AOT (Ahead-of-Time) and JIT (Just-in-Time) are two compilation methods used in Angular. Here's a comparison of the two: JIT (Just-in-Time): 1. Compilation: JIT compilation happens at runtime in the user's browser. The Angular compiler runs in the browser and compiles the application's templates and components into JavaScript during the application's bootstrap process. 2. Development mode: JIT is primarily used during development as it allows for rapid iterations and immediate feedback. It supports features like hot module replacement, which speeds up the development process. 3. Performance: JIT compilation can impact the initial load time of the application because the compilation process happens at runtime. The browser needs to download the Angular compiler and perform the compilation process, which can lead to a slower startup time. 4. Debugging: JIT allows for better debugging experience as the browser can map the compiled code to the original TypeScript source files, enabling developers to debug directly in the browser's developer tools. AOT (Ahead-of-Time): 1. Compilation: AOT compilation occurs before the application is deployed. The Angular compiler runs on the developer's machine during the build process and generates pre-compiled JavaScript code. The compiled code includes templates and components converted into efficient JavaScript code. 2. Production mode: AOT is primarily used in production deployments to optimize the performance and load time of the application. It eliminates the need for the Angular compiler in the browser, resulting in faster startup times and smaller bundle sizes. 3. Performance: AOT significantly improves the initial load time of the application. The browser downloads pre-compiled JavaScript code, reducing the amount of work needed to be done at runtime. 4. Security: AOT provides a level of security by pre-compiling the templates and removing the Angular compiler from the client-side code. This mitigates the risk of template injection attacks. 5. Smaller bundle size: AOT allows for tree shaking, a process that eliminates unused code during the compilation phase. This leads to smaller bundle sizes and reduces the overall download size for users. 6. Limited dynamic behavior: AOT introduces some limitations on dynamic behaviors, such as dynamic template generation or dynamic component loading, as the templates and components are pre-compiled during the build process. In summary, JIT compilation is used during development, provides a better debugging experience, but can impact the initial load time. On the other hand, AOT compilation is primarily used in production deployments, improves performance and security, and reduces bundle sizes, but has limitations on dynamic behaviors.