rollup-plugin-svelte

Compile Svelte components.

Installation

npm install --save-dev svelte rollup-plugin-svelte

Note that we need to install Svelte as well as the plugin, as it's a 'peer dependency'.

Usage

// rollup.config.js
import svelte from 'rollup-plugin-svelte';

export default {
  input: 'src/main.js',
  output: {
    file: 'public/bundle.js',
    format: 'iife'
  },
  plugins: [
    svelte({
      // By default, all ".svelte" files are compiled
      extensions: ['.my-custom-extension'],

      // You can restrict which files are compiled
      // using `include` and `exclude`
      include: 'src/components/**/*.svelte',

      // By default, the client-side compiler is used. You
      // can also use the server-side rendering compiler
      generate: 'ssr',

      // ensure that extra attributes are added to head
      // elements for hydration (used with generate: 'ssr')
      hydratable: true,

      // Optionally, preprocess components with svelte.preprocess:
      // https://svelte.dev/docs#svelte_preprocess
      preprocess: {
        style: ({ content }) => {
          return transformStyles(content);
        }
      },

      // Emit CSS as "files" for other plugins to process
      emitCss: true,

      // You can optionally set 'customElement' to 'true' to compile
      // your components to custom elements (aka web elements)
      customElement: false,

      // Extract CSS into a single bundled file (recommended).
      // See note below
      css: function (css) {
        console.log(css.code); // the concatenated CSS
        console.log(css.map); // a sourcemap

        // creates `main.css` and `main.css.map`
        // using a falsy name will default to the bundle name
        // — pass `false` as the second argument if you don't want the sourcemap
        css.write('main.css');
      },

      // Warnings are normally passed straight to Rollup. You can
      // optionally handle them here, for example to squelch
      // warnings with a particular code
      onwarn: (warning, handler) => {
        // e.g. don't warn on <marquee> elements, cos they're cool
        if (warning.code === 'a11y-distracting-elements') return;

        // let Rollup handle all other warnings normally
        handler(warning);
      }
    })
  ]
}

Preprocessing and dependencies

If you are using the preprocess feature, then your callback responses may — in addition to the code and map values described in the Svelte compile docs — also optionally include a dependencies array. This should be the paths of additional files that the preprocessor result in some way depends upon. In Rollup 0.61+ in watch mode, any changes to these additional files will also trigger re-builds.

pkg.svelte

If you're importing a component from your node_modules folder, and that component's package.json has a "svelte" property...

{
  "name": "some-component",

  // this means 'some-component' resolves to 'some-component/src/SomeComponent.svelte'
  "svelte": "src/MyComponent.svelte"
}

...then this plugin will ensure that your app imports the uncompiled component source code. That will result in a smaller, faster app (because code is deduplicated, and shared functions get optimized quicker), and makes it less likely that you'll run into bugs caused by your app using a different version of Svelte to the component.

Conversely, if you're publishing a component to npm, you should ship the uncompiled source (together with the compiled distributable, for people who aren't using Svelte elsewhere in their app) and include the "svelte" property in your package.json.

If you are publishing a package containing multiple components, you can create an index.js file that re-exports all the components, like this:

export { default as Component1 } from './Component1.svelte';
export { default as Component2 } from './Component2.svelte';

and so on. Then, in package.json, set the svelte property to point to this index.js file.

Extracting CSS

If your Svelte components contain <style> tags, by default the compiler will add JavaScript that injects those styles into the page when the component is rendered. That's not ideal, because it adds weight to your JavaScript, prevents styles from being fetched in parallel with your code, and can even cause CSP violations.

A better option is to extract the CSS into a separate file. Using the css option as shown above would cause a public/main.css file to be generated each time the bundle is built (or rebuilt, if you're using rollup-watch), with the normal scoping rules applied.

If you have other plugins processing your CSS (e.g. rollup-plugin-scss), and want your styles passed through to them to be bundled together, you can use emitCss: true.

Alternatively, if you're handling styles in some other way and just want to prevent the CSS being added to your JavaScript bundle, use css: false.

License

MIT