Vue

Composition API is the default. <script setup> is the default syntax. If you reach for Options API, you need a reason.

Vue 3 rewards explicit, composable code. Prefer ref() over reactive(), composables over mixins, and typed props over runtime-only validation. References contain extended examples, rationale, and edge cases for each topic.

References

Reactivity

Choosing a Reactive Primitive

ref() Is the Primary API

• Works with primitives (string, number, boolean).
• Can be destructured from composable returns without losing reactivity.
• Can be reassigned (count.value = newValue).
• Consistent .value access pattern everywhere in script.
• Access .value in script, omit in template — templates auto-unwrap top-level refs.

reactive() Limitations

• Cannot hold primitives.
• Reassignment loses reactivity — state = reactive({...}) breaks tracking.
• Destructuring primitives loses reactivity — use toRefs() if you must destructure.
• Do not use reactive() as the primary primitive. Use ref().

Ref Unwrapping Rules

• Top-level refs in templates are auto-unwrapped: {{ count }} works.
• Non-top-level refs in plain objects are NOT unwrapped: {{ obj.id + 1 }} breaks if obj.id is a ref. Destructure to top level to fix.
• Refs nested inside reactive() objects are unwrapped automatically.
• Refs inside reactive arrays/collections are NOT unwrapped — need .value.

Computed Properties

• Keep computed getters pure — no side effects.
• Split complex computed into smaller ones.
• Computed caches its value; only recalculates when dependencies change.
• Computed stability (3.4+): only triggers effects when the returned value actually changes. Avoid returning new objects from computed — each new object is "different".
• Writable computed is rare — use sparingly. Requires get/set form.

Watchers

watch() vs watchEffect():

watch() options:

• { immediate: true } — run callback on creation (like watchEffect).
• { deep: true } — watch all nested properties (expensive, use sparingly).
• Watch a getter for specific property: watch(() => obj.specificProp, callback).
• Watch multiple sources: watch([a, b], ([newA, newB]) => {...}).

Cleanup: Both watch and watchEffect support cleanup via the onCleanup parameter. Use it for aborting fetch requests, clearing timers, removing listeners.

DOM Update Timing

Reactive state changes batch DOM updates to the next tick. Use nextTick() for post-DOM-update logic.

Single-File Components

Block Order

Always: <script setup> first, <template> second, <style> last.

Organization Within <script setup>

Order declarations logically:

1. Imports — Vue APIs, components, composables, types
2. Props and emits — defineProps, defineEmits
3. Composable calls — useRouter(), custom composables
4. Reactive state — ref(), reactive(), computed()
5. Functions — event handlers, helpers
6. Watchers — watch(), watchEffect()
7. Lifecycle hooks — onMounted(), onUnmounted()
8. Expose — defineExpose() (rare)

Compiler Macros

Key macros available without import in <script setup>: defineProps(), defineEmits(), defineModel(). Use defineOptions() for options that <script setup> doesn't natively support (name, inheritAttrs: false). Full macro catalog in references/sfc.md.

Template Syntax

Directive shorthands — use consistently, don't mix:

Conditional rendering:

• v-if / v-else-if / v-else for conditional blocks.
• v-show for frequent toggles (CSS display: none, avoids mount/unmount cost).
• Use v-if for conditions that rarely change, v-show for frequent toggles.

Template refs:

• 3.5+: useTemplateRef<HTMLInputElement>('input') with matching ref attribute.
• Pre-3.5: ref<HTMLInputElement | null>(null) with matching ref name.

Scoped Styles

• Use <style scoped> by default. Global styles only in App.vue or layouts.
• Child component root elements are affected by both parent and child scoped styles.
• Deep selectors: .parent :deep(.child-class) to style child internals (use sparingly).
• CSS modules: <style module> generates unique class names, accessed via $style.
• v-bind(color) in <style> uses reactive values in CSS.

Components

Naming

• Multi-word names always — TodoItem not Item. Avoids HTML element conflicts.
• PascalCase in SFC templates: <TodoItem />. Kebab-case only in in-DOM templates.
• PascalCase filenames: TodoItem.vue.
• Base component prefix for presentational components: BaseButton, BaseIcon.
• Parent prefix for tightly coupled children: TodoListItem, TodoListItemButton.
• Highest-level word first to group related: SearchButtonClear, SearchInputQuery.
• Full words always — no abbreviations.
• One component per file. No inline registration.
• Self-closing tags for components without children: <MyComponent />.
• PascalCase when importing in JS/TS.

Props

• Use defineProps<T>() (type-based) in TypeScript projects. Object syntax in JS projects. Never array syntax in committed code.
• Declare in camelCase, use in templates as kebab-case — Vue converts automatically.
• Never mutate props. Use computed() for transformations, ref() + initial value for local copies.
• One-way data flow — props down, events up.
• Reactive props destructure (3.5+): destructured props are reactive, usable in watch/computed directly.
• Pass destructured props to composables via getter: useComposable(() => id).

Emits

• Declare all emits with defineEmits() — preferably type-based.
• Emit in camelCase: emit('someEvent'). Listen in kebab-case: @some-event="handler". Vue converts automatically.
• Use named tuple syntax (3.3+) for type-based emits: defineEmits<{ change: [id: number] }>().

v-model

• Use defineModel<T>() for two-way binding shorthand.
• Named v-model: defineModel<string>('firstName') with v-model:first-name="first" on parent.

Slots

• Default slot: <slot /> in child, content between tags in parent.
• Named slots: <slot name="header" /> in child, <template #header> in parent.
• Scoped slots: pass data via slot props — <slot :item="item" />, consume with <template #default="{ item }">.

Provide / Inject

• Use Symbol keys (InjectionKey<T>) for type safety — avoid string collisions.
• Export keys from a shared keys.ts file.
• Provide readonly() refs to prevent consumers from mutating state.
• Provide updater functions when consumers need to change provided state.
• Always provide a default or handle undefined in the consumer.

Template Rules

• v-for always has :key — stable, unique identifiers.
• Never v-if on same element as v-for. Use computed to filter, or wrap with <template v-for>.
• Simple expressions only in templates. Move logic to computed or functions.
• Multi-attribute elements span multiple lines — one attribute per line.

Composables

Design

• Always prefix with use: useMouse, useFetch, useAuth.
• camelCase naming: useEventListener not use-event-listener.
• The use prefix signals the function uses Vue reactivity and must be called within setup() or <script setup>.

Structure

Follow this order: (1) reactive state, (2) logic that modifies state, (3) lifecycle hooks for side effects, (4) return refs.

Return Values

Always return a plain object containing refs. Never return a reactive() object — it loses reactivity on destructure. If the consumer wants an object, they can wrap: const mouse = reactive(useMouse()).

Input Arguments

Accept refs, getters, and raw values. Use toValue() (with MaybeRefOrGetter<T> type) to normalize inputs inside watchEffect so reactive sources are tracked.

Side Effects and Cleanup

• Always clean up side effects in onUnmounted() — event listeners, timers, subscriptions.
• SSR safety: DOM-specific effects go in onMounted() / onUnmounted(), not at top level.

Composition

Composables can call other composables — build complex logic by composing simple hooks.

Usage Restrictions

• Must be called inside <script setup> or setup() function.
• Must be called synchronously — not inside async callbacks or promises.
• Exception: <script setup> restores the active instance after await, so composables work after await in <script setup>.
• Inside lifecycle hooks like onMounted() is acceptable.

Composables vs Alternatives

Composables are explicit, namespaced through destructuring, and integrate with Vue's reactivity and lifecycle.

TypeScript

Props

• Use defineProps<T>() with an interface.
• Defaults with 3.5+ destructure: const { title, count = 0 } = defineProps<Props>().
• Defaults with 3.4 and below: withDefaults(defineProps<Props>(), { count: 0 }). Mutable reference defaults (arrays, objects) must use factory functions with withDefaults.
• Imported types work since Vue 3.3.

Emits

Use named tuple syntax (3.3+): defineEmits<{ change: [id: number] }>().

Refs

• Refs infer types from initial values — ref(0) is Ref<number>.
• Use explicit type for union types: ref<string | number>('2024').
• Nullable: ref<User | null>(null).
• Without initial value: ref<ResponseData>() yields Ref<ResponseData | undefined>.

Computed

Types are inferred. Use explicit generic only when inference falls short: computed<string>(() => ...).

Reactive

Annotate with interface, do not use reactive<T>() generic — the returned type handles ref unwrapping differently from the generic parameter.

Template Refs

• 3.5+: useTemplateRef<HTMLInputElement>('input').
• Component refs: useTemplateRef<InstanceType<typeof MyComponent>>('comp').
• Pre-3.5: ref<HTMLInputElement | null>(null).

Provide / Inject

Use InjectionKey<T> for type-safe keys. String keys require generic annotation: inject<string>('key').

Generic Components

Use generic attribute on <script setup>: <script setup lang="ts" generic="T extends string | number">.

Event Handlers

Type DOM events explicitly: (event: Event) then cast target: event.target as HTMLInputElement.

Utility Types

Use Vue's typed helpers: Ref<T>, ComputedRef<T>, MaybeRefOrGetter<T>, InjectionKey<T>, PropType<T>, ComponentPublicInstance. Full utility types table in references/typescript.md.

Performance

Page Load

• Use dynamic imports for route-level code splitting.
• Use defineAsyncComponent() for component-level splitting.
• Prefer tree-shakable dependencies (lodash-es over lodash).
• <script setup> compiles to more minification-friendly code than Options API.

Update Performance

• Props stability — pass derived booleans (active="item.id === activeId") instead of raw IDs to prevent unnecessary child re-renders.
• v-once — render once, skip all future updates. For truly static content.
• v-memo — conditionally skip sub-tree updates. Accepts a dependency array; only re-renders when a dependency changes. Use on v-for lists where most items don't change.
• Computed stability (3.4+) — computed only triggers effects when value actually changes. Avoid returning new objects from computed (each new object is "different").

Large Lists

• Virtual scrolling for 1000+ items — render only visible items (vue-virtual-scroller, vueuc/VVirtualList).
• shallowRef() for large immutable datasets — mutations must replace the whole value.
• Avoid unnecessary wrapper components in large lists — every component instance has overhead.

Watch Performance

• Avoid { deep: true } on large objects — watch specific properties instead.
• Throttle/debounce watchers for expensive callbacks (e.g., useDebounceFn from VueUse).

Profiling

• Enable app.config.performance = true for Vue-specific performance markers.
• Use Chrome DevTools Performance panel and Vue DevTools profiler.
• Measure before and after — don't optimize without data.

Application

When writing Vue code:

• Apply all conventions silently — don't narrate each rule being followed.
• If an existing codebase contradicts a convention, follow the codebase and flag the divergence once.
• Always use <script setup> unless there is a documented reason not to.

When reviewing Vue code:

• Cite the specific violation and show the fix inline.
• Don't lecture or quote the rule — state what's wrong and how to fix it.

Integration

This skill provides Vue-specific conventions. The coding skill governs workflow; language skills govern JS/TS choices; this skill governs component architecture, reactivity patterns, and framework API usage.