svelte-subcomponent-preprocessor

The svelte-subcomponent-preprocessor allows you to write more than one component within a svelte file. The subcomponents are written in a {#component} block and remain local to the svelte file. Here's an example

<!-- List.svelte -->
<script>
  export let items = ['svelte', 'subcomponent', 'preprocessor'];
</script>

<ul>
  {#each items as item}
    <Item {item} />
  {/each}
</ul>

{#component Item}
  <script>
    import { onMount } from 'svelte';
    export let item;

    onMount(() => {
      console.log(item);
    });
  </script>

  <li>{item}!!!</li>

  <style>
    li {
      color: red;
    }
  </style>
{/component}

Installation and Configuration

npm install --save-dev svelte-subcomponent-preprocessor

In your svelte config import the preprocessor and add it to the preprocess array.

import subcomponentPreprocessor from 'svelte-subcomponent-preprocessor';
import sveltePreprocess from 'svelte-preprocess';

svelte({
  preprocess: [
    subcomponentPreprocessor(),
    sveltePreprocess() // must come after subcomponentPreprocessor
  ]
})

If you're using svelte-preprocess it must run after svelte-subcomponent-preprocessor.

svelte-subcomponent-preprocessor works by extracting the {#component} blocks from your svelte code and writing them to disk. By default they are written to ./node_modules/.svelte-subcomponent-preprocessor/. This can be changed with a configuration object passed to the preprocessor.

subcomponentPreprocessor({ out: './subcomponents' });

Config with Vite or SvelteKit

If you're using Vite or SvelteKit you'll need a bit of extra configuration to get the subcomponents to work with the dev server. You need the following in your vite config.

{
  // ...
  server: {
    watch: {
      ignored: ['!**/node_modules/.svelte-subcomponent-preprocessor/**']
    }
  },
  optimizeDeps: {
    exclude: ['.svelte-subcomponent-preprocessor']
  },
  // ...
}

Full vite configuration.

import { defineConfig } from "vite";
import { svelte } from "@sveltejs/vite-plugin-svelte";

// https://vitejs.dev/config/
export default defineConfig({
  plugins: [svelte()],
  server: {
    watch: {
      ignored: ['!**/node_modules/.svelte-subcomponent-preprocessor/**']
    }
  },
  optimizeDeps: {
    exclude: ['.svelte-subcomponent-preprocessor']
  }
});

Or in SvelteKit the server and optimizeDeps config would go inside the svelte.config.js vite object.

Config with Snowpack

With snowpack the default out configuration will not work. Change the out location to someplace that will be watched by the snowpack dev server.

Usage

To define a subcomponent put your component code inside a #component block. Pass the name of the subcomponent to the block, like so...

{#component ComponentName}    
  <h1>My Component</h1>
{/component}

<div>
  <ComponentName />
<div>

Limitations

• Does not currently work with snowpack. It may be possible to change the snowpack config to get it to work, but I'm not sure how.
• Subcomponents cannot have interdependencies. i.e. Only the default export component can use the subcomponents.