Middleware

Using middleware in Aurelia Store to intercept and transform state changes

Middleware act as an intermediary step in your application to allow you to intercept, modify, or block state changes. They run either before or after your action handlers execute, giving you powerful control over state transitions.

How Middleware Works

Middleware functions receive:

• The current state

• The dispatched action

• Optional settings you provide during registration

They can:

• Transform state by returning a new state object

• Block actions by returning false

• Pass through by returning undefined or nothing

• Run asynchronously by returning a Promise

Creating Middleware

A middleware function has the following signature:

import { IStateMiddleware } from '@aurelia/state';

// Basic middleware (no settings)
const loggingMiddleware: IStateMiddleware<MyState> = (state, action, settings) => {
  console.log('Action dispatched:', action);
  console.log('Current state:', state);
  // Return undefined to pass through without changes
};

// Middleware with settings
interface ValidationSettings {
  strict: boolean;
}

const validationMiddleware: IStateMiddleware<MyState, ValidationSettings> = (state, action, settings) => {
  if (settings.strict && !isValidAction(action)) {
    return false; // Block the action
  }
  // Pass through
};

Registering Middleware

Use registerMiddleware() on the store to add middleware. Specify whether it should run 'before' or 'after' action handlers:

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

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

  bound() {
    // Run before action handlers
    this.store.registerMiddleware(loggingMiddleware, 'before');

    // Run after action handlers, with settings
    this.store.registerMiddleware(validationMiddleware, 'after', { strict: true });
  }

  unbinding() {
    // Clean up middleware when component is destroyed
    this.store.unregisterMiddleware(loggingMiddleware);
    this.store.unregisterMiddleware(validationMiddleware);
  }
}

Common Middleware Patterns

Logging Middleware

import { IStateMiddleware } from '@aurelia/state';

const loggerMiddleware: IStateMiddleware = (state, action) => {
  console.group('State Change');
  console.log('Action:', action);
  console.log('State before:', state);
  console.groupEnd();
  // Return undefined to pass through
};

// Register as 'before' to log before state changes
store.registerMiddleware(loggerMiddleware, 'before');

Validation Middleware

import { IStateMiddleware } from '@aurelia/state';

interface AppState {
  user: { name: string; email: string } | null;
  items: any[];
}

const validationMiddleware: IStateMiddleware<AppState> = (state, action) => {
  // Block actions when user is not authenticated
  if (!state.user && action.type !== 'LOGIN') {
    console.warn('Action blocked: User not authenticated');
    return false;
  }
  // Pass through
};

store.registerMiddleware(validationMiddleware, 'before');

Transform Middleware

import { IStateMiddleware } from '@aurelia/state';

// Add timestamps to state after every action
const timestampMiddleware: IStateMiddleware<AppState> = (state, action) => {
  return {
    ...state,
    lastUpdated: Date.now(),
    lastAction: action.type
  };
};

// Register as 'after' to transform state after handlers run
store.registerMiddleware(timestampMiddleware, 'after');

Async Middleware

import { IStateMiddleware } from '@aurelia/state';

// Middleware that persists state to localStorage
const persistMiddleware: IStateMiddleware<AppState> = (state, action) => {
  try {
    localStorage.setItem('app-state', JSON.stringify(state));
  } catch (e) {
    console.error('Failed to persist state:', e);
  }
  // Return undefined to pass through unchanged
};

store.registerMiddleware(persistMiddleware, 'after');

Analytics Middleware

import { IStateMiddleware } from '@aurelia/state';

interface AnalyticsSettings {
  endpoint: string;
  enabled: boolean;
}

const analyticsMiddleware: IStateMiddleware<AppState, AnalyticsSettings> = async (state, action, settings) => {
  if (!settings.enabled) return;

  await fetch(settings.endpoint, {
    method: 'POST',
    body: JSON.stringify({
      action: action.type,
      timestamp: Date.now()
    })
  });
};

store.registerMiddleware(analyticsMiddleware, 'after', {
  endpoint: '/api/analytics',
  enabled: true
});

Middleware Execution Order

When multiple middleware are registered for the same placement, they execute in registration order:

store.registerMiddleware(middleware1, 'before'); // Runs first
store.registerMiddleware(middleware2, 'before'); // Runs second
store.registerMiddleware(middleware3, 'before'); // Runs third

Each middleware receives the state returned by the previous middleware:

const addOneMiddleware: IStateMiddleware<{ count: number }> = (state) => {
  return { ...state, count: state.count + 1 };
};

const doubleMiddleware: IStateMiddleware<{ count: number }> = (state) => {
  return { ...state, count: state.count * 2 };
};

// If initial count is 5:
// addOneMiddleware runs → count becomes 6
// doubleMiddleware runs → count becomes 12
store.registerMiddleware(addOneMiddleware, 'after');
store.registerMiddleware(doubleMiddleware, 'after');

Blocking Actions

Returning false from middleware prevents the action from completing:

• Before middleware returns false: Action handlers don't run, state isn't updated

• After middleware returns false: State changes from handlers are discarded

const rateLimitMiddleware: IStateMiddleware = (() => {
  let lastDispatch = 0;

  return (state, action) => {
    const now = Date.now();
    if (now - lastDispatch < 100) {
      console.warn('Rate limited');
      return false; // Block rapid dispatches
    }
    lastDispatch = now;
  };
})();

Type Safety

Use TypeScript generics for type-safe middleware:

import { IStateMiddleware } from '@aurelia/state';

interface AppState {
  user: User | null;
  todos: Todo[];
  settings: Settings;
}

interface MiddlewareSettings {
  logLevel: 'debug' | 'info' | 'error';
}

// Fully typed middleware
const typedMiddleware: IStateMiddleware<AppState, MiddlewareSettings> = (state, action, settings) => {
  // state is typed as AppState
  // settings is typed as MiddlewareSettings
  if (settings.logLevel === 'debug') {
    console.log('User:', state.user?.name);
    console.log('Todo count:', state.todos.length);
  }
};

Best Practices

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

2. Avoid side effects in 'before' middleware — Save side effects (API calls, logging) for 'after' middleware when possible

3. Clean up middleware — Unregister middleware when components are destroyed to prevent memory leaks

4. Handle errors gracefully — Errors in middleware are caught and logged, but the state passes through unchanged

5. Use settings for configuration — Pass configuration via settings rather than closures for better testability

6. Consider async implications — Async middleware can delay state updates; use sparingly for time-sensitive operations