Svelte

Svelte

CSS/SASS

Svelte supports writing CSS directly in a component within <style> tags, as well as writing compiled-to-CSS languages like SASS within <style lang="scss"> tags.

CSS written this way is automatically scoped to the component: read more in the Svelte docs.

This SASS to CSS compilation is handled by svelte-preprocess, and the resulting CSS is handed to Wepback for further processing. Since Webpack isn't involved in SASS compilation within Svelte components, care must be taken in a few areas:

1. Because Webpack isn't handling @use and @import resolution, it's not possible to use its aliases to reference paths to other SASS files. Instead paths relative to the Svelte component, or full node module paths, must be used. This doesn't include paths not processed by the SASS compiler, such as paths to images in url() functions, as these are still handled by Webpack. To illustrate:

@use "@mozilla-protocol/core/protocol/css/includes/lib" as p;
@use "../../kitsune/sumo/static/sumo/scss/config/typography-mixins";
// ^ here we must use a full or relative path, as svelte-preprocess is resolving this

div {
    background: url("protocol/img/icons/reader-mode.svg");
    // ^ here we can use a webpack alias, as webpack is resolving this
    background-size: p.$spacing-md;
    @include typography-mixins.text-display-sm;
}

1. As a knock-on effect of the above, any partials used in both our main CSS bundle, as well as a Svelte component (like the typography-mixins above) must not use Webpack aliases in their own imports, otherwise svelte-preprocess won't be able to resolve them.

2. Partials should be careful to not accidentally include or import CSS blocks outside of mixin defintions. This is because neither svelte-preprocess nor the Webpack sass-loader are able to chunk split @imports and @uses, or even de-duplicate their use across Svelte components (due to the scoped nature of the CSS within). Not doing this will lead to unnecessarily duplicated code.

Pre-rendering

To pre-render Svelte components a two-step process is required.

First the components must pass through the Svelte compiler, with the appropriate flags enabled to compile components for server-side rendering (SSR). Then those compiled components must be rendered into static HTML. We do both these steps in Webpack using the webpack.pre-render.js config file.

In order to pre-render a route, add it to the config object passed to svelte-pre-render-plugin in webpack.pre-render.js, referencing the entrypoint containing the component you want to render:

entry: {
    foobar: "./svelte/SomeComponent",
    barfoo: "./svelte/AnotherComponent",
},
plugins: [
    new SveltePreRenderPlugin({
        "foobar.js": [
            "/route-1",
            "/route-2",
            "/route-2/subroute",
        ],
        "barfoo.js": [
            "/route-3",
        ],
    }),
],