🌐 svelte-web-components-template

A base template for building a shareable web components library using Vite, Svelte and TypeScript.

This templates generates vanilla web components than can be used with plain HTML or within any major frameworks, such as React, Angular, Vue or Svelte (see compatibility).

How to use this template

You can directly create a new GitHub repo from this template by selecting the Use this template button on GitHub.

You can also clone it locally with the following commands:

npx degit sinedied/svelte-web-components-template#main my-component-lib
cd my-component-lib
npm install

Your components source code lives in lib/ folder. Only components with the .wc.svelte extension will be exported as web components and available in your library. This means that you can also use regular Svelte components with the .svelte extension as child components for your implementation details.

You can add additional components by adding them to the lib folder and editing lib/index.js.

Testing your components

You can start a development server with:

npm start

Then open your browser to localhost:5173.

This will build the demo application located in the demo/ folder, in which you can use and test your web components during development.

If you need unit tests, you can take a look at Jest and Jest testing library.

Using the built web components with the demo app

The demo application is provided for development and testing of your components, that's why it imports the .svelte files from the lib/ folder directly by default.

If you prefer, you can import the built web components from the dist/ folder instead, by editing demo/src/App.svelte and replacing the import '../../lib'; statement with import '../../../dist/lib'; if you have the bundleComponents option enabled, or individually import your components with import import '../../dist/MyComponent.wc.js'; otherwise.

You'll also have to make sure to run the npm run build script to generate the dist/lib/ folder first.

Building the library

The command npm run build will create the web components library in the dist/lib/ folder. It creates both an ES module (dist/lib/<your-lib>.js) suitable for bundler (non-minified), a minified ES module (dist/lib/<your-lib>.min.js) and a regular UMD script (dist/lib/<your-lib>.umd.js).

The build is automatically called when executing npm publish to distribute your library, thanks to the prepublishOnly script entry in package.json.

Notes and limitations

This template does not provide any web components polyfills for older browsers support. It's usually best to leave that task to the host application, hence why they're left out.

Props

Any props accepted by your web component are automatically transformed to element attributes. Since camelCase or PascalCase does not work in HTML, you have to make sure to name your props in lowercase.

<script>
  export let myvalue = "Default";
</script>

Events

The Svelte syntax event for listening to events like on:myevent doesn't work with events dispatched from a Svelte web component (#3119).

You need to use a workaround for that, by creating a CustomEvent and dispatching it.

Here's an example:

// MyComponent.wc.svelte
<svelte:options tag="my-component" />
<script>
  import { get_current_component } from "svelte/internal";
  const component = get_current_component();
  
  // example function for dispatching events
  const dispatchEvent = (name, detail) =>
    component.dispatchEvent(new CustomEvent(name, { detail }));
</script>
<button on:click={() => dispatchEvent("test", "Hello!")}>
  Click to dispatch event
</button>

Building each component into its own module

By default this template will build all components into a single module.

If you prefer to build each component into its own module, you can do so by setting the environment variable BUNDLE_COMPONENTS to false, or editing vite.config.js and setting the bundleComponents option to false.

Then you also need to replace the content of packages/lib/index.ts with:

export default () => {
  import('./MyComponent.wc.svelte');
  // Import each of your component this way
};

This will enable code-splitting and will generate an ES module for each component in the dist/lib/ folder.

As you changed the way components are exported, you also need to replace the import '../../lib'; statement in demo/src/App.svelte to import '../../lib/MyComponent.wc.svelte';.

Why enable allowJs in the TS template?

While allowJs: false would indeed prevent the use of .js files in the project, it does not prevent the use of JavaScript syntax in .svelte files. In addition, it would force checkJs: false, bringing the worst of both worlds: not being able to guarantee the entire codebase is TypeScript, and also having worse typechecking for the existing JavaScript. In addition, there are valid use cases in which a mixed codebase may be relevant.

Why is HMR not preserving my local component state?

HMR state preservation comes with a number of gotchas! It has been disabled by default in both svelte-hmr and @sveltejs/vite-plugin-svelte due to its often surprising behavior. You can read the details here.

If you have state that's important to retain within a component, consider creating an external store which would not be replaced by HMR.

// store.ts
// An extremely simple external store
import { writable } from 'svelte/store'
export default writable(0)

svelte-webcomponents