What is metadata in Angular?

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Metadata in Angular is the configuration attached to a class via a decorator ( @Component , @NgModule , @Injectable , @Directive ). Tells Angular how to use the class. Component metadata — selector , template , styles , changeDetection , standalone , imports . Module metadata — declarations , imports , providers , exports , bootstrap . Service metadata — providedIn on @Injectable ; controls where the service lives in the DI tree. Read at compile time — the Angular compiler uses metadata to wire up DI, change detection, and template references. Reflect-metadata — the underlying library that lets decorators carry runtime info. Without metadata — the class is just a plain TypeScript class; Angular doesn't know what to do with it. Metadata = the config object passed to a decorator. Compiler reads it **statically** at build time — function calls in metadata historically failed AOT. Naming `reflect-metadata` signals depth.