The NgUpgrade is an Angular 8 library mainly used to integrate both AngularJS and Angular components in the application. The NgUpgrade library is also used to bridge the gap between the Dependency Injection Systems in both AngularJS and Angular.

In Angular 8, NgUpgrade is a built-in Angular module that facilitates the seamless coexistence of AngularJS (Angular 1.x) and Angular (Angular 2+). It allows developers to upgrade their AngularJS applications gradually to Angular, enabling them to migrate piece by piece rather than having to rewrite the entire application at once.

The main usage of NgUpgrade includes:

Parallel Development: NgUpgrade allows developers to continue adding features and making updates to their AngularJS application while simultaneously integrating Angular components. This enables a smoother transition process without disrupting the existing functionality.
Interoperability: NgUpgrade provides APIs for bootstrapping Angular components within AngularJS applications and vice versa. This interoperability allows components from both frameworks to interact seamlessly within the same application.
Code Sharing: With NgUpgrade, developers can share code between AngularJS and Angular components, reducing duplication and easing the migration process. This enables a gradual transition strategy where components can be migrated incrementally, starting with the most critical ones.
Migration Path: NgUpgrade offers a clear migration path for transitioning from AngularJS to Angular. Developers can gradually refactor their codebase, replacing AngularJS components with Angular components as needed, while maintaining the application’s functionality and usability.

In summary, NgUpgrade is a crucial tool in the migration process from AngularJS to Angular, allowing developers to upgrade their applications incrementally, maintain interoperability between the two frameworks, and ensure a smooth transition for users.