Inertia Rails TypeScript Setup

Type-safe shared props, flash, and errors using InertiaConfig module augmentation. Works identically across React, Vue, and Svelte — the globals.d.ts and InertiaConfig setup is the same for all frameworks.

Before adding TypeScript types, ask:

• Shared props (auth, flash)? → Update SharedProps/FlashData in index.ts — InertiaConfig in globals.d.ts propagates them globally via usePage()
• Page-specific props? → type Props = { ... } in the page file only — never include shared props here

InertiaConfig Module Augmentation

Define shared props type ONCE globally — never in individual page components.

InertiaConfig property names are EXACT — do not rename them:

• sharedPageProps (NOT sharedProps)
• flashDataType (NOT flashProps, NOT flashData)
• errorValueType (NOT errorBag, NOT errorType)

// app/frontend/types/globals.d.ts
import type { FlashData, SharedProps } from '@/types'

declare module '@inertiajs/core' {
  export interface InertiaConfig {
    sharedPageProps: SharedProps   // EXACT name — auto-typed for usePage().props
    flashDataType: FlashData      // EXACT name — auto-typed for usePage().flash
    errorValueType: string[]      // EXACT name — errors are arrays of strings
  }
}

// app/frontend/types/index.ts
export interface FlashData {
  notice?: string
  alert?: string
}

export interface SharedProps {
  auth: { user?: { id: number; name: string; email: string } }
}

Convention: Use auth: { user: ... } as the shared props key — this matches the Rails inertia_share community convention ({ auth: { user: current_user } }). The auth namespace separates authentication data from page props, preventing collisions when a page has its own user prop. Do NOT use current_user: or user: as top-level keys — they collide with page-specific props and break the convention that other Inertia skills and examples assume.

BAD vs GOOD Patterns

// BAD — passing shared props as generics:
// usePage<{ users: User[], auth: AuthData, flash: FlashData }>()

// BAD — extending a SharedProps interface into page props:
// interface Props extends SharedData { users: User[] }

// BAD — declaring PageProps interface:
// interface PageProps { auth: AuthData; flash: FlashData }

// BAD — using current_user or user as top-level shared key:
// interface SharedProps { current_user: User }

// BAD — destructuring auth directly from usePage() (TS2339: 'auth' does not exist on Page):
// const { auth } = usePage()
// usePage() returns a Page object with { props, flash, component, url, ... }
// auth lives inside props, not on the Page itself

// BAD — duplicating InertiaConfig in index.ts (it belongs in globals.d.ts):
// declare module '@inertiajs/core' { ... }  ← in index.ts

// GOOD — props from usePage().props, flash from usePage().flash:
const { props, flash } = usePage()
// props.auth is typed (from SharedProps via InertiaConfig)
// flash.notice is typed (from FlashData via InertiaConfig)

Important: globals.d.ts configures InertiaConfig ONCE. When adding a new shared prop, only update index.ts — do NOT touch globals.d.ts:

// BEFORE — app/frontend/types/index.ts
export interface SharedProps {
  auth: { user?: { id: number; name: string; email: string } }
}

// AFTER — add the new key here, NOT in globals.d.ts
export interface SharedProps {
  auth: { user?: { id: number; name: string; email: string } }
  notifications: { unread_count: number }
}

InertiaConfig in globals.d.ts references SharedProps by name — it picks up the change automatically. Adding a second declare module '@inertiajs/core' causes conflicts.

Page-Specific Props

Page components type ONLY their own props. Shared props (like auth) and flash come from InertiaConfig automatically.

type vs interface for page props (React-specific)

This constraint applies to React only. Vue's defineProps<T>() and Svelte's $props() do not use usePage<T>() generics, so interface works fine there.

usePage<T>() requires T to have an index signature. type aliases have one implicitly; interface declarations do not. Using interface with usePage causes TS2344 at compile time.

// React
type Props = {
  users: User[]         // page-specific only
  // auth is NOT here — it comes from InertiaConfig globally
}

export default function Index({ users }: Props) {
  // Access shared props separately:
  const { props, flash } = usePage()
  // props.auth is typed via InertiaConfig
  // flash.notice is typed via InertiaConfig
  return <UserList users={users} />
}

Accessing shared props in Vue and Svelte

Vue and Svelte use different patterns to access shared props, but InertiaConfig typing works the same way.

<!-- Vue 3 — usePage() returns reactive object; use computed() for derived values -->
<script setup lang="ts">
import { usePage } from '@inertiajs/vue3'
import { computed } from 'vue'

const page = usePage()
const userName = computed(() => page.props.auth.user?.name) // typed via InertiaConfig
</script>

<!-- Svelte — page store from @inertiajs/svelte -->
<script lang="ts">
  import { page } from '@inertiajs/svelte'
  // $page.props.auth is typed via InertiaConfig
  // $page.flash.notice is typed via InertiaConfig
</script>

Common TypeScript Errors

Typelizer Integration

If using the typelizer gem (see alba-inertia skill), SharedProps are auto-generated from your serializer — do NOT manually write the SharedProps interface in index.ts. You only write globals.d.ts once (the InertiaConfig augmentation). When you add a new attribute to SharedPropsResource, Typelizer regenerates index.ts and the types propagate via InertiaConfig — no manual type updates needed.

Related Skills

• Shared props setup → inertia-rails-controllers (inertia_share)
• Flash config → inertia-rails-controllers (flash_keys)
• Auto-generated types → alba-inertia (Typelizer + Alba resources)
• Page component props → inertia-rails-pages (type Props pattern)