Facebook’s mobile apps are predominantly written in JavaScript and Java. However, given the massive scale and complexity of Facebook’s codebase, there are components written in a variety of other languages as well.
The Primary Languages
The core of Facebook’s mobile apps is written in JavaScript and Java:
- JavaScript – This is used to build the UI and application logic of the apps. Facebook uses the React Native framework for rendering mobile UI components with JavaScript.
- Java – This is used primarily on the Android version of the Facebook app. It compiles to native Android bytecode and allows tight integration with the Android OS.
So in summary, the main languages are:
| Platform | Language |
|---|---|
| iOS | JavaScript (React Native) |
| Android | Java + JavaScript (React Native) |
JavaScript powers the UI and business logic of the app, while Java handles lower-level integration on Android. But the app is more than just these two languages.
Other Languages Used
Given the massive scale of Facebook, the mobile app codebase also contains components written in other languages, including:
- C++ – For performance-critical components.
- Objective-C – Some legacy iOS code is written in Obj-C.
- Swift – Some newer iOS code is written in Swift.
- Python – For backend services and machine learning models.
- PHP – Some backend code and tooling is written in PHP.
So in summary, while JS and Java account for most of the app code, there are components written in many other languages as well for specialized purposes.
Why JavaScript and Java?
There are several key reasons why JavaScript and Java are the primary languages used for Facebook’s mobile apps:
- Portability – JavaScript allows code reuse across iOS and Android via React Native.
- Performance – Java and C++ allow high performance for native apps.
- Developer Productivity – JS and Java have huge developer communities and ecosystem.
- Full-Stack Integration – JS on front-end and Java on backend ease full-stack work.
Some key advantages of JavaScript:
| Advantage | Description |
|---|---|
| Portability | JS allows sharing code across platforms through React Native. |
| Productivity | Highly productive thanks to modern JS frameworks like React. |
| Full Stack | JS front-end and Node.js backend enable full stack in one language. |
And some key advantages of Java:
| Advantage | Description |
|---|---|
| Performance | Java code compiles to high performance native code. |
| Android Integration | Tight integration with Android OS and APIs. |
| Community | Massive Java community and ecosystem. |
Development Environment
Facebook uses a customized development environment to build and maintain its mobile apps:
- Java code is built using Gradle as the build system.
- JavaScript code is transpiled from JSX and ES6/7 to vanilla JavaScript using Babel.
- Source code is managed in a massive customized git repository.
- Testing is done through Jest (JS) and JUnit (Java).
- Facebook has internal frameworks and tools for logging, analytics, networking etc.
The core languages of JavaScript and Java allow leveraging open source tools like React Native, Gradle, Jest etc. But Facebook heavily customizes the environment with internal frameworks, build tools, testing tools, etc.
Custom Build Tools
Facebook has created many custom build tools and systems to manage code for its apps, including:
- BUCK – a custom build system optimized for large projects.
- SoLoader – for loading native libraries from JS.
- Codemod – for automated code refactors.
Testing and Debugging
For testing, Facebook leverages industry standard tools:
- Jest – For testing JavaScript code.
- JUnit – For testing Java code.
But they have also created many custom tools:
- Jest Workers – A custom Jest runner for parallel testing.
- react-devtools – Custom devtools for React Native.
Technical Challenges
Some key technical challenges Facebook faces building and maintaining their apps include:
- Scale – Supporting billions of users on low-powered mobile devices.
- Performance – Optimizing JSON parsing, image loading, etc for slow networks.
- Reliability – Ensuring stability across different OS versions and devices.
- Automation – Automating builds, tests, and releases for rapid iteration.
- Security – Guarding against vulnerabilities like XSS attacks or account hijacking.
Scalability
To handle massive scale, Facebook has invested heavily in solutions like:
- Asynchronous Programming – Using futures/promises for async code.
- Code Splitting – Only loading required parts of the app.
- Pagination – Fetching small chunks of content.
- Caching – Caching API responses and images.
Performance
Some key performance optimizations include:
- Lazy Loading – Images, tabs, and other content.
- Network Optimizations – Caching, retries, request optimization.
- Native Modules – Performance critical parts written in C++.
Conclusion
In summary:
- Facebook’s mobile apps are primarily written in JavaScript (React Native) and Java.
- JS is used for UI and app logic, Java for tight Android integration.
- But many other languages are used for specialized purposes like C++, Python, Objective-C.
- Facebook heavily invests in custom tooling and infrastructure around JS and Java.
- Major technical challenges include scale, performance, automation and reliability.
So while leveraging standard languages like JS and Java, Facebook pours tremendous resources into customizing the development environment and scaling infrastructure to support billions of users on a wide variety of mobile devices and networks.