This PR allows you to type-hint shared page props in a global type definition file, for example, resources/js/types/globals.d.ts.

declare module '@inertiajs/core' {
  export interface InertiaConfig {
    sharedPageProps: {
      auth: { user: { id: number; name: string } | null }
      flash: { success?: string; error?: string }
    }
  }
}

Now the shared page props are automatically type-hinted throughout the app when you use usePage().

const page = usePage()
const successMessage = page.props.flash.success 

In the example above, successMessage will be typed as string | undefined. In Svelte and Vue, this also works in the template.

<template>
  <p>{{ $page.props.flash.error }}</p>

  <!-- @vue-expect-error - 'message' does not exist on flash -->
  {{ $page.props.flash.message }}
</template>

You can still pass an interface to usePage() for page-specific props, which will automatically be merged with the declared sharedPageProps.

type PageProps = {
  posts: { id: number; title: string }[]
}

const page = usePage<PageProps>()
const successMessage = page.props.flash.success
const postTitles = page.props.posts.map((post) => post.title)

In this case, postTitles will be typed as string[].

For reference, previously, you could already partially do this by extending the PageProps interface.

type SharedPageProps = {
  flash: { error?: string; success?: string }
}

declare module '@inertiajs/core' {
  interface PageProps extends PageProps, SharedPageProps {}
}

The problem with this approach is that the shared page props are overwritten when you pass a specific interface to usePage().

type PageProps = {
  posts: { id: number; title: string }[]
}

const page = usePage<PageProps>()
const successMessage = page.props.flash.success // <== No longer typed! (shared props are lost)
const postTitles = page.props.posts.map((post) => post.title)

This was originally planned in PR #2549, but I decided to split it up into multiple PRs for clarity.