Making web components in svelte 4

I came across this BBC article where they have a scrolly inside a webpage and looking through the DOM, they are using a <template> and using the template as a web-component which then inserts stuff onto a shadow-root. Here’s a good MDN article about templates and slots. I first heard about web-components when datawrapper said they had a new way of embedding charts using web-components.

When we tried to do a scrolly embedded in an article using an iframe, the page and iframe scrolly separately so was a bit janky and quite an unsatisfying experience. Since we use Svelte as our main framework, I did some exploring and found Svelte can compile components to web components.

There is a way of using the old svelte template and rollup to generate web-components that can be imported as standalone .js files (See phptuts or logrocket for a walkthrough) but this version uses vite and svelte 4.

The how

Svelte can set up projects to be like a component library using npm create svelte@latest and this is the recommended way from comments.

For library projects, your interesting stuff is in lib and then how it works is in routes. I’ve created a svelte component inside lib and added this line to the to the .svelte file.

<svelte:options customElement="count-er> 

Also added compilerOptions to svelte.config.js


When you run npm run package, this creates files in a dist folder.

Following instructions from this recipe you can run vite again on the dist/index.js to create the standalone iife .js files. See the vite.webcomponents.config.js and the modified build command in the package.json file. These standalone files can be then loaded from a webpage and then the web-component can be used. See the test-page folder for an example.

The finish result

Here’s my github repo for svelte 4 web components. I used this template as a good starting point and followed this recipe which I found from a stackoverflow answer.