Persistent State Utilities

A tiny, type-safe toolkit for persisting React state to different backends using declarative schemas. Define how each field is serialized/deserialized once, and reuse the same schema with:

• LocalStorage
• URL Search Params
• Custom storages

Full TypeScript inference included.

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Features

• 🔒 Type-safe schemas with DataType<T> (serialize/deserialize per-field)
• 💾 Multiple backends: localStorage, URL query, or your own
• ⚛️ React hooks: usePersistentState, useLocalStorageState, useSearchParamsState
• 🧩 Default values supported per-field
• 📦 Utility fns for direct read/write without hooks

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Installation

npm i @sasha.p/use-persistent-state

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Core Concepts

DataType<T, D>

Defines how a single field is persisted:

export type DataType<T, D = null> = {
  serialize(value: NonNullable<T>): string;
  deserialize(serializedValue?: string | null | undefined): T | D;
};

• T — runtime type you want to work with
• D — fallback type when there is no value (defaults to null)
• deserialize should return either a value of T or the fallback D

Creating DataTypes

Use the provided factory helper to build consistent data types with optional defaults:

export const createDataTypeCtr = <T>(
  impl: (options?: { defaultValue?: T }) => DataType<T, any>,
) => {
  /* ... */
};

This yields curried creators like $String, $Number, etc., each supporting:

• No options → fallback is null
• { defaultValue } → fallback is that value

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Built-in Data Types

• $String – persists strings
• $StringArray – persists string arrays
• $Number – persists numbers
• $NumberArray – persists number arrays
• $Boolean – persists booleans

Example with defaults:

const name = $String({ defaultValue: 'John' });
const age = $Number({ defaultValue: 30 });
const tags = $StringArray({ defaultValue: [] });

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Schema

Schemas describe the shape of your persistent state:

const schema = {
  name: $String({ defaultValue: 'Guest' }),
  age: $Number(),
  tags: $StringArray({ defaultValue: [] }),
};

This produces a fully typed object when loaded.

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React Hooks

usePersistentState

Backend-agnostic hook that powers all others:

const [state, setState] = usePersistentState(schema, {
  get: (key) => storage.getItem(key),
  save: (data) => storage.setItem('myKey', JSON.stringify(data)),
});

• state → typed object matching schema
• setState(update) → updates and persists values

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useLocalStorageState

Persist state in localStorage:

const schema = {
  theme: $String({ defaultValue: 'light' }),
  count: $Number({ defaultValue: 0 }),
};

const [state, setState] = useLocalStorageState(schema, { key: 'app-settings' });

// Usage
console.log(state.theme); // "light"
setState({ theme: 'dark' });

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useSearchParamsState

Persist state in URL query params:

const schema = {
  page: $Number({ defaultValue: 1 }),
  filter: $String(),
};

const [state, setState] = useSearchParamsState(schema);

// Usage
console.log(state.page); // 1 (from ?page=1)
setState({ page: 2 }); // updates URL to ?page=2

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Non-hook Utilities

getPersistedValues

Read persisted values directly:

const values = getPersistedValues(schema, { key: 'app-settings' });

persistValues

Write values directly:

persistValues(schema, { theme: 'dark', count: 5 }, { key: 'app-settings' });

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Custom Storages

Implement the Storage interface to create your own backend:

export interface Storage<Key = string> {
  get(key: Key): string | null;
  save(data: object): void;
}

Example: sessionStorage adapter:

const sessionStorageAdapter: Storage<string> = {
  get: (key) => sessionStorage.getItem(key),
  save: (data) => sessionStorage.setItem('state', JSON.stringify(data)),
};

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Example

const schema = {
  username: $String({ defaultValue: 'Anonymous' }),
  darkMode: $Boolean({ defaultValue: false }),
};

function App() {
  const [state, setState] = useLocalStorageState(schema, { key: 'settings' });

  return (
    <div>
      <p>Hello, {state.username}</p>
      <button onClick={() => setState({ darkMode: !state.darkMode })}>
        Toggle Dark Mode
      </button>
    </div>
  );
}

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License

MIT © Your Name