01st June 2024

Understanding the Difference Between AOT and JIT Compilation in Angular


Angular is a popular framework for building web applications, known for its powerful features and performance. One of the key aspects of Angular that impacts performance is its compilation process. Angular uses two main types of compilers: Ahead-of-Time (AOT) and Just-in-Time (JIT). Understanding the differences between these compilers can help developers make informed decisions about their build process and optimize their applications for speed and efficiency.

1. Introduction to Angular Compilation

In Angular, the code you write is not directly executed by the browser. Instead, it is first compiled into JavaScript that the browser can understand. This compilation process can happen at different stages, either during development (JIT) or before deployment (AOT). The choice of compilation method affects various aspects of your application, including its performance, load time, and development workflow.

2. What is JIT Compilation?


Just-in-Time (JIT) compilation is a method where the Angular application is compiled in the browser at runtime. The code is compiled just before it is executed, allowing for dynamic changes during development.

How JIT Works
  • Loading the Application: When the user accesses the Angular application, the browser downloads the Angular framework and the application’s source code.
  • Compilation: The Angular JIT compiler takes the downloaded code and compiles the Angular templates into JavaScript at runtime.
  • Execution: The compiled code is executed, rendering the application in the browser.
Advantages of JIT
  • Faster Development Cycle: Since the compilation happens in the browser, developers can see the changes they make almost instantly without needing to recompile the entire application.
  • Dynamic Changes: Allows for dynamic loading of components and modules, making it flexible for development and debugging.
Disadvantages of JIT
  • Slower Initial Load Time: Because the compilation happens in the browser, it adds extra time to the initial load of the application.
  • Larger Bundle Size: The JIT compiler needs to be included in the application bundle, increasing its size.
  • Performance Overhead: The runtime compilation adds overhead, which can affect the performance of the application.

3. What is AOT Compilation?


Ahead-of-Time (AOT) compilation is a method where the Angular application is compiled at build time, before being served to the browser. This means the browser receives the pre-compiled JavaScript code, ready to be executed immediately.

How AOT Works
  • Compilation: During the build process, the Angular AOT compiler compiles the Angular templates into JavaScript. This happens once, at build time.
  • Deployment: The pre-compiled JavaScript code is bundled and deployed to the server.
  • Execution: When the user accesses the application, the browser downloads and executes the pre-compiled code, skipping the compilation step.
Advantages of AOT
  • Faster Initial Load Time: Since the code is already compiled, the browser can execute it immediately, leading to faster load times.
  • Smaller Bundle Size: The AOT compiler removes unnecessary code and only includes the necessary parts, reducing the bundle size.
  • Improved Performance: Eliminates the runtime compilation overhead, resulting in better performance.
  • Early Detection of Errors: Compilation errors are caught at build time, reducing the chances of runtime errors.
Disadvantages of AOT
  • Longer Build Times: The build process takes longer as the compilation happens before deployment.
  • Complex Build Process: Requires more configuration and setup compared to JIT.

4. Detailed Comparison of AOT and JIT

Compilation Timing
  • JIT Compilation Happens at runtime in the browser. The Angular application is compiled every time it is loaded.
  • AOT Compilation: Happens at build time. The Angular application is compiled once during the build process, and the pre-compiled code is deployed.
  • JIT Compilation: Slower initial load time due to runtime compilation. Adds performance overhead during execution.
  • AOT Compilation: Faster initial load time as the code is pre-compiled. Eliminates runtime compilation overhead, resulting in better performance.
Bundle Size
  • JIT Compilation: Larger bundle size as the JIT compiler needs to be included in the application bundle.
  • AOT Compilation: Smaller bundle size as the unnecessary code is removed and only the necessary parts are included.
Debugging and Development Experience
  • JIT Compilation: Provides a faster development cycle with instant feedback on changes. Suitable for development and debugging.
  • AOT Compilation: Requires longer build times, making the development cycle slower. However, it catches errors at build time, improving code quality.
  • JIT Compilation: May expose more code to potential attacks as the compilation happens in the browser.
  • AOT Compilation: Enhances security by reducing the amount of exposed code and eliminating the need for runtime compilation.

5. When to Use AOT or JIT

  • Development Phase: Use JIT compilation during development and debugging for a faster development cycle and immediate feedback on changes.
  • Production Phase: Use AOT compilation for deploying the application to production. It provides better performance, smaller bundle size, faster load times, and improved security.

6. Conclusion

Understanding the differences between JIT and AOT compilation in Angular is crucial for optimizing your application's performance and development workflow. JIT compilation offers a flexible and fast development experience, making it ideal for development and debugging. On the other hand, AOT compilation provides faster load times, smaller bundle sizes, better performance, and enhanced security, making it the preferred choice for production deployment.

By leveraging the strengths of both compilation methods, you can create efficient, high-performance Angular applications that deliver a great user experience.

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