State

A guide on working with the Aurelia State plugin.

This guide aims to help you become familiar with the Aurelia State plugin, a means of managing the state in your Aurelia applications.

When should you use state management?

Before we delve too deeply into Aurelia State and how it can help manage state in your Aurelia applications, we should discuss when to use state management and when not to.

• When you need to reuse data in other parts of your application — State management shines when it comes to helping keep your data organized for cross-application reuse.

• When dealing with complex data structures — Ephermal state is great for simple use cases. Still, when working with complex data structures (think multi-step forms or deeply structured data), state management can help keep it consistent.

Think about the state plugin as an event aggregator but customized for state management.

Aurelia State guides

Installing Aurelia State

To install the Aurelia State plugin, open up a Command Prompt/Terminal and install it:

npm i @aurelia/state

Setup the initial state

When registering the Aurelia State plugin, you must pass in your application's initial state. This is an object which defines the data structure of your application.

Create a new file in the src directory called initialstate.ts with your state object inside:

export const initialState = {
  keywords: '',
  items: []
};

As you can see, it's just a plain old Javascript object. In your application, your properties would be called something different, but you can see we have a mixture of empty values and some defaults.

This state will be stored in the global state container for bindings to use in the templates.

Setup the action handlers

The initial state above isn't meant to be mutated directly. In order to produce a state change, a mutation request should be dispatched instead. An action handler is a function that is supposed to combine the current state of the state container and the action parameters of the function call to produce a new state:

const actionHandler = (state, action) => newState

An example of an action handler that produces a new state with the updated keyword on a keyword action:

export function keywordsHandler(currentState, action) {
  return action.type === 'newKeywords'
    ? { ...currentState, keywords: action.value }
    : currentState
}

Create a new file in the src directory called action-handlers.ts with the above code.

Configuration

To use the Aurelia State plugin in Aurelia, it needs to be imported and registered. Inside main.ts, the plugin can be registered as follows:

import Aurelia from 'aurelia';
import { StateDefaultConfiguration } from '@aurelia/state';

import { initialState } from './initialstate';
import { keywordsHandler } from './action-handlers';

Aurelia
  .register(
    StateDefaultConfiguration.init(
      initialState,
      keywordsHandler
    )
  )
  .app(MyApp)
  .start();

The above imports the StateDefaultConfiguration object from the plugin and then calls init, passing the initial state object and action handlers for your application.

For advanced use cases with middleware, the second parameter can be an options object:

import { MiddlewarePlacement } from '@aurelia/state';
import { loggingMiddleware } from './middleware';

Aurelia
  .register(
    StateDefaultConfiguration.init(
      initialState,
      {
        middlewares: [
          { middleware: loggingMiddleware, placement: MiddlewarePlacement.Before }
        ]
      },
      keywordsHandler
    )
  )
  .app(MyApp)
  .start();

If you are familiar with Redux, you'll find this plugin familiar. The most obvious difference will be around the action handler (similar to reducer in Redux) function signature.

Template binding

With .state and .dispatch commands

The Aurelia State Plugin provides state and dispatch binding commands that simplify binding with the global state. Example usages:

// bind value property of the input to `keywords` property on the global state
<input value.state="keywords">

// dispatch an action object `{ type: 'clearKeywords' }` to request state mutation
<button click.dispatch="{ type: 'clearKeywords' }">Clear keywords</button>

// bind value property of the input to `keywords` property on the global state
// and dispatch an action with type `newKeywords` on input event
<input value.state="keywords" input.dispatch="{ type: 'newKeywords', value: $event.target.value }">

With & state binding behavior

In places where it's not possible to use the .state binding command, the global state can be connected via & state binding behavior, like the following example:

<p>Found ${items.length & state} results for keyword: "${keyword & state}"</p>
<div repeat.for="item of items & state">
  ${item.name}
</div>

Accessing view model

Note: by default, bindings created from .state and .dispatch commands will only allow you to access the properties on the global state. If it's desirable to access the property of the view model containing those bindings, use $parent like the following example:

// access the property `prefix` on the view model, and `keywords` property on the global state
<input value.state="$parent.prefix + keywords">

View model binding

With @fromState decorator

Sometimes, it's also desirable to connect a view model property to the global state. The Aurelia State Plugin supports this via the @fromState decorator. An example usage is as follows:

export class AutoSuggest {
  @fromState(state => state.keywords)
  keywords: string;
}

With the above, whenever the state changes, it will ensure the keywords property of the view model stays in sync with the keywords property on the global state.

Memoizing derived state

Expensive computations in @fromState selectors will run on every state change by default. To avoid unnecessary work, the createStateMemoizer helper allows you to memoize derived values so they are recomputed only when their dependencies actually change.

import { fromState, createStateMemoizer } from '@aurelia/state';

interface State { items: number[]; }

const selectTotal = createStateMemoizer(
  (s: State) => s.items,
  items => items.reduce((a, b) => a + b, 0)
);

export class Summary {
  @fromState(selectTotal)
  total!: number;
}

In the example above, the selectTotal function executes only when items changes by reference. Other state updates won't trigger a recalculation, keeping the component performant and giving derived logic a clear place to live.

When you only need to read a value from state or perform a cheap calculation, passing a simple function directly to @fromState is usually adequate. The decorated property will update on every state change, which keeps things straightforward.

createStateMemoizer shines when deriving data is expensive or shared across multiple components. Because the selector remembers its last inputs, recalculation happens only when those inputs change by reference. This reduces wasted work and centralizes complex logic.

Here is another example using multiple selectors:

interface State { items: string[]; search: string; }

const selectFiltered = createStateMemoizer(
  (s: State) => s.items,
  (s: State) => s.search,
  (items, term) => items.filter(i => i.includes(term))
);

export class Results {
  @fromState(selectFiltered)
  results!: string[];
}

In contrast, writing the filter inline like @fromState(s => s.items.filter(i => i.includes(s.search))) would rerun on every single state update, even when neither items nor search changed.

Authoring action handlers

As mentioned at the start of this guide, action handlers are the way to handle mutation of the global state. They are expected to return to a new state instead of mutating it. Even though normal mutation works, it may break future integration with devtool.

Action handlers can be either synchronous or asynchronous. An application may have one or more action handlers, and if one action handler is asynchronous, a promise will be returned for the dispatch call.

An action handler should return the existing state (first parameters) if the action is unnecessary.

Middleware

Middleware provides a way to intercept and process actions before or after they reach your action handlers. This is particularly useful for cross-cutting concerns such as logging, validation, authentication, error handling, and performance monitoring.

What is middleware?

Middleware functions are executed before or after actions are processed by action handlers. Middleware can be either synchronous or asynchronous. When a middleware returns a Promise, the store will await it and use the resolved value (if any) before continuing the dispatch pipeline. This makes it possible to perform tasks such as API calls, logging to remote endpoints, reading from IndexedDB, etc. without blocking the UI thread.

Middleware can:

• Log actions for debugging and auditing

• Validate actions before processing

• Transform or modify actions

• Handle authentication and authorization

• Monitor performance

• Process state after action handlers complete

Creating middleware

A middleware function has the following signature:

type IStateMiddleware<TState = any, TSettings = any> = (
  state: TState,
  action: unknown,
  settings?: TSettings
) => TState | undefined | false | void | Promise<TState | undefined | false | void>;

Here's a simple logging middleware example:

export const loggingMiddleware = (state, action, settings) => {
  console.log('Action dispatched:', action);
  console.log('Current state:', state);
  return state; // Return the state unchanged
};

And an example of an asynchronous middleware:

export const asyncLoggingMiddleware = async (state, action, settings) => {
  await api.log(action);
  return state; // Return the state unchanged
};

Middleware return values

Middleware can return different values to control execution:

• Return state object: Continue execution with the returned state

• Return undefined or void: Continue with the current state unchanged

• Return false: Block the action from proceeding further

export const validationMiddleware = (state, action) => {
  if (!action.type) {
    console.error('Action must have a type');
    return false; // Block invalid actions
  }
  return state; // Continue with valid actions
};

Registering middleware

Middleware is registered during state plugin configuration using the MiddlewarePlacement enum to specify when the middleware should run:

import Aurelia from 'aurelia';
import { StateDefaultConfiguration, MiddlewarePlacement } from '@aurelia/state';

import { initialState } from './initialstate';
import { keywordsHandler } from './action-handlers';
import { loggingMiddleware, validationMiddleware } from './middleware';

Aurelia
  .register(
    StateDefaultConfiguration.init(
      initialState,
      {
        middlewares: [
          { middleware: loggingMiddleware, placement: MiddlewarePlacement.Before },
          { middleware: validationMiddleware, placement: MiddlewarePlacement.Before }
        ]
      },
      keywordsHandler
    )
  )
  .app(MyApp)
  .start();

Middleware placement

Use MiddlewarePlacement to control when middleware executes:

• MiddlewarePlacement.Before: Runs before action handlers

• MiddlewarePlacement.After: Runs after action handlers

// Before middleware - good for validation, logging incoming actions
{ middleware: validationMiddleware, placement: MiddlewarePlacement.Before }

// After middleware - good for logging final state, cleanup
{ middleware: stateLoggerMiddleware, placement: MiddlewarePlacement.After }

Middleware execution order

Middleware functions are executed in the order they are registered within their placement group:

const firstMiddleware = (state, action) => {
  console.log('First middleware');
  return state;
};

const secondMiddleware = (state, action) => {
  console.log('Second middleware');
  return state;
};

// Registration order determines execution order
middlewares: [
  { middleware: firstMiddleware, placement: MiddlewarePlacement.Before },
  { middleware: secondMiddleware, placement: MiddlewarePlacement.Before }
]

// Output:
// First middleware
// Second middleware
// [Action handlers execute]

Common middleware patterns

1. Validation middleware

export const validationMiddleware = (state, action) => {
  // Validate action structure
  if (!action.type) {
    console.error('Action must have a type property');
    return false; // Block invalid action
  }

  // Validate specific action types
  if (action.type === 'updateUser' && !action.payload?.id) {
    console.error('updateUser action must include user id');
    return false;
  }

  return state; // Continue with valid action
};

2. Authentication middleware

export const authMiddleware = (state, action) => {
  const protectedActions = ['deleteUser', 'updateSettings', 'createPost'];

  if (protectedActions.includes(action.type) && !state.user?.isAuthenticated) {
    console.error('Authentication required for this action');
    return false; // Block unauthorized action
  }

  return state;
};

3. Action transformation middleware

export const transformMiddleware = (state, action) => {
  // Add timestamp to all actions
  const enhancedAction = {
    ...action,
    timestamp: Date.now()
  };

  // Note: This modifies the action object for subsequent middleware/handlers
  Object.assign(action, enhancedAction);

  return state;
};

4. Error handling middleware

export const errorHandlingMiddleware = (state, action) => {
  try {
    // Validate action payload
    if (action.type === 'complexOperation' && !isValidPayload(action.payload)) {
      throw new Error('Invalid payload for complexOperation');
    }

    return state;
  } catch (error) {
    console.error('Middleware error:', action.type, error);

    // Return modified state with error information
    return {
      ...state,
      error: {
        message: error.message,
        action: action.type,
        timestamp: new Date().toISOString()
      }
    };
  }
};

5. Performance monitoring middleware

export const performanceMiddleware = (state, action) => {
  // Log slow action types (this is a simple example)
  const slowActions = ['heavyComputation', 'dataProcessing'];

  if (slowActions.includes(action.type)) {
    console.time(`Action: ${action.type}`);
    // Note: In a real scenario, you'd need after middleware to log completion
  }

  return state;
};

// Corresponding after middleware
export const performanceAfterMiddleware = (state, action) => {
  const slowActions = ['heavyComputation', 'dataProcessing'];

  if (slowActions.includes(action.type)) {
    console.timeEnd(`Action: ${action.type}`);
  }

  return state;
};

Middleware with settings

Middleware can accept custom settings through the third parameter:

export const loggingMiddleware = (state, action, settings) => {
  const { logLevel = 'info', prefix = '[STATE]' } = settings || {};

  if (logLevel === 'debug') {
    console.debug(`${prefix} Action:`, action);
    console.debug(`${prefix} State:`, state);
  } else {
    console.log(`${prefix} ${action.type}`);
  }

  return state;
};

// Register with settings
middlewares: [
  {
    middleware: loggingMiddleware,
    placement: MiddlewarePlacement.Before,
    settings: { logLevel: 'debug', prefix: '[APP]' }
  }
]

Middleware factory pattern

For more advanced scenarios, you can create factory functions that return middleware:

export const createLoggingMiddleware = (actionTypeFilter) => (state, action, settings) => {
  const { logLevel = 'info', prefix = '[STATE]' } = settings || {};

  if (!actionTypeFilter || action.type === actionTypeFilter) {
    if (logLevel === 'debug') {
      console.debug(`${prefix} Action:`, action);
      console.debug(`${prefix} State:`, state);
    } else {
      console.log(`${prefix} ${action.type}`);
    }
  }

  return state;
};

// Register factory-created middleware
middlewares: [
  {
    middleware: createLoggingMiddleware('updateUser'),
    placement: MiddlewarePlacement.Before,
    settings: { logLevel: 'debug', prefix: '[APP]' }
  }
]

Runtime middleware management

You can add and remove middleware at runtime using the store instance:

import { resolve } from 'aurelia';
import { IStore, MiddlewarePlacement } from '@aurelia/state';

export class MyComponent {
  private store: IStore = resolve(IStore);

  addDebugMiddleware() {
    const debugMiddleware = (state, action) => {
      console.log('Debug:', action.type);
      return state;
    };

    this.store.registerMiddleware(debugMiddleware, MiddlewarePlacement.Before);
  }

  removeDebugMiddleware() {
    // Note: You need to keep a reference to the middleware function
    this.store.unregisterMiddleware(this.debugMiddleware);
  }
}

Best practices for middleware

1. Keep middleware focused: Each middleware should have a single responsibility

2. Handle errors gracefully: Use try-catch blocks and return appropriate values

3. Consider performance: Avoid heavy computations in frequently executed middleware

4. Use TypeScript: Leverage type safety for better development experience

5. Order matters: Register middleware in the correct execution order for your use case

6. Return appropriate values: Use false to block actions, return state to continue

7. Test thoroughly: Write unit tests for your middleware functions

8. Consider async cost: Async middleware is supported but will delay the completion of dispatch until the Promise settles. Keep any network / long-running work minimal or move it off the critical path when possible.

Debugging middleware

When troubleshooting middleware issues, you can add debug middleware to trace execution:

export const debugMiddleware = (state, action) => {
  console.group(`Middleware Debug: ${action.type}`);
  console.log('Current state:', state);
  console.log('Action:', action);
  console.groupEnd();

  return state;
};

Example of type declaration for application stores

Applications can enforce strict typings with view model-based dispatch calls via the 2nd type parameter of the store.

For example, if the store only accepts two types of actions, that has the following type:

export type EditAction = { type: 'edit'; value: string }
export type ClearAction = { type: 'clear' }

Then the store can be declared like this:

import { resolve } from 'aurelia';
import { IStore } from '@aurelia/state';

class MyEl {
  store: IStore<{}, EditAction | ClearAction> = resolve(IStore);

  onSomeUserAction() {
    this.store.dispatch({ type: 'edit', value: 'hi' }); // good
    this.store.dispatch({ type: 'edit' }); // error 💥
    this.store.dispatch({ type: 'clear' }); // good
  }
}