-svelte-store-previous-behaviour

Yarn

yarn add @drtrt/give-svelte-store-previous-behaviour

And then giveSvelteStorePreviousBehaviour can be used thusly:

ECMAScript Modules (ESM)

import { giveSvelteStorePreviousBehaviour } from "@drtrt/give-svelte-store-previous-behaviour";

CommonJS (CJS)

const { giveSvelteStorePreviousBehaviour } = require("@drtrt/give-svelte-store-previous-behaviour");

Types

A full set of types is available for TypeScript consumers.

Full detail is available in the dedicated Types documentation.

Questions

Why did you spell 'behaviour' wrong?

Answer: I didn't. 😉

Release History

The Change Log for this package is available in the GitHub Repo, here.