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.
npm install state-guard
Here's how to use StateGuard to define a simple state machine for fetching website content:
- Import
createMachine
function from the StateGuard package.
import { createMachine } from 'state-guard';
- Create a
websiteContent
machine using thecreateMachine
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` },
},
});
- Subscribe to
websiteContent
to start fetching if in thefetching
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);
}
}
}
});
- Subscribe to
websiteContent
to log the current state and value.
websiteContent.subscribe(() => {
const { state, value } = websiteContent.get();
console.log(state, value);
});
- Trigger the
fetch
action in theresetted
state.
websiteContent.assert(`resetted`).actions.fetch(`https://example.com`);
- 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.
};
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.
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.