This Little-Known JavaScript API Changes How We Handle Async States Across Threads

1-2-App.jsx

// App.jsx

import { AtomicState, useAtomicValue } from "./shared-atomic-state";
import './App.css';

// Create a shared state instance (could be moved to a context or module)
const atomicState = new AtomicState();

// Create two atomic values
const counterA = atomicState.createAtomicValue(0, "int");
const counterB = atomicState.createAtomicValue(100, "int");

function CounterA() {
  const value = useAtomicValue(counterA);

  return (
    <div>
      <h3>Counter A: {value}</h3>
      <button onClick={() => counterA.setValue(value + 1)}>Increment A</button>
    </div>
  );
}

function CounterB() {
  const value = useAtomicValue(counterB);

  return (
    <div>
      <h3>Counter B: {value}</h3>
      <button onClick={() => counterB.setValue(value + 10)}>Increment B</button>
    </div>
  );
}

function AtomicDemo() {
  return (
    <div>
      <h2>Atomic State Demo</h2>
      <CounterA />
      <CounterB />
    </div>
  );
}

function App() {
  return (
    <AtomicDemo />
  )
}

export default App;

2-2-App.jsx

// App.jsx

import React, { useRef } from "react";
import { CoordinationHub, useChannel } from "./coordination-hub";

// A broadcaster component that updates the channel state
function Broadcaster({ hub, channelName }) {
  const inputRef = useRef();

  const handleBroadcast = () => {
    const value = parseInt(inputRef.current.value, 10) || 0;
    const channel = hub.getChannel(channelName) || hub.createChannel(channelName);
    channel.broadcast(value);
  };

  return (
    <div style={{ marginBottom: 20 }}>
      <h2>Broadcaster</h2>
      <input ref={inputRef} type="number" defaultValue={0} />
      <button onClick={handleBroadcast}>Broadcast State</button>
    </div>
  );
}

// A listener component that subscribes to channel updates
function Listener({ hub, channelName }) {
  const state = useChannel(hub, channelName);

  return (
    <div>
      <h2>Listener</h2>
      <div>Current Channel State: {state}</div>
    </div>
  );
}

// Main App component
export default function App() {
  // Only create one hub instance for the app
  const hubRef = useRef();
  if (!hubRef.current) hubRef.current = new CoordinationHub();

  return (
    <div style={{ padding: 40 }}>
      <h1>Coordination Hub Demo</h1>
      <Broadcaster hub={hubRef.current} channelName="demo" />
      <Listener hub={hubRef.current} channelName="demo" />
    </div>
  );
}

coordination-hub.js

import { useEffect, useState } from "react";

// coordination-hub.js
export class CoordinationHub {
  constructor() {
    this.buffer = new SharedArrayBuffer(256);
    this.view = new Int32Array(this.buffer);
    this.channels = new Map();
    this.nextChannelId = 0;
  }

  createChannel(name) {
    const channelId = this.nextChannelId++;
    const stateIndex = channelId * 2;     // Current state
    const signalIndex = channelId * 2 + 1; // Signal counter
    
    Atomics.store(this.view, stateIndex, 0);
    Atomics.store(this.view, signalIndex, 0);
    
    const channel = {
      name,
      channelId,
      stateIndex,
      signalIndex,
      
      broadcast: (state) => {
        Atomics.store(this.view, stateIndex, state);
        const signalCount = Atomics.add(this.view, signalIndex, 1) + 1;
        Atomics.notify(this.view, signalIndex);
        return signalCount;
      },
      
      waitForBroadcast: async (lastSignalCount = 0, timeout = 10000) => {
        const result = Atomics.waitAsync(
          this.view, 
          signalIndex, 
          lastSignalCount, 
          timeout
        );
        
        if (result.async) {
          const status = await result.value;
          return status === 'ok' ? {
            state: Atomics.load(this.view, stateIndex),
            signalCount: Atomics.load(this.view, signalIndex)
          } : null;
        }
        
        return null;
      },
      
      getCurrentState: () => ({
        state: Atomics.load(this.view, stateIndex),
        signalCount: Atomics.load(this.view, signalIndex)
      })
    };
    
    this.channels.set(name, channel);
    return channel;
  }

  getChannel(name) {
    return this.channels.get(name);
  }
}

// React hook for channel subscription
export function useChannel(hub, channelName) {
  const [channelState, setChannelState] = useState(null);
  
  useEffect(() => {
    const channel = hub.getChannel(channelName) || hub.createChannel(channelName);
    let lastSignalCount = channel.getCurrentState().signalCount;
    
    async function listenForBroadcasts() {
      while (true) {
        const update = await channel.waitForBroadcast(lastSignalCount);
        if (update) {
          setChannelState(update.state);
          lastSignalCount = update.signalCount;
        }
      }
    }
    
    // Set initial state
    setChannelState(channel.getCurrentState().state);
    listenForBroadcasts();
  }, [hub, channelName]);

  return channelState;
}

demo.js

const ResilientAtomicState = require('./resilient-state');

// Instantiate the state manager
const state = new ResilientAtomicState();

// Example: Update state with a function
async function demoUpdate() {
  try {
    const result = await state.updateState((current) => current + 5);
    console.log('Update result:', result);
  } catch (err) {
    console.error('Update failed:', err);
  }
}

// Example: Listen for checkpoints (in background)
async function listenCheckpoints() {
  for await (const checkpoint of state.listenForCheckpoints()) {
    console.log('Checkpoint reached:', checkpoint);
  }
}

// Example: Trigger multiple updates
async function runDemo() {
  // Start checkpoint listener
  listenCheckpoints();

  // Perform several updates
  for (let i = 0; i < 15; i++) {
    await demoUpdate();
  }

  // Get current state
  const current = state.getState();
  console.log('Current state:', current);
}

runDemo();

resilient-state.js

// resilient-state.js
class ResilientAtomicState {
  constructor() {
    // State management buffer:
    // [0]: primary state value
    // [1]: backup state value  
    // [2]: state version/generation
    // [3]: error count
    // [4]: last error code
    // [5]: recovery attempts
    // [6]: health check flag
    // [7]: checkpoint trigger
    
    this.buffer = new SharedArrayBuffer(32);
    this.coords = new Int32Array(this.buffer);
    
    // Initialize state
    Atomics.store(this.coords, 0, 0); // primary state
    Atomics.store(this.coords, 1, 0); // backup state
    Atomics.store(this.coords, 2, 1); // version
    Atomics.store(this.coords, 3, 0); // error count
    Atomics.store(this.coords, 4, 0); // last error
    Atomics.store(this.coords, 5, 0); // recovery attempts
    Atomics.store(this.coords, 6, 1); // health (1 = healthy)
    Atomics.store(this.coords, 7, 0); // checkpoint trigger
  }

  // Resilient state update with automatic recovery
  async updateState(updateFn, maxRetries = 3, backoffMs = 100) {
    for (let attempt = 0; attempt < maxRetries; attempt++) {
      try {
        // Check system health before attempting update
        const health = Atomics.load(this.coords, 6);
        if (health === 0) {
          await this.waitForRecovery(5000);
        }
        
        // Read current state
        const currentState = Atomics.load(this.coords, 0);
        const currentVersion = Atomics.load(this.coords, 2);
        
        // Calculate new state
        const newState = await updateFn(currentState);
        
        // Attempt atomic update with version check
        const success = Atomics.compareExchange(
          this.coords, 0,      // primary state index
          currentState,        // expected current value
          newState            // new value
        ) === currentState;
        
        if (success) {
          // Update successful, increment version and backup state
          const newVersion = Atomics.add(this.coords, 2, 1) + 1;
          Atomics.store(this.coords, 1, newState); // Update backup
          
          // Trigger checkpoint if needed
          if (newVersion % 10 === 0) {
            Atomics.store(this.coords, 7, newVersion);
            Atomics.notify(this.coords, 7); // Notify checkpoint handlers
          }
          
          // Reset error count on success
          Atomics.store(this.coords, 3, 0);
          
          return { success: true, value: newState, version: newVersion };
        } else {
          // CAS failed, another thread updated state
          if (attempt < maxRetries - 1) {
            await this.sleep(backoffMs * Math.pow(2, attempt));
            continue;
          }
        }
        
      } catch (error) {
        // Handle errors and update error metrics
        const errorCount = Atomics.add(this.coords, 3, 1) + 1;
        Atomics.store(this.coords, 4, error.code || -1);
        
        // Trigger recovery if error threshold exceeded
        if (errorCount >= 5) {
          await this.triggerRecovery();
        }
        
        if (attempt === maxRetries - 1) {
          throw new Error(`State update failed after ${maxRetries} attempts: ${error.message}`);
        }
        
        await this.sleep(backoffMs * Math.pow(2, attempt));
      }
    }
    
    return { success: false, error: 'Max retries exceeded' };
  }

  // Trigger recovery process
  async triggerRecovery() {
    // Mark system as unhealthy
    Atomics.store(this.coords, 6, 0);
    
    // Increment recovery attempt counter
    Atomics.add(this.coords, 5, 1);
    
    // Attempt to restore from backup
    const backupState = Atomics.load(this.coords, 1);
    const primaryState = Atomics.load(this.coords, 0);
    
    // If primary state is corrupted, restore from backup
    if (this.validateState(primaryState) === false) {
      Atomics.store(this.coords, 0, backupState);
      console.log('Restored primary state from backup');
    }
    
    // Reset error count
    Atomics.store(this.coords, 3, 0);
    
    // Mark system as healthy
    Atomics.store(this.coords, 6, 1);
    
    // Notify waiting operations
    Atomics.notify(this.coords, 6);
  }

  // Wait for system recovery
  async waitForRecovery(timeout = 10000) {
    const result = Atomics.waitAsync(this.coords, 6, 0, timeout);
    
    if (result.async) {
      const status = await result.value;
      return status === 'ok';
    }
    
    return false;
  }

  // Listen for checkpoint triggers
  async *listenForCheckpoints() {
    let lastCheckpoint = 0;
    
    while (true) {
      const result = Atomics.waitAsync(this.coords, 7, lastCheckpoint, 30000);
      
      if (result.async) {
        const status = await result.value;
        if (status === 'ok') {
          lastCheckpoint = Atomics.load(this.coords, 7);
          yield {
            version: lastCheckpoint,
            state: Atomics.load(this.coords, 0),
            backup: Atomics.load(this.coords, 1),
            timestamp: Date.now()
          };
        }
      }
    }
  }

  // Get current state safely
  getState() {
    return {
      primary: Atomics.load(this.coords, 0),
      backup: Atomics.load(this.coords, 1),
      version: Atomics.load(this.coords, 2),
      healthy: Atomics.load(this.coords, 6) === 1,
      errorCount: Atomics.load(this.coords, 3),
      recoveryAttempts: Atomics.load(this.coords, 5)
    };
  }

  // Validate state integrity (implement based on your needs)
  validateState(state) {
    // Add your validation logic here
    return state !== null && state !== undefined && !isNaN(state);
  }

  sleep(ms) {
    return new Promise(resolve => setTimeout(resolve, ms));
  }
}

module.exports = ResilientAtomicState;

shared-atomic-state.js

import { useEffect, useReducer } from 'react';

// shared-atomic-state.js
export class AtomicState {
  constructor(size = 1024) {
    this.buffer = new SharedArrayBuffer(size);
    this.intView = new Int32Array(this.buffer);
    this.floatView = new Float64Array(this.buffer);
    this.nextIndex = 0;
  }

  createAtomicValue(initialValue = 0, type = 'int') {
    const index = this.nextIndex++;
    const view = type === 'int' ? this.intView : this.floatView;
    
    Atomics.store(view, index, initialValue);
    
    return {
      index,
      view,
      getValue: () => Atomics.load(view, index),
      setValue: (newValue) => {
        Atomics.store(view, index, newValue);
        Atomics.notify(view, index);
      },
      waitForChange: async (currentValue, timeout = 5000) => {
        const result = Atomics.waitAsync(view, index, currentValue, timeout);
        return result.async ? await result.value : result.value;
      }
    };
  }
}

// React hook for surgical updates
export function useAtomicValue(atomicValue) {
  const [, forceUpdate] = useReducer(x => x + 1, 0);
  
  useEffect(() => {
    async function watchForChanges() {
      while (true) {
        const currentValue = atomicValue.getValue();
        const status = await atomicValue.waitForChange(currentValue);
        
        if (status === 'ok') {
          forceUpdate(); // Only THIS component re-renders
        }
      }
    }
    
    watchForChanges();
  }, [atomicValue]);

  return atomicValue.getValue();
}

thread-integration.js

const { Worker, isMainThread, workerData, parentPort } = require('worker_threads');

if (isMainThread) {
  // --- Main Thread: Coordinator ---
  const coordinationBuffer = new SharedArrayBuffer(1024);
  const coords = new Int32Array(coordinationBuffer);

  // Indices: 0 = head, 1 = tail, 2 = active worker count
  Atomics.store(coords, 0, 0); // Task queue head
  Atomics.store(coords, 1, 0); // Task queue tail
  Atomics.store(coords, 2, 0); // Active worker count

  const cpuCount = require('os').cpus().length;
  const workers = [];

  for (let i = 0; i < cpuCount; i++) {
    const worker = new Worker(__filename, {
      workerData: { coordinationBuffer, workerId: i }
    });

    worker.on('message', (msg) => {
      console.log(`Worker ${i}: ${msg}`);
    });

    worker.on('error', (err) => {
      console.error(`Worker ${i} error:`, err);
    });

    worker.on('exit', (code) => {
      if (code !== 0) console.error(`Worker ${i} stopped with exit code ${code}`);
    });

    workers.push(worker);
  }

  // Periodically distribute random work
  setInterval(() => {
    const taskCount = Math.floor(Math.random() * 10) + 1;
    distributeWork(coords, taskCount);
  }, 1000);

  function distributeWork(coords, taskCount) {
    // Atomically add tasks to queue
    const oldTail = Atomics.add(coords, 1, taskCount);

    // Notify waiting workers
    const awakened = Atomics.notify(coords, 1);
    console.log(`Added ${taskCount} tasks, awakened ${awakened} workers`);
  }

} else {
  // --- Worker Thread ---
  const coords = new Int32Array(workerData.coordinationBuffer);

  runWorker(coords, workerData.workerId, parentPort);

  async function runWorker(coords, workerId, parentPort) {
    Atomics.add(coords, 2, 1); // Increment active worker count

    while (true) {
      const currentTail = Atomics.load(coords, 1);
      const currentHead = Atomics.load(coords, 0);

      if (currentTail <= currentHead) {
        // No work available, wait for new tasks
        const result = Atomics.waitAsync(coords, 1, currentTail, 5000);

        if (result.async) {
          const status = await result.value;
          if (status === 'timed-out') {
            continue; // Check again
          }
        }
      }

      // Try to claim a task
      const taskId = Atomics.add(coords, 0, 1); // Increment head atomically
      const availableTasks = Atomics.load(coords, 1);

      if (taskId < availableTasks) {
        // Successfully claimed task
        const taskResult = await processTask(taskId, workerId);
        parentPort.postMessage(`Completed task ${taskId}: ${taskResult}`);
      }
    }
  }

  async function processTask(taskId, workerId) {
    // Simulate work
    await new Promise(resolve => setTimeout(resolve, Math.random() * 1000));
    return `Task-${taskId}-by-Worker-${workerId}`;
  }
}