Question 1

What is a composable in Vue?

Accepted Answer

A function starting with use that encapsulates reusable stateful logic via Composition API. Similar spirit to React hooks but different rules. Return refs/functions. Document side effects.

Question 2

Lifecycle hooks in setup / script setup?

Accepted Answer

onMounted, onUpdated, onUnmounted, etc., register when called synchronously during setup. Don't register hooks async after await without care—call before await or use other patterns.

Question 3

How do provide/inject work with Composition API?

Accepted Answer

provide(key, value) in ancestor; inject(key) in descendants; can be reactive if value is. Prefer explicit props for public component APIs; provide/inject for app-level concerns.

Question 4

Top-level await in <script setup> — implications?

Accepted Answer

Makes the component async; use with Suspense; be mindful of SSR and error handling. Great for gated data loading. Provide fallbacks.

Question 5

Sharing state with a module-level ref composable?

Accepted Answer

A singleton ref outside the function creates shared state across callers. Intentional for global stores; accidental for per-component state—create refs inside the function.

Question 6

Composable design best practices?

Accepted Answer

Accept refs/getters for inputs, return consistent APIs, clean up side effects, avoid hidden globals. Keep composables focused. Name clearly. Test independently.

Question 7

How do you migrate an Options API component to Composition?

Accepted Answer

Move data→ref/reactive, methods→functions, computed/watch similarly; use script setup gradually. Don't big-bang rewrite. Extract composables as you go.

Question 8

defineExpose — when do you need it?

Accepted Answer

With script setup, bindings are closed by default; expose selectively for parent template refs. Prefer props/emits over imperative parent calls when possible.

Question 9

TypeScript with defineProps/defineEmits?

Accepted Answer

Use generic type params or runtime declaration with types; prefer type-based for clarity. defineProps<{ title: string }>(). Avoid duplicating runtime + types inconsistently.

Question 10

Composables vs Pinia stores?

Accepted Answer

Composables for local/reusable logic; Pinia for shared app state with tooling. Don't put every bit of state in Pinia. Use composables inside stores too.

Question 11

How do you unit test a composable?

Accepted Answer

Call it inside a setup wrapper or @vue/test-utils; assert returned refs after actions. Mock dependencies. Test cleanup on unmount.

Question 12

Lead: standards for composables in a large Vue codebase?

Accepted Answer

Lint naming, folder conventions (composables/), review for singleton leaks, docs with examples. Encourage composition over mixins. Ban new mixins for greenfield.

Composition API Vue.js Interview Questions

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