How to use createRawSnippet to test components?

[unikitty37]

In #2588, we're told that we can use Svelte 5's createRawSnippet to programmatically create child snippets (or "slots", in the old language). The docs show it being used as {@render greet(name)} but this doesn't give much of a clue on how to use it in a Vitest/Testing Library test.

Additionally, despite the switch at lower left being set to TypeScript, the example code is pure JavaScript and I don't know what types to use.

The result is that this code produces the error "Argument of type '[string]' is not assignable to parameter of type 'never'" and the test doesn't work as expected.

import { describe, it } from 'vitest'
import { render, screen } from '@testing-library/svelte'
import { createRawSnippet } from 'svelte'

import Badge from './Badge.svelte'

const badge = createRawSnippet((body) => {
  return {
    render: () => `<Badge>${body}</Badge>`,
    setup: (node) => {
      $effect(() => {
        node.innerHTML = `${body}`
      })
    },
  }
})

describe('Badge', () => {
  it('shows a badge with the slot', () => {
    render(badge('SAUSAGE'))

    screen.debug()
  })
})

I also get "'Badge' is defined but never used", because TypeScript isn't able to parse the template string. I'd rather have a way of rendering the component that doesn't use template strings, TBH — if I'd wanted to pepper my code with those, I'd use React 😁

What should I be putting here? (It would be really helpful if the docs actually covered the use of this for writing tests, as that's likely to be one of the main uses…)

[7nik]

render() should return an ordinary HTML, while in setup() you make it interactive (add event listeners or mount a component).

Usage is like this:

<script>
	import { createRawSnippet, mount, unmount } from 'svelte';
	import Component from "./Component.svelte";
	import Container from "./Container.svelte"

	const props = $state({ name: "Test" })
	
	const snip = createRawSnippet((name) => {
		return {
			render: () => `<div></div>`,
			setup: (target) => {
				const comp = mount(Component, { target, props }); 
				return () => unmount(comp);
			}	
		};
	});
</script>

<input bind:value={props.name}><br>
<Container children={snip} />

[unikitty37]

Thanks, but I still can't see how to integrate that into a vitest test. When I try running it, it complains that 'mount(…)' is not available on the server.

It would really have helped if the comment on #2588 had not just said it was possible to do this, but how to do this.

[7nik]

There are docs about testing; just the site with docs isn't deployed.

[unikitty37]

Ah, looks like the resolve section was missing from the vitest config. Thanks!