# JavaScript Screen Wake Lock API Complete Guide

The Screen Wake Lock API allows web pages to prevent the device screen from entering sleep mode. This article covers its usage and practical applications.

Basic Concepts

Detecting Support

// Check API support
if ('wakeLock' in navigator) {
  console.log('Screen Wake Lock API is supported');
} else {
  console.log('Screen Wake Lock API is not supported');
}

Requesting Wake Lock

// Request screen wake lock
async function requestWakeLock() {
  try {
    const wakeLock = await navigator.wakeLock.request('screen');
    console.log('Wake lock activated');
    return wakeLock;
  } catch (error) {
    console.error('Could not activate wake lock:', error.message);
    return null;
  }
}

// Usage example
let wakeLock = null;

document.getElementById('start-btn').addEventListener('click', async () => {
  wakeLock = await requestWakeLock();
});

Releasing Wake Lock

// Release wake lock
async function releaseWakeLock(wakeLock) {
  if (wakeLock) {
    await wakeLock.release();
    console.log('Wake lock released');
  }
}

// Usage example
document.getElementById('stop-btn').addEventListener('click', async () => {
  await releaseWakeLock(wakeLock);
  wakeLock = null;
});

Listening for Release Events

async function requestWakeLock() {
  try {
    const wakeLock = await navigator.wakeLock.request('screen');
    
    // Listen for release event
    wakeLock.addEventListener('release', () => {
      console.log('Wake lock was released');
      // Possible reasons:
      // 1. Called release()
      // 2. Page became hidden
      // 3. Low battery
    });
    
    console.log('Wake lock activated');
    return wakeLock;
  } catch (error) {
    console.error('Could not activate wake lock:', error);
    return null;
  }
}

Wake Lock Manager

Wrapper Class

class WakeLockManager {
  constructor() {
    this.wakeLock = null;
    this.isSupported = 'wakeLock' in navigator;
    this.listeners = new Set();
    
    // Auto-reacquire when page becomes visible
    document.addEventListener('visibilitychange', () => {
      this.handleVisibilityChange();
    });
  }
  
  get isActive() {
    return this.wakeLock !== null && !this.wakeLock.released;
  }
  
  async request() {
    if (!this.isSupported) {
      console.warn('Screen Wake Lock API not supported');
      return false;
    }
    
    if (this.isActive) {
      console.log('Wake lock already active');
      return true;
    }
    
    try {
      this.wakeLock = await navigator.wakeLock.request('screen');
      
      this.wakeLock.addEventListener('release', () => {
        this.notify('release');
      });
      
      this.notify('acquire');
      console.log('Wake lock activated');
      return true;
    } catch (error) {
      console.error('Failed to request wake lock:', error);
      this.notify('error', error);
      return false;
    }
  }
  
  async release() {
    if (!this.isActive) return;
    
    try {
      await this.wakeLock.release();
      this.wakeLock = null;
      console.log('Wake lock released');
    } catch (error) {
      console.error('Failed to release wake lock:', error);
    }
  }
  
  async handleVisibilityChange() {
    if (document.visibilityState === 'visible' && this.wakeLock?.released) {
      // Re-request wake lock when page becomes visible
      console.log('Page visible, re-requesting wake lock');
      await this.request();
    }
  }
  
  onChange(callback) {
    this.listeners.add(callback);
    return () => this.listeners.delete(callback);
  }
  
  notify(event, data = null) {
    this.listeners.forEach(callback => callback(event, data));
  }
}

// Usage
const wakeLockManager = new WakeLockManager();

wakeLockManager.onChange((event, data) => {
  switch (event) {
    case 'acquire':
      console.log('Wake lock acquired');
      break;
    case 'release':
      console.log('Wake lock released');
      break;
    case 'error':
      console.error('Wake lock error:', data);
      break;
  }
});

// Request wake lock
wakeLockManager.request();

Auto-managed Wake Lock

class AutoWakeLock {
  constructor(options = {}) {
    this.manager = new WakeLockManager();
    this.autoReacquire = options.autoReacquire !== false;
    this.shouldBeActive = false;
    
    if (this.autoReacquire) {
      document.addEventListener('visibilitychange', () => {
        if (document.visibilityState === 'visible' && this.shouldBeActive) {
          this.manager.request();
        }
      });
    }
  }
  
  async enable() {
    this.shouldBeActive = true;
    return await this.manager.request();
  }
  
  async disable() {
    this.shouldBeActive = false;
    return await this.manager.release();
  }
  
  get isActive() {
    return this.manager.isActive;
  }
  
  get isSupported() {
    return this.manager.isSupported;
  }
}

// Usage
const autoWakeLock = new AutoWakeLock({ autoReacquire: true });

// Enable (will auto-reacquire when page becomes visible)
autoWakeLock.enable();

// Disable
// autoWakeLock.disable();

Practical Applications

Reading Mode

class ReadingMode {
  constructor() {
    this.wakeLock = new AutoWakeLock();
    this.isEnabled = false;
    this.scrollPosition = 0;
  }
  
  async enable() {
    if (this.isEnabled) return;
    
    this.isEnabled = true;
    await this.wakeLock.enable();
    
    // Save scroll position
    this.scrollPosition = window.scrollY;
    
    // Apply reading mode styles
    document.body.classList.add('reading-mode');
    
    // Hide unnecessary elements
    this.hideElements();
    
    console.log('Reading mode enabled');
  }
  
  async disable() {
    if (!this.isEnabled) return;
    
    this.isEnabled = false;
    await this.wakeLock.disable();
    
    // Remove reading mode styles
    document.body.classList.remove('reading-mode');
    
    // Restore elements
    this.showElements();
    
    console.log('Reading mode disabled');
  }
  
  hideElements() {
    document.querySelectorAll('.hide-in-reading').forEach(el => {
      el.dataset.originalDisplay = el.style.display;
      el.style.display = 'none';
    });
  }
  
  showElements() {
    document.querySelectorAll('.hide-in-reading').forEach(el => {
      el.style.display = el.dataset.originalDisplay || '';
    });
  }
  
  toggle() {
    if (this.isEnabled) {
      return this.disable();
    }
    return this.enable();
  }
}

// Usage
const readingMode = new ReadingMode();

document.getElementById('reading-mode-btn').addEventListener('click', () => {
  readingMode.toggle();
});

Video Player

class VideoPlayerWithWakeLock {
  constructor(videoElement) {
    this.video = videoElement;
    this.wakeLock = new AutoWakeLock();
    
    this.setupEventListeners();
  }
  
  setupEventListeners() {
    // Activate wake lock on play
    this.video.addEventListener('play', () => {
      this.enableWakeLock();
    });
    
    // Release on pause or end
    this.video.addEventListener('pause', () => {
      this.disableWakeLock();
    });
    
    this.video.addEventListener('ended', () => {
      this.disableWakeLock();
    });
    
    // Ensure wake lock is active in fullscreen
    document.addEventListener('fullscreenchange', () => {
      if (document.fullscreenElement && !this.video.paused) {
        this.enableWakeLock();
      }
    });
  }
  
  async enableWakeLock() {
    const success = await this.wakeLock.enable();
    if (success) {
      console.log('Video playing, screen will stay awake');
    }
  }
  
  async disableWakeLock() {
    await this.wakeLock.disable();
    console.log('Video paused/ended, screen can sleep');
  }
}

// Usage
const video = document.getElementById('video');
const player = new VideoPlayerWithWakeLock(video);

Navigation App

class NavigationApp {
  constructor() {
    this.wakeLock = new AutoWakeLock();
    this.isNavigating = false;
    this.route = null;
  }
  
  async startNavigation(route) {
    this.route = route;
    this.isNavigating = true;
    
    // Enable wake lock
    await this.wakeLock.enable();
    
    // Start position tracking
    this.watchId = navigator.geolocation.watchPosition(
      (position) => {
        this.updatePosition(position);
      },
      (error) => {
        console.error('Position error:', error);
      },
      {
        enableHighAccuracy: true,
        maximumAge: 0
      }
    );
    
    console.log('Navigation started, screen will stay awake');
  }
  
  async stopNavigation() {
    this.isNavigating = false;
    this.route = null;
    
    // Stop position tracking
    if (this.watchId) {
      navigator.geolocation.clearWatch(this.watchId);
      this.watchId = null;
    }
    
    // Release wake lock
    await this.wakeLock.disable();
    
    console.log('Navigation ended');
  }
  
  updatePosition(position) {
    if (!this.isNavigating) return;
    
    const { latitude, longitude } = position.coords;
    
    // Update map position
    this.updateMap(latitude, longitude);
    
    // Check if arrived at destination
    if (this.hasArrived(latitude, longitude)) {
      this.onArrival();
    }
  }
  
  updateMap(lat, lng) {
    // Update map display
    console.log('Current position:', lat, lng);
  }
  
  hasArrived(lat, lng) {
    if (!this.route?.destination) return false;
    
    const distance = this.calculateDistance(
      lat, lng,
      this.route.destination.lat,
      this.route.destination.lng
    );
    
    return distance < 50; // Within 50 meters is considered arrived
  }
  
  onArrival() {
    console.log('You have arrived!');
    this.stopNavigation();
    
    // Play notification sound
    this.playArrivalSound();
  }
  
  calculateDistance(lat1, lon1, lat2, lon2) {
    const R = 6371e3;
    const φ1 = lat1 * Math.PI / 180;
    const φ2 = lat2 * Math.PI / 180;
    const Δφ = (lat2 - lat1) * Math.PI / 180;
    const Δλ = (lon2 - lon1) * Math.PI / 180;
    
    const a = Math.sin(Δφ/2) * Math.sin(Δφ/2) +
              Math.cos(φ1) * Math.cos(φ2) *
              Math.sin(Δλ/2) * Math.sin(Δλ/2);
    const c = 2 * Math.atan2(Math.sqrt(a), Math.sqrt(1-a));
    
    return R * c;
  }
  
  playArrivalSound() {
    // Play arrival notification sound
  }
}

// Usage
const navigation = new NavigationApp();

// Start navigation
navigation.startNavigation({
  destination: { lat: 40.7128, lng: -74.0060 }
});

Presentation Mode

class PresentationMode {
  constructor() {
    this.wakeLock = new AutoWakeLock();
    this.fullscreen = null;
    this.isPresenting = false;
  }
  
  async start(element) {
    if (this.isPresenting) return;
    
    this.isPresenting = true;
    
    // Activate wake lock
    await this.wakeLock.enable();
    
    // Enter fullscreen
    if (element.requestFullscreen) {
      await element.requestFullscreen();
    }
    
    // Hide mouse cursor
    document.body.style.cursor = 'none';
    
    // Listen for fullscreen exit
    document.addEventListener('fullscreenchange', () => {
      if (!document.fullscreenElement) {
        this.stop();
      }
    });
    
    console.log('Presentation mode started');
  }
  
  async stop() {
    if (!this.isPresenting) return;
    
    this.isPresenting = false;
    
    // Release wake lock
    await this.wakeLock.disable();
    
    // Exit fullscreen
    if (document.fullscreenElement) {
      await document.exitFullscreen();
    }
    
    // Restore mouse cursor
    document.body.style.cursor = '';
    
    console.log('Presentation mode ended');
  }
}

// Usage
const presentation = new PresentationMode();

document.getElementById('present-btn').addEventListener('click', () => {
  presentation.start(document.getElementById('slides'));
});

Timer Application

class TimerWithWakeLock {
  constructor() {
    this.wakeLock = new AutoWakeLock();
    this.timerId = null;
    this.remainingSeconds = 0;
    this.onTick = null;
    this.onComplete = null;
  }
  
  async start(seconds) {
    if (this.timerId) {
      this.stop();
    }
    
    this.remainingSeconds = seconds;
    
    // Activate wake lock
    await this.wakeLock.enable();
    
    this.timerId = setInterval(() => {
      this.tick();
    }, 1000);
    
    console.log('Timer started, screen will stay awake');
  }
  
  tick() {
    this.remainingSeconds--;
    
    this.onTick?.(this.remainingSeconds, this.formatTime(this.remainingSeconds));
    
    if (this.remainingSeconds <= 0) {
      this.complete();
    }
  }
  
  async complete() {
    this.stop();
    
    // Release wake lock
    await this.wakeLock.disable();
    
    // Play alarm sound
    this.playAlarm();
    
    this.onComplete?.();
  }
  
  async stop() {
    if (this.timerId) {
      clearInterval(this.timerId);
      this.timerId = null;
    }
    
    await this.wakeLock.disable();
  }
  
  pause() {
    if (this.timerId) {
      clearInterval(this.timerId);
      this.timerId = null;
    }
  }
  
  async resume() {
    if (this.remainingSeconds > 0 && !this.timerId) {
      await this.wakeLock.enable();
      
      this.timerId = setInterval(() => {
        this.tick();
      }, 1000);
    }
  }
  
  formatTime(seconds) {
    const mins = Math.floor(seconds / 60);
    const secs = seconds % 60;
    return `${String(mins).padStart(2, '0')}:${String(secs).padStart(2, '0')}`;
  }
  
  playAlarm() {
    // Play alarm sound
    const audio = new Audio('/sounds/alarm.mp3');
    audio.play();
  }
}

// Usage
const timer = new TimerWithWakeLock();

timer.onTick = (remaining, formatted) => {
  document.getElementById('timer-display').textContent = formatted;
};

timer.onComplete = () => {
  alert('Time is up!');
};

// Start 5 minute countdown
timer.start(300);

Best Practices Summary

Screen Wake Lock API Best Practices:
┌─────────────────────────────────────────────────────┐
│                                                     │
│   Use Cases                                         │
│   ├── Video/audio playback                         │
│   ├── Navigation apps                              │
│   ├── Reading mode                                 │
│   ├── Timers/alarms                                │
│   └── Presentations/slideshows                     │
│                                                     │
│   Battery Management                                │
│   ├── Only request when needed                     │
│   ├── Release immediately when not needed          │
│   ├── Handle release events for edge cases         │
│   └── Auto-release when page hidden                │
│                                                     │
│   User Experience                                   │
│   ├── Inform user screen stays awake              │
│   ├── Provide manual control option                │
│   ├── Auto-reacquire when page visible            │
│   └── Graceful fallback for unsupported browsers  │
│                                                     │
└─────────────────────────────────────────────────────┘

Scenario	Need Wake Lock?
Video playback	Yes
Music playback (background)	No (audio already prevents sleep)
Reading long articles	Yes
Map navigation	Yes
Normal browsing	No

---

Use the Screen Wake Lock API wisely to keep the screen awake when needed.