Architect reusable form validation in Vue vs Angular

WHAT THIS TESTS: The interviewer wants to see if you understand framework-level reactivity and ownership models rather than just API syntax. Vue composables are functions that leverage the Composition API to encapsulate stateful logic; they return reactive refs and can hook into the owner component's lifecycle automatically. Angular services with FormGroup are class-based dependencies that centralize state and rely on RxJS and dependency injection. This question reveals whether you can choose the right abstraction for each framework's mental model.

A GOOD ANSWER COVERS: First, explain that a Vue composable like useForm is a factory function that creates reactive state with ref or reactive, returns it to the component, and can register side effects via onMounted or onUnmounted. Second, note that the composable is scoped to the component instance; each call gets its own state, so multiple forms on the same page do not collide. Third, describe the Angular side as a service that injects FormBuilder, constructs a FormGroup, and exposes it along with validation helpers; the component injects the service and binds the FormGroup in the template using formGroup and formControlName directives. Fourth, highlight that Angular's approach pushes state into a separate class and uses RxJS for async validation and value changes, while Vue keeps the logic colocated with the component through the composable. Fifth, mention that in Vue you would likely pass initial config into the composable as arguments, whereas in Angular you might configure the service through its constructor or a factory provider.

COMMON WRONG ANSWERS: Treating the Vue composable as a singleton by defining state outside the function body; this causes shared mutable state across components. Suggesting that Angular services automatically clean up subscriptions without calling unsubscribe or using takeUntilDestroyed. Claiming that Vue composables cannot be tested independently because they rely on a component; in fact they are just functions and can be tested in isolation. Proposing to put template-level DOM logic inside the Angular service, which breaks separation of concerns. Ignoring the difference in how templates consume each: Vue binds to returned refs directly, while Angular binds to the FormGroup object and its controls.

LIKELY FOLLOW-UPS: How would you handle async validators in each approach? What happens if two components use the same Vue composable on the same page? How do you reset form state without mutating references in Vue versus calling reset on the FormGroup? Would you ever inject a Vue composable, and how does that compare to Angular's DI? How do you share the same form instance across sibling components in Angular versus Vue?

ONE CONCRETE EXAMPLE: In Vue, you would write a useLoginForm composable that takes an onSubmit callback, creates refs for email and password, attaches a watch for password strength, and returns email, password, errors, and submit. The component imports it and destructures the returned object. In Angular, you would create a LoginFormService that injects FormBuilder, builds a FormGroup with email and password controls and their sync and async validators, and exposes the group plus a submit method. The component injects LoginFormService in its constructor and assigns the FormGroup to a local property that the template references.