Vue component patterns

Vue Component Patterns

A Vue component is a contract: typed props in, declared events out, everything else private. Most tangled Vue codebases broke exactly that contract.

When to use

• Creating or reviewing Vue 3 components (<script setup>)
• "Where should this state live?" — local, parent, composable, or Pinia
• Extracting logic shared between components
• Reactivity bugs (stale values, lost reactivity, surprise re-renders)

Principles

• Props down, events up — no exceptions. Mutating a prop, reaching into $parent, or passing callbacks-as-props breaks traceability. defineModel for two-way intent.
• State lives at the lowest level that all readers share. Local until two components need it; lift to the common parent; Pinia only for genuinely app-wide state (auth, cart) — not as a prop-drilling shortcut.
• Composables own logic, components own presentation. When a component grows watchers + fetches + transforms, the logic wants to be a useX() composable with its own test.
• Types are the documentation. Typed props, typed emits, typed composable returns. An any prop is an undocumented API.

Process

1. Define the contract first

<script setup lang="ts">
const props = defineProps<{
  order: Order
  editable?: boolean
}>()

const emit = defineEmits<{
  save: [order: Order]
  cancel: []
}>()
</script>

• Props: data the component renders or branches on. Don't pass a whole object when it uses two fields — but don't explode into 12 scalar props either; group cohesive data
• Events: named after what happened (save, item-selected), not what the parent should do (refetchList)

2. Place the state

State	Lives in
UI-only (open/closed, hover, input draft)	Component-local ref
Shared by siblings	Common parent, passed down
Form state across a multi-step flow	Parent or a scoped composable
Server data	Data-fetch layer (composable / query lib), cached — not copied into Pinia
True app-globals (auth user, locale, cart)	Pinia store

3. Extract composables (when, not how)

Extract useX() when: the same logic appears twice, or a component mixes 2+ concerns (fetching + debouncing + formatting).

• Composable returns refs + functions, no DOM or component references
• One concern per composable: useOrderSearch(), not useOrderStuff()
• Side-effectful composables clean up after themselves (onUnmounted, abort controllers)

4. Respect reactivity rules

• Don't destructure props or a reactive() (loses reactivity) — toRefs/computed or access via the object
• computed for derived data; a watch that just recomputes a value should be a computed
• watch is for side effects (fetch on param change, sync to localStorage); keep them few — watcher webs are where reactivity bugs breed
• Lists: stable :key (id, never index when reordering/filtering)

5. Keep renders cheap

• Heavy lists: pagination/virtual scrolling before memo-tricks
• Expensive pure subtrees: v-memo / split into a child so its re-render scope shrinks
• shallowRef for large immutable structures (big API payloads) you replace wholesale
• Defer below-the-fold weight: defineAsyncComponent

6. Review pass

# Smells worth grepping for
grep -rn "props\.\w* =" src/            # prop mutation
grep -rn '\$parent\|getCurrentInstance' src/
grep -rn ":key=\"index\"\|:key=\"i\"" src/

Plus: components > ~200 lines of script (extract composable), stores imported by everything (Pinia as global junk drawer), emits not declared/typed.

Output format

## Vue review: <component/feature>

### Contract
Props: … | Emits: … | v-model: … — typed: ✅/❌

### State placement
| State | Now | Should be |
|-------|-----|-----------|
| …     | Pinia | parent-local |

### Extractions
- `useX()` from <component> — owns: …

### Findings (ranked)
1. <file:line> — <issue> → <fix>

Anti-patterns

• ❌ Mutating props, or "syncing" a prop into a local ref and editing the copy
• ❌ Events named as commands to the parent (refetch) instead of facts (saved)
• ❌ Pinia as a prop-drilling workaround — every component coupled to global state
• ❌ Server data copied into a store, then drifting from the server
• ❌ Watcher chains where A watches B watches C — replace with computed
• ❌ :key="index" on a filterable/sortable list
• ❌ One 600-line component with fetching, transforming, and three modals inside