What is Angular Ivy's 'locality' and its benefits over View Engine?

WHAT THIS TESTS: Whether you understand the Ivy compilation model's core principle of locality and how it differs from View Engine's global program analysis. Interviewers want to see that you know Ivy compiles Angular decorators into self-contained static properties per file, and that you can articulate the concrete build-time and bundle-size benefits that result.

A GOOD ANSWER COVERS: First, a crisp definition of locality. Explain that under Ivy, decorators like @Injectable compile to static properties on the class itself, for example ɵprov, using only the metadata present in that single file. Second, acknowledge the one exception to pure locality: @Component compilation requires knowledge of the declaring @NgModule's scope, specifically the transitive closure of selectors from that module's imports and exports, to generate the ɵcmp definition. Third, contrast this with View Engine, which relied on global program knowledge and shipped .metadata.json sidecars alongside compiled JavaScript to preserve decorator information. Fourth, enumerate the three practical benefits asked for: tree-shaking improves because unused classes and their metadata are no longer entangled in global sidecars and can be dropped by bundlers; compilation speed increases because the compiler can work incrementally file-by-file without re-analyzing the whole program; and bundle size shrinks because dead code elimination is more effective and the extra metadata files are eliminated.

COMMON WRONG ANSWERS: Confusing locality with style encapsulation or component-level DOM scoping. Claiming that Ivy still requires .metadata.json for all libraries. Asserting that @Component is fully local without mentioning the @NgModule selector scope exception. Describing the benefits only in vague terms like faster without explaining the mechanism, or attributing bundle size wins solely to the runtime rather than the compiler model.

LIKELY FOLLOW-UPS: How does ngcc bridge the gap for pre-Ivy libraries on NPM? What happens if a component is declared in multiple NgModules? How does the locality principle affect library authors shipping Angular Package Format? Can you explain the difference between ngtsc and ngcc?

ONE CONCRETE EXAMPLE: In View Engine, publishing an @Injectable service to NPM required a .metadata.json file so that downstream compilers could understand its provider configuration. If an application imported the library but never injected that service, the metadata still had to be parsed and shipped because it was part of a global graph. In Ivy, the service carries its provider definition as a static ɵprov property directly on the class. If the application never references the class, a bundler like Webpack or Rollup can tree-shake the entire class and its definition away, reducing parse time and bundle size with no orphaned metadata files.

Source: angular/packages/compiler/design/architecture.md