Inside +page.ts and +layout.ts SvelteKit provides parent function to access the data returned from a parent load function, a couple of things to notice when using the parent function.

Load function#

SvelteKit handles data loading for page component (+page.svelte) inside +page.ts (or +page.js) file, exporting a load function in which and the returned value will be available through data prop inside page:

// src/routes/note/[slug]/+page.ts
import type { PageLoad } from './$types'

export const load: PageLoad = function(({ params }) {
  return {
    note: {
      title: `Title for ${params.slug} goes here`,
      content: `Content for ${params.slug} goes here`
    }
  }
})

<!-- src/routes/note/[slug]/+page.svelte -->
<script lang="ts">
  import type { PageLoad } from './$types'

  export let data: PageLoad
</script>

<h1>{data.note.title}</h1><div>{@html data.note.content}</div>

Using Parent data#

Sometimes it might be useful for a load function to use data from a parent load function. For example, a parent /project/+page.ts routes return a list of tags and their amount to the project list page. Inside each project /project/[slug]/+page.ts we might also need to access this same information, this is where parent function is useful:

// src/routes/+layout.ts
import type { LayoutLoad } from './$types'

export const load: LayoutLoad = function () {
  return { a: 1 }
}

///src/routes/abc/+layout.ts
import type { LayoutLoad } from './$types'

export const load: LayoutLoad = async function ({ parent }) {
  const { a } = await parent()
  return { b: a + 1 }
}

//src/routes/abc/+page.ts
import type { PageLoad } from './$types'

export const load: PageLoad = async function ({ parent }) {
  const { a, b } = await parent()
  return { c: a + b }
}

Things to notice#

1. The load funciton inside +page.ts will receive the merged data from both parent +page.ts and +layout.ts and the sibling +layout.ts, not just the parent.
2. The load function inside +layout.ts will receive the merged data from its parent load in +page.ts and +layout.ts.
3. A missing +layout.ts is treated as ({ data }) => data.
4. The load funciton inside +page.server.ts and +layout.server.ts will receive data from only parent +layout.server.ts

Refereneces#

• SvelteKit Docs