100 Fundamental PWA Interview Questions

You can also find all 100 answers here 👉 Devinterview.io - PWA

1. What is a Progressive Web App (PWA)?

Progressive Web Apps (PWAs) combine the best features of web and mobile applications. They deliver a seamless, fast, and engaging user experience across devices and networks. Key to their functionality is enabling offline access, push notifications, and device hardware interaction - features that were traditionally associated with native apps.

PWA Components

1. Progressive: Works for all users, regardless of browser choice.

2. Discoverable: Indexed by search engines and shareable via URLs.

3. Re-Engageable: Supports push notifications.

4. Responsive: Adapts to various screen sizes and orientations.

5. App-like Interactions: Navigates seamlessly with smooth animations and gestures.

6. Fresh: Updates content automatically.

7. Safe: Served over HTTPS to prevent tampering and ensure user security.

Key Technologies

• Service Workers: Background scripts enabling features like offline access and push notifications.
• Web App Manifest: JSON file providing app details to browsers, such as the icon to display on the home screen or its starting URL.

Benefits

• Cross-Platform: Works on desktops, laptops, tablets, and mobile devices.
• Linkable: Can be shared and accessed through URLs, avoiding the need for app store installations.
• Cost-Effective: Eliminates the expenses associated with app store submissions or multi-platform development.
• Auto-Updates: Updates automatically when users are online, preventing version fragmentation.
• Offline Functionality: Continues to function in the absence of a stable internet connection, offering reliability and speed.
• Engagement Features: Allows for push notifications and home screen installations, promoting user engagement.
• SEO-friendly: Content can be indexed by search engines, enhancing discoverability.

Practical Use Cases

1. Twitter: After adopting PWA technology, Twitter witnessed a 65% increase in page sessions and a 75% rise in tweets viewed.
2. Pinterest: Embracing PWAs led to a 60% rise in user engagement and core interactions across diverse platforms.
3. Starbucks: The Starbucks PWA, designed for speed and reliability, is utilized by customers to browse menus, manage rewards, and place orders.

2. How do PWAs differ from traditional web applications?

Progressive Web Applications (PWAs) substantially enhance the web browsing experience to a degree that's akin to native mobile applications. Let's investigate the key areas in which PWAs diverge from traditional web applications.

Key Distinctions

Installability

• PWA: Offer the choice to users to install them on their device, showing up on their home screen or in the app drawer.
• Web App: Generally, users access web apps through a web browser.

Integrations

• PWA: Emulate features typically associated with native apps, such as push notifications and device hardware access.
• Web App: Limited or no capability to integrate with device-specific functionalities.

Connectivity

• PWA: Work offline or with a poor internet connection by caching resources.
• Web App: Require a steady and reliable internet connection.

Discoverability

• PWA: Register themselves in app marketplaces (e.g., Google Play Store).
• Web App: Largely dependent on traditional search engine visibility.

Supported Technologies

PWA

• Service Workers: A script that runs in the background, enabling features like push notifications, offline support, and caching strategies.
• Web App Manifest: A JSON file that provides metadata about the web application, creating the experience of a standalone app.

Web Apps

• Progressive Enhancement: A universal design approach that starts with basic functionality and progressively enhances based on the capabilities of the client or user.
• Responsive Web Design (RWD): Ensures websites look and feel optimal across various devices and screen sizes.

Code Example: PWA Service Worker

Here is the JavaScript code:

// A simple example of a service worker that caches resources for offline use
self.addEventListener('install', function(event) {
    event.waitUntil(
        caches.open('static-v1').then(function(cache) {
            return cache.addAll([
                '/styles/main.css',
                '/script/main.js',
                '/images/logo.png',
                // Additional resources to cache
            ]);
        })
    );
});

self.addEventListener('fetch', function(event) {
    event.respondWith(
        caches.match(event.request).then(function(response) {
            return response || fetch(event.request);
        })
    );
});

Code Example: PWA Manifest File

Here is the JSON manifest:

{
    "name": "My PWA",
    "short_name": "PWA",
    "start_url": "/",
    "display": "standalone",
    "background_color": "#fff",
    "theme_color": "#000",
    "icons": [
        {
            "src": "/icon-192x192.png",
            "type": "image/png",
            "sizes": "192x192"
        },
        // Add multiple icon sizes
    ]
}

3. Can you list the core pillars of a PWA?

Progressive Web Applications adhere to a set of core principles to deliver an enhanced user experience that is both reliable and engaging across various platforms.

Core Elements of a PWA

App-shell and Shell-First Navigation

• What is it? It is a design pattern that separates the visual part of your app from the data. This visual part will be cached and the data fetched dynamically.
• Benefits: Quick navigation and a consistently fast experience once the initial content is loaded.

Service Workers

• What is it?: A script that the browser runs in the background, separate from a web page, opening the door to features like caching and push notifications.
• Benefits: Caching for offline use and faster load times, push notifications

Web App Manifest

• What is it?: A simple JSON file that gives you the ability to control how your app appears to the user in areas where they would expect to see apps.
• Benefits: Add to home screen, splash screen, and more consistent in UI across screens

Responsive Design

• What is it?: Design principle that ensures web content adapts to any device on which it is displayed.
• Benefits: Compatibility across various devices and screen sizes.

HTTPS

• What is it?: PWA must be served over a secure network.
• Benefits: Data protection and integrity.

Recently, some additional fundamental requirements have been added:

Fast

• What is it?: Deliver quickly, making sure the first load is fast.
• Benefits: Improved user experience and SEO ranking.

Safe

• What is it?: Must be served over secure HTTPS.
• Benefits: Security for users.

Engaging

• What is it?: Engage the user. Use features like push notifications.
• Benefits: Better user experience.

4. What are the advantages of developing a PWA over a native app?

Both Progressive Web Apps (PWAs) and Native Apps come with distinctive benefits and limitations. Let's examine them in detail.

Unique Benefits of PWAs

• Cross-Platform Compatibility: PWAs seamlessly work across various operating systems and devices.
• Update Convenience: Users access the latest version of the PWA without manual updates, enhancing security and performance.
• Web Technology Base: Developers leverage standardized web technologies for PWA development.
• No App Store Dependencies: PWAs don't necessarily require listing in app stores. This simplifies deployment and eliminates associated fees, although they might still benefit from being listed in app stores for discoverability.
• Faster Development: With a single codebase, PWA development can be swifter than building multi-platform native apps.
• Lower Storage Requirements: PWAs can be "lighter" in size, especially compared to large native apps.

Unique Benefits of Native Apps

• Optimized Performance: Native apps excel in performance, especially for complex tasks such as 3D graphics or high-fidelity video.
• Rich in Features: Native apps can harness the full range of device-specific features and hardware, delivering highly tailored experiences.
• Robust Offline Functionality: While PWAs offer some offline capabilities, native apps, particularly those with local databases, can function fully offline.

Shared Advantages

• Access to Device Features: Both PWAs and native apps can tap into device-specific functionalities like geolocation, camera, and more.
• Engaging User Experiences: Both app types are primed to offer engaging user interfaces, driving user retention and satisfaction.

5. What is the role of the service worker in a PWA?

The service worker is a key component in the Progressive Web App (PWA) architecture. It presents a unique approach to web application development, focusing on offline capability, performance enhancement, and seamless user experience.

Core Functions of the Service Worker

• Network Proxy: Acts as a middleman, intercepting network requests and allowing the app to utilize cached data when a network connection is unavailable.

• Cache Management: Maintains a distinct cache, streamlining the storage and retrieval of assets like HTML, CSS, JS, and media files.

• Background Synchronization: Allows for data synchronization even when the app is not actively in use.

• Push Notifications: Facilitates direct communication with the user through notifications, keeping them informed about relevant app updates.

Service Worker Lifecycle

1. Registration: The web app registers the service worker for the first time. The worker is then downloaded and installed.
2. Installation: New service workers are installed in the background, but they don't take over operational control until all tabs using the earlier service worker are closed.
3. Activation: Upon successful installation, the new service worker is activated, replacing the previous worker.
4. Update: When there are significant changes to the service worker file, a new worker is installed in the background. It becomes active only after all tabs using the existing worker are closed.

Key Role in Offline Functionality

• Caching: Service workers store content in a local cache, ensuring that a PWA can function without a live internet connection.

• Fallback Content: When online resources are inaccessible, the service worker can serve cached content, ensuring a seamless user experience.

• Background Sync: The service worker enables apps to queue specific tasks, such as form submissions, until an internet connection becomes available.

Tools for Performance Optimizations

• Pre-Caching: Service workers can preemptively cache assets, making them available for rapid loading.

• Runtime Caching: Content can be cached dynamically based on user interactions or other events, enhancing the app's responsiveness.

Enhanced User Engagement

• Push Notifications: The service worker enables the delivery of push notifications to users, driving re-engagement with the PWA.

• Rich Offline UI: By integrating 'Background sync,' service workers elevate the offline experience, preparing and updating UI before a lost network connectivity is encountered.

6. How do you make a web app installable on a user's home screen?

By harnessing the capabilities of modern web browsers and adhering to specific criteria through the Web App Manifest, developers can enable a fast, reliable and engaging web app experience, compatible with the user's home screen.

Web App Manifest

The Web App Manifest is a configuration file in JSON format that affirms an app's identity and defines its behavior when installed.

For a website to be considered an installable PWA, it must:

• Be served over HTTPS
• Include a Web App Manifest file hosted at the root level
• Conform to critical attributes like "short_name", "start_url", and "icons"

A straightforward example of a manifest JSON:

{
  "short_name": "My App",
  "name": "My Progressive Web App",
  "start_url": "/",
  "background_color": "#3367D6",
  "theme_color": "#3367D6",
  "display": "standalone",
  "icons": [
    {
      "src": "icon-192x192.png",
      "type": "image/png",
      "sizes": "192x192"
    },
    {
      "src": "icon-512x512.png",
      "type": "image/png",
      "sizes": "512x512"
    }
  ]
}

Service Workers

Service Workers are the backbone for many PWA features, including the ability to work offline. This characteristic is key to ensuring that once installed, PWAs provide a consistent user experience, independent of network availability.

The core tasks of a Service Worker in this context involve:

• Precaching key assets
• Managing runtime caching
• Implementing a robust fetch event handler

Registering the Service Worker

To begin reaping the benefits of a Service Worker, it must first be registered. This action is usually performed via your web app's main JavaScript file:

if ('serviceWorker' in navigator) {
  window.addEventListener('load', function() {
    navigator.serviceWorker.register('/sw.js')
    .then(function(registration) {
      console.log('ServiceWorker registration successful with scope: ', registration.scope);
    })
    .catch(function(err) {
      console.error('ServiceWorker registration failed: ', err);
    });
  });
}

Post-Installation Behavior

After successful installation, a PWA is expected to present consistent UX standards, in line with what the user has grown accustomed to on their home screen. This would typically entail:

• Opening as a standalone app without a browser UI
• Releasing clear, legible icons
• Working offline or under unreliable network conditions

Browser Compatibility

While most modern browsers support the core features required for PWA installation, you should remember to triple-check the latest compatibility tables based on the functionality you incorporate.

Responsive Design Elements

PWA developers incorporate responsive web design strategies to ensure the visual fidelity and usability of their app across diverse devices and screen sizes. Such designs guarantee a seamless user experience, crucial for availing the benefits of PWAs on various form factors.

7. What is a manifest file and what is its significance in PWAs?

The Web App Manifest is a JSON file enabling developers to provide rich, app-like experiences for Progressive Web Apps (PWAs). It ensures consistent behavior across platforms and devices.

Key Manifest Properties

• name: Application's display name.
• short_name: A shorter name, beneficial for space-restricted environments.
• start_url: Defines the initial URL when the app is launched.
• display: Determines the app's layout and launch mode.
• icons: Specifies various sizes of the app icon for display consistency.
• background_color: Sets the color users see upon app launch, providing a seamless experience during loading.
• theme_color: Governs the color of the web browser's UI.

Code Example: Web App Manifest

Here is the JSON representation of a manifest.json file:

{
  "name": "Sample PWA",
  "short_name": "Sample",
  "start_url": "/",
  "display": "fullscreen",
  "icons": [
    {
      "src": "/icons/icon-72x72.png",
      "type": "image/png",
      "sizes": "72x72"
    },
    {
      "src": "/icons/icon-96x96.png",
      "type": "image/png",
      "sizes": "96x96"
    }
  ],
  "background_color": "#f0f0f0",
  "theme_color": "#3367d6"
}

8. What is the purpose of the 'Add to Home Screen' feature?

The "Add to Home Screen" feature is one of the key progressive web app (PWA) benefits. It empowers web applications to be installed and used like native mobile apps, directly from the user's device home screen.

Key Advantages of the Feature

• Improved User Experience: The feature streamlines accessibility, making the app readily available with a single tap on the home screen, much like a native app.

• Enhanced Engagement: By offering a permanent presence, the app remains on the user's device. This can motivate users to engage more frequently.

• Offline Capabilities: The app can be tailored to work consistently and flexibly even when there's no internet connection. This aspect is beneficial for both user and developer alike.

Technologies Supporting "Add to Home Screen"

The Cache Storage API in JavaScript, for example, is foundational to PWAs and, consequently, the "Add to Home Screen" feature. Nonetheless, the feature's availability across devices and browsers can differ.

The Service Worker, another PWA-centric technology, equips the app to function even when offline. It achieves this by intercepting network requests, allowing the app to respond with its cached resources - a technique known as "cache first" strategy.

Should internet connectivity be restored, the app can then update its cache with the latest data.

Considerations for "Add to Home Screen"

Investing in the feature demands a nuanced understanding of its advantages and challenges. For instance, while PWAs on iOS devices support "Add to Home Screen," native app storefronts like the Apple App Store often garner more initial user trust and visibility. This can impact the visibility of your app, and it's essential to consider your target audience and how they discover and use mobile applications.

Furthermore, the space on a smartphone's home screen is competitive. Users are selective about what earns a coveted spot here. Therefore, your app must provide clear, discernible value to entice them to "add" it.

Code example: 'Service Worker'

Below is the JavaScript code:

// Service worker for cache-first strategy
self.addEventListener('fetch', (event) => {
  event.respondWith(caches.match(event.request)
    .then((cachedResponse) => cachedResponse || fetch(event.request))
  );
});

// On installation, populate cache
self.addEventListener('install', (event) => {
  event.waitUntil(
    caches.open('my-app-cache').then((cache) => {
      return cache.addAll([
        '/',
        '/styles/main.css',
        '/script/main.js',
      ]);
    })
  );
});

9. How can you detect if your web app has been added to the user's home screen?

To detect if a Progressive Web App (PWA) has been added to a user's home screen, you can use JavaScript in combination with other browser-specific methods:

Using Web APIs

• Web App Manifest: Access the display property in the manifest file to check if the app is running in "standalone" mode.

• Web Application Install Prompt: Identify whether the browser displayed an installation prompt.

Code Example: Web App Manifest

"manifest.json"
{
  "display": "fullscreen"
}

// main.js
const isStandalone = window.matchMedia('(display-mode: standalone)').matches;

Code Example: Web Application Install Prompt

// main.js
window.addEventListener('beforeinstallprompt', () => {
  // Installation is possible
});

Browser-Specific Methods

• Chrome: Use window.matchMedia('(display-mode: standalone)').matches to detect standalone mode.

• Firefox: Utilize document.hidden and check for false to determine if the app is in standalone mode.

Code Example: App in Standalone Mode

// main.js
if ('standalone' in navigator && navigator.standalone) {
  console.log('Launched from home screen');
}

Code Example: Safari Home Screen Detection

// main.js
if (window.matchMedia('(display-mode: standalone)').matches) {
  console.log('Launched from home screen using Safari');
}

10. Explain how PWAs achieve offline functionality.

Progressive Web Applications (PWAs) leverage various technologies and strategies to maintain core functionality even in offline or low connectivity scenarios. Key methods involved include caching with Service Workers, adapting UI/UX, and data synchronization.

Service Workers & Caching

• Role: Service Workers act as a bridge between the PWA and the network, allowing resources to be cached for future use.

• Caching Mechanics: Service Workers implement a caching strategy to save essential assets and data, such as HTML, CSS, JavaScript, and API responses.

• Cache Storage: Resources can be stored in different types of caches, such as the Application, Navigation, or Data cache.

• Cache Durability: Cached resources include a versioning or cache-busting mechanism to ensure they update when necessary.

Offline First Strategies

• Data First Approach: PWAs safeguard the user experience by focusing on ensuring data integrity and then syncing it with the backend. This method ensures users can at least access cached data when offline.

• Interactive Pre-Caching: PWAs can proactively cache elements or sections that a user is likely to interact with, further enhancing the offline experience.

Synchronization Mechanisms

• Event Listeners: Service Workers monitor certain network events and trigger corresponding actions. For instance, when network status changes or when an online connection is established.

• Background Sync: PWAs can leverage background sync functionality to queue data for later synchronization if the user operates in an offline mode. Once the user is back online, the data is automatically synced.

Real-Time Databases and Offline Storage

• IndexedDB: This is a client-side storage system for application data, tailored primarily for large amounts of data or structured data storage.

• Web Storage: Consisting of localStorage and sessionStorage, this provides simple key-value storage along with storage segregation based on the session.

• Real-Time Databases: Databases such as Firebase or CouchDB integrate real-time data synchronization capabilities, reducing the manual effort required for offline data management.

User Guidance and Consistency

• Offline Mode Indicators: To keep users informed about their current connectivity status, PWAs might display visual cues (like an alert or a dedicated offline page) to indicate when the app is offline and what capabilities are limited.

• Data Consistency: When operating offline, PWAs try to maintain data consistency, ensuring that operations, when committed, are, if possible, also committed to the backend data source when the app gets back online.

Addressing Security Concerns

• Access Validation: When online access is restored, PWAs perform validations to ensure the authenticity and integrity of client-side and server-side data before syncing.

• Validation Mechanisms: They might use unique transaction identifiers or entity versions to track data transactions and ensure that no inconsistent or malicious changes are applied to the data sources.

11. What are the security requirements for a PWA?

PWAs integrate the best of the web and mobile applications, focusing holistically on security and user experience.

Key Features

• HTTPS: HTTPS ensures the integrity and confidentiality of data. It's non-negotiable for PWAs.

• Service Workers: Intercepts network requests and provides offline capabilities while preventing unauthorized access to resources.

• Content Security Policy (CSP): Defines the content sources that the browser can load resources from to defend against cross-site scripting (XSS) attacks.

• Sandboxing: Restricts the execution context of individual components like iframes for better security.

• Push Notifications: Require user permission, putting data control in users' hands.

• App Transport Security: Ensures secure data transfers between the app and web servers.

These features combine with others, such as reliable internet connections via Background Sync, to form a multifaceted security approach.

12. Can you delineate between App Shell and content in a PWA context?

In the context of Progressive Web Apps (PWAs), the App Shell and the content are two fundamental components that work synergistically to deliver an enhanced user experience.

App Shell

The App Shell is the PWA's core architectural design, resembling the frame of a single-page application. It is composed of static elements such as the header, navigation bar, and footer that remain consistent across the app.

Purpose

• Enhances Performance: Caching the App Shell optimizes load times, while dynamic or personalized content is fetched when required.
• Framework for Navigation: Offers a seamless navigational experience, especially in offline or low-connectivity scenarios.

Content

The Content of a PWA refers to the distinct data or interface components other than the static elements provided by the App Shell.

Dynamism and Interactivity

• Real-time Data: It's responsible for displaying dynamic data fetched from APIs, databases, or other sources.
• User Interactions: Handles user input and dynamic changes within the app.

Fetch Strategy

Unlike the App Shell, which is typically cached for quick access, the content uses tailored fetch strategies based on the data required, user context, and network conditions.

Code Example: App Shell and Content

Here is a HTML code:

<!-- App Shell -->
<header id="appHeader">...</header>
<nav id="appNav">...</nav>
<main id="appContent">...</main>

<!-- Content -->
<section id="dynamicSection">...</section>

In this example:

• The elements within <header>, <nav>, and <main> contribute to the App Shell.
• <section id="dynamicSection"> represents Content that might be fetched dynamically and updated based on user interactions or data changes.

13. How does a PWA function on a low-bandwidth or offline network?

PWAs are designed to work seamlessly under less-than-ideal network conditions or even in offline mode. This is achieved through a suite of innovative techniques and patterns coupled with the Service Worker.

Service Worker Primer

• Role: Acts as a lightweight, programmable proxy between the web app and the network.
• Functionality: It provides resource caching, background sync, and push notifications.
• Note: Service Workers require HTTPS for security reasons.

Techniques for Offline Support

• Cache-First Strategy: When the network isn't available, the browser serves resources (like HTML, CSS, and JavaScript files) from the cache, effectively maintaining the app's core functionality. Requests to the network are made only for resources not available in the cache.

• Cache Storage:

  • The Cache Storage API, employed by Service Workers, provides a centralized location for app-centric resource caching.
  • Resources are manually cached, giving developers granular control over the caching strategy.

• Real-time Database Synchronization:

  • A traditional database often involves direct server interactions, making it unsuitable for offline use.
  • Solutions may utilize local databases that sync with a remote source when the network is accessible. One example is IndexedDB, a low-level API for client-side storage.

• Lazy Loading and Pre-caching:

  • Resources are divided into essential and non-essential categories, ensuring core elements are swiftly accessible. Non-essential resources can be "lazily" loaded based on user interaction.
  • Pre-caching secures necessary resources in the cache for rapid retrieval.

• Background Sync: Allows users to interact with the app, even when offline, and then synchronize the changes with the server once the network is restored.

• Persistent Storage: By using techniques like Service Worker Caching and IndexedDB, a PWA can provide persistent storage that retains data across browsing sessions.

Code Example: Service Worker Installation

Here is the JavaScript code:

// Define a list of resources to pre-cache
const preCacheResources = ['index.html', 'styles.css', 'app.js'];

// Service Worker installation event
self.addEventListener('install', event => {
  // Perform pre-caching of resources
  event.waitUntil(caches.open('preCache').then(cache => cache.addAll(preCacheResources)));
});

14. What are push notifications in the context of PWAs?

Push Notifications enable web applications, including Progressive Web Apps, to send real-time updates to users even when the app isn't open.

They are a powerful tool to engage users and can be particularly valuable in scenarios like news or social media apps, e-commerce platforms, or for personalized offers and reminders.

Key Components

• Service Worker: Acts as a bridge between the server and the user's device.

• Push API: Facilitates communication between the web app and a Push Service.

• Push Service: Operated by a third-party, it's responsible for routing notifications to the intended client device.

• User Interface: The device displays notifications, and users can interact with them.

Essential Steps for Push Notifications in PWAs

1. Request Permission: The app must first ask for the user's consent to send push notifications. This is usually done with a prompt.

2. Register Service Worker: This enables the Service Worker to handle incoming push messages.

3. Subscribe to Push and Obtain a Push Subscription: The web app requests the user's device to subscribe to the push service (like Firebase Cloud Messaging), Afterward, it sends the push subscription to the server.

4. Send Push Notification: The app's server, using the key from the push subscription, can send a push message to the push service, which in turn delivers it to the user's device.

5. Handle Push Event: When a push message is received, the Service Worker wakes up and invokes an event, allowing the app to process the notification and potentially show an in-app notification or take other relevant actions.

6. Use Data Payload: The received push message can include a data payload, carrying information for the app to use and act upon. This is particularly useful for handling actionable notifications.

Code Example: Requesting Push Notification Permission

Here is the code:

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <script>
        if ('serviceWorker' in navigator && 'PushManager' in window) {
            // Register the service worker
            navigator.serviceWorker.register('sw.js')
                .then(function(registration) {
                    console.log('Service Worker registered with scope:', registration.scope);
                    // Request push notification permission
                    return registration.pushManager.permissionState({ userVisibleOnly: true });
                })
                .then(function(permissionState) {
                    if (permissionState === 'granted') {
                        console.log('Push notifications are allowed');
                        // The user has granted permission
                        // Now you can subscribe to the push service, obtain the push subscription, and send it to your server
                    } else {
                        console.log('Push notifications are not allowed');
                        // The user has declined or has not yet granted permission
                        // You might want to inform the user about the benefits of allowing push notifications.
                    }
                })
                .catch(function(err) {
                    console.error('Service Worker registration failed:', err);
                });
        } else {
            console.warn('Push messaging is not supported');
        }
    </script>
    <title>Push Notification Permission Request</title>
</head>
<body>
    <!-- Your app's UI elements can go here -->
</body>
</html>

In this code example:

• We check if the browser supports both service workers and push notifications.
• If supported, we register the service worker, and then use the PushManager to check or request permission for push notifications.

15. Can you explain the concept of background sync in PWAs?

Background sync in Progressive Web Applications (PWAs) enables data updates or actions, often generated offline, to synchronize with web servers as soon as the device reconnects to the internet.

This provides users with a seamless experience, regardless of their online status, and is especially useful for unexpectedly interrupted actions, like submitting forms or uploading files.

Key Advantages

• Seamless and Reliable: Users do not have to worry about their data or tasks being lost due to an intermittent internet connection.
• Improved Engagement: Users are emancipated from the necessity of staying connected to perform tasks. Developer and IT Concerns
• Resource Efficient: Syncing occurs when network resources are available, avoiding unnecessary data usage.
• Data Integrity and Security: Synchronized data undergoes server-side validation, ensuring integrity and security.

Underlying Mechanisms

• Service Worker: This script acts as a bridge between the browser and the network, handling sync operations in the background.
• Sync Manager: A built-in Chrome feature that queues sync events. These are later executed when the network is accessible.
• Backoff mechanisms: Inherent to the sync process, these mechanisms regulate the timing of sync retries. The Process
• Queueing: The service worker puts data needing sync into the sync queue when the device is offline.
• Monitor & Execute: Even when the app isn't active or open, the service worker continues to observe the sync queue. It executes queued tasks as soon as the device re-establishes a connection.

Code Example: Syncing Files with Service Worker

Here is the JavaScript code:

self.addEventListener('sync', function(event) {
  if (event.tag == 'syncFiles') {
    event.waitUntil(syncFiles());
  }
});

function syncFiles() {
  return new Promise((resolve, reject) => {
    fetch('/sync/files')
      .then(() => resolve())
      .catch(() => reject());
  });
}

To manually trigger sync, use the following JavaScript:

navigator.serviceWorker.ready.then(function(swRegistration) {
  return swRegistration.sync.register('syncFiles');  
});

Explore all 100 answers here 👉 Devinterview.io - PWA