Facebook is one of the most popular social media platforms in the world, with over 2.8 billion monthly active users as of 2020. Behind the scenes, Facebook relies heavily on JavaScript to create the dynamic and interactive user experience that people have come to expect.
Client-Side JavaScript
The majority of JavaScript used by Facebook is client-side code that runs in the user’s web browser. This allows the Facebook interface to react and update in real-time without requiring a full page reload. Some examples of how client-side JavaScript is used on Facebook include:
- Updating the News Feed as new posts arrive
- Displaying notifications dynamically
- Allowing users to like, comment on, and share posts
- Creating pop-ups and dialog boxes
- Scrolling through the News Feed and pagination
- Animations and transitions
- Form input validation
All of these features utilize JavaScript to manipulate the DOM (Document Object Model) and CSS to provide a smooth user experience. The use of client-side JavaScript allows Facebook to feel fast and responsive compared to traditional server-rendered websites.
Ajax Requests
Facebook makes heavy use of Ajax (Asynchronous JavaScript and XML) requests to fetch data from its servers asynchronously. This means the browser can update parts of the page without needing to reload the entire thing.
Some examples of Ajax requests on Facebook include:
- Loading more posts in the News Feed
- Updating likes and comments on a post
- Fetching notifications
- Autocompleting friend tags in posts
- Searching for groups, events, and more
Without Ajax, the Facebook experience would be very disjointed and require constant full page reloads. Ajax allows new data to be seamlessly merged into the existing page.
Front-End Frameworks
Facebook engineers rely on JavaScript frameworks like React and Relay to build complex user interfaces and manage data flow. Some benefits of using frameworks include:
- React’s component model makes it easy to encapsulate UI elements and manage state.
- Relay provides a declarative framework for fetching and managing GraphQL data.
- Redux helps manage state across React components in a predictable way.
- Flux architecture enforces unidirectional data flow between views and stores.
By leveraging frameworks like these, Facebook can write modular front-end code that scales well as new features are added.
GraphQL
For its data layer, Facebook developed GraphQL, a query language for APIs. GraphQL replaced Facebook’s previous REST APIs in 2012. With GraphQL, the client can specify exactly what data it needs from the server. This allows the Facebook apps to request only the data they need and nothing more.
Some benefits of GraphQL for Facebook include:
- Avoiding over-fetching data like with REST
- Making changes to the API without breaking existing clients
- Powering React components with data from Relay
- Enabling precise data queries from mobile apps
GraphQL is served over HTTP, so JavaScript makes for a good client language. The declarative nature of GraphQL fits well with React and Relay on the front-end.
JavaScript Across Platforms
In addition to web, Facebook utilizes JavaScript across its mobile apps for iOS and Android. Languages like React Native allow product engineers to write mobile UI code in JavaScript. Benefits include:
- Write once, render natively on both platforms
- Share code and skills across iOS, Android, and Web teams
- Use JavaScript patterns not available in Swift or Java
- Faster development than native languages
Facebook is heavily invested in making JavaScript a viable language for building fast, high-quality mobile apps. React Native brings the benefits of React’s architecture to mobile.
Performance and Optimization
To achieve good performance, Facebook optimizes its JavaScript code through techniques like:
- Lazy loading – Only load code needed for the initial page
- Code splitting – Split bundles into smaller chunks that can be loaded on demand
- Tree shaking – Remove unused module exports to reduce bundle size
- Immutable data – Use immutable data structures to avoid expensive deep equality checks
- Service workers – Cache assets for better performance on repeat views
Facebook’s web app is quite large, so it’s crucial for them to ship less code to the client. Optimizing the JavaScript that powers Facebook is an ongoing priority.
Testing and Quality
To maintain quality at scale, Facebook writes extensive tests for its JavaScript code including:
- Unit tests – Isolate and test individual functions and components
- Integration tests – Verify interactions between components
- Snapshot tests – Serialize React tree and compare against previous snapshots
- End-to-end tests – Simulate user interactions and test the full stack
Test automation helps Facebook detect regressions and catch bugs before releasing new code. Test coverage expectations are high for both web and mobile code.
Conclusion
JavaScript powers the dynamic experience that makes Facebook feel responsive for more than 2 billion people worldwide. Both on the web and in mobile apps, JavaScript handles the client-side rendering while back-end services provide the data.
Performance and quality are ongoing priorities for Facebook’s engineers. By leveraging frameworks like React and GraphQL, Facebook’s product teams can innovate quickly while keeping the site reliable for its massive user base. JavaScript will continue to be a strategic language invested in at Facebook for the foreseeable future.