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 scenarios where snapshots are used following asynchronous operations, it's critical to validate their freshness to ensure actions are based on the current state. To achieve this, use the isFresh() method on an existing snapshot instead of acquiring a new one via get(). This approach is preferred because even though a new snapshot might represent a state with the same name, it might not reflect the same execution context you're working within. A snapshot that remains fresh ensures that your code's execution branch is still active and relevant to the current state of the application.

Importantly, when performing such checks, avoid using await within blocks guarded by isFresh(). The reason is that during the delay introduced by await, the snapshot's state could have been altered by other operations, rendering it stale once the asynchronous operation completes. This could potentially lead to actions being taken based on outdated information.

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.