StateGuard

Type-safe, deterministic state management featuring state machines and automatic stale snapshot invalidation.

StateGuard is a JavaScript library for managing state with an emphasis on type safety, enabling seamless integration with TypeScript. It facilitates deterministic behavior by offering an encapsulated state machine, user-defined actions and state transformers, as well as automatic stale snapshot invalidation.

✅ 536 B with all dependencies, minified and gzipped.

Installation

npm install state-guard

Usage Example

Here's how to use StateGuard to define a simple state machine for fetching website content:

1. Import createMachine function from the StateGuard package.

import { createMachine } from 'state-guard';

2. Create a websiteContent machine using the createMachine function, with the initial state, value, a transformer map, and transitions map.

const websiteContent = createMachine({
  initialState: `resetted`,
  initialValue: undefined,

  transformerMap: {
    resetted: () => undefined,
    fetching: (url: string) => ({ url }),
    resolved: (text: string) => ({ text }),
    rejected: (error: unknown) => ({ error }),
  },

  transitionsMap: {
    resetted: { fetch: `fetching` },
    fetching: { resolve: `resolved`, reject: `rejected` },
    resolved: { reset: `resetted` },
    rejected: { reset: `resetted` },
  },
});

3. Subscribe to websiteContent to start fetching if in the fetching state.

websiteContent.subscribe(async () => {
  const fetching = websiteContent.get(`fetching`);

  if (fetching) {
    try {
      const response = await fetch(fetching.value.url);
      const text = await response.text();

      if (fetching.isFresh()) {
        fetching.actions.resolve(text);
      }
    } catch (error) {
      if (fetching.isFresh()) {
        fetching.actions.reject(error);
      }
    }
  }
});

4. Subscribe to websiteContent to log the current state and value.

websiteContent.subscribe(() => {
  const { state, value } = websiteContent.get();

  console.log(state, value);
});

5. Trigger the fetch action in the resetted state.

websiteContent.assert(`resetted`).actions.fetch(`https://example.com`);

6. Implement a React component using the useSyncExternalStore hook for state synchronization.

import * as React from 'react';

const YourComponent = () => {
  const websiteContentSnapshot = React.useSyncExternalStore(websiteContent.subscribe, () =>
    websiteContent.get(),
  );

  // Your component logic and rendering.
};

Ensuring Snapshot Freshness

In some cases, a snapshot taken can become stale, for example, when used after the result of an asynchronous operation. Using a stale snapshot will lead to exceptions being thrown, and it is crucial to ensure that this does not happen. The StateGuard API enables you to avoid such issues by allowing you to check the freshness of a snapshot or get an updated one before proceeding.

Avoiding State Transitions in Subscription Listeners

Performing state transitions directly within a subscription listener is prohibited in StateGuard. Using actions to change the state within a listener will lead to exceptions being thrown. This enforcement helps prevent cascading updates, exponential state changes, and potential violation of the unidirectional data flow principle.