Rollup-plugin-svelte-hot NPM

rollup-plugin-svelte-hot

This is a fork of rollup-plugin-svelte that integrates rollup-plugin-svelte-hmr to compile Svelte components and provide support for HMR over Nollup.

rollup-plugin-svelte only compiles Svelte components
rollup-plugin-svelte-hmr only adds HMR support for Svelte components
rollup-plugin-svelte-hot (this plugin) combines both functionality

The main reason why this plugin exists is to add support to preserve local component state over HMR updates. Svelte makes it possible (even easy), but the HMR-only plugin can't currently leverage it because it requires a change in Svelte dev API. This plugin is able to workaround this limitation because it has access to Svelte compiler's output (that contains useful metadata about components).

This plugin can be used as a drop-in replacement for rollup-plugin-svelte. It adds one additional option to enable HMR: hot.

Quick start / Template

Use svelte-template-hot to quickly bootstrap a new project, or as a reference to add this to your project (more detailed instructions to come).

Installation

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

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

You'll also need Nollup, Rollup, etc...

Usage

// rollup.config.js
import * as fs from 'fs';
import svelte from 'rollup-plugin-svelte-hot';

export default {
  input: 'src/main.js',
  output: {
    file: 'public/bundle.js',
    format: 'iife'
  },
  plugins: [
    svelte({
      // Use `hot: true` to use default options (as follow)
      hot: {
        // Prevent preserving local component state
        noPreserveState: false,
        // Prevent doing a full reload on next HMR update after fatal error
        noReload: false,
        // Try to recover after runtime errors in component init
        optimistic: false
      },

      // By default, all .svelte and .html 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',

      // 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,

      // Extract CSS into a separate 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` — pass `false`
        // as the second argument if you don't want the sourcemap
        css.write('public/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.