esm-loader-svelte

Node.js ESModule Loader for importing and loading Svelte (.svelte) and SvelteKit files, and transpiling on the fly.

Warning! Using experimental Node.js features and flags, API will likely change. This may be helpful for development and testing, but should not be used in production.

Usage

npm install --save-dev esm-loader-svelte

We want to import a .svelte file with Node.js:

<!-- Component.svelte -->
<script>
  const words = 'Hello'
</script>

<h1>{words} World!</h1>

<style>
  h1 {
    color: blue;
  }
</style>

// index.js
import Component from './Component.svelte'

// render(Component) to DOM, etc.

Standalone

# node >= 20.7
cat << EOF > ./register.js
import { register } from 'node:module'
register('esm-loader-svelte', import.meta.url)
EOF
NODE_OPTIONS="--import ./register.js" node index.js

# node < 20.7
NODE_OPTIONS="--loader esm-loader-svelte" node index.js

Chainable

This loader can be configured, and chained with other loaders, using node-esm-loader.

npm install --save-dev node-esm-loader

// .loaderrc.js
export default {
  loaders: ['esm-loader-svelte'],
}

# node >= 20.7
NODE_OPTIONS="--import node-esm-loader/register" node index.js

# node < 20.7
NODE_OPTIONS="--loader node-esm-loader" node index.js

Options

Debug

// .loaderrc.js
export default {
  loaders: [
    {
      loader: 'esm-loader-svelte',
      options: {
        debug: true,
      },
    },
  ],
}

Preprocess

Preprocessing options can be supplied, for usage with something like SvelteKit's svelte-preprocess.

Supply preprocessing options via node-esm-loader config file .loaderrc.js:

// .loaderrc.js
import preprocess from 'svelte-preprocess'
import { resolve } from 'node:path'

export default {
  loaders: [
    {
      loader: 'esm-loader-svelte',
      options: {
        preprocess: preprocess({
          postcss: true,
          typescript: {
            tsconfigDirectory: resolve('./'),
            tsconfigFile: 'tsconfig.json',
          },
        }),
      },
    },
  ],
}

SvelteKit

If options.preprocess is NOT found in .loaderrc.js, then we will try to load a SvelteKit svelte.config.js file, and use the preprocess settings found therein:

// svelte.config.js
import preprocess from 'svelte-preprocess'
import { resolve } from 'node:path'

export default {
  kit: ...,
  preprocess: preprocess({
    postcss: true,
    typescript: {
      tsconfigDirectory: resolve('./'),
      tsconfigFile: 'tsconfig.json',
    }
  })
}

To further support loading SvelteKit, you may be interested in chaining additional loaders. If you are testing a SvelteKit app, we suggest using vitest instead.

Caveats

Svelte does not run some lifecycle events on the server under Node.js: onMount, beforeUpdate, afterUpdate.

License

MIT