carbon-preprocess-svelte

A zero-dependency library providing Svelte preprocessors and build plugins for the Carbon Design System.

Installation

Install carbon-preprocess-svelte as a development dependency.

# npm
npm i -D carbon-preprocess-svelte

# pnpm
pnpm i -D carbon-preprocess-svelte

# Yarn
yarn add -D carbon-preprocess-svelte

# Bun
bun add -D carbon-preprocess-svelte

Usage

• optimizeImports: Svelte preprocessor that rewrites Carbon Svelte imports to their source path in the script block, making development compile times dramatically faster.
• optimizeCss: Vite/Rollup plugin that removes unused Carbon styles, resulting in smaller CSS bundles.
• OptimizeCssPlugin: The corresponding optimizeCss plugin for Webpack that removes unused Carbon styles.

optimizeImports

optimizeImports is a Svelte preprocessor that rewrites barrel imports from Carbon components/icons/pictograms packages to their source Svelte code paths. This can significantly speed up development and build compile times while preserving typeahead and autocompletion offered by integrated development environments (IDE) like VS Code.

The preprocessor optimizes imports from the following packages:

• carbon-components-svelte
• carbon-icons-svelte
• carbon-pictograms-svelte

- import { Button } from "carbon-components-svelte";
+ import Button from "carbon-components-svelte/src/Button/Button.svelte";

- import { Add } from "carbon-icons-svelte";
+ import Add from "carbon-icons-svelte/lib/Add.svelte";

- import { Airplane } from "carbon-pictograms-svelte";
+ import Airplane from "carbon-pictograms-svelte/lib/Airplane.svelte";

[!NOTE] When this preprocessor was first created, there was no workaround to optimize slow cold start times with Vite in development. Today, @sveltejs/vite-plugin-svelte enables prebundleSvelteLibraries: true by default. However, this preprocessor is still useful for non-Vite bundlers, like Rollup and Webpack. Also, it can further improve cold start development times even with prebundleSvelteLibraries: true.

SvelteKit

See examples/sveltekit.

// svelte.config.js
import adapter from "@sveltejs/adapter-static";
import { vitePreprocess } from "@sveltejs/vite-plugin-svelte";
import { optimizeImports } from "carbon-preprocess-svelte";

/** @type {import('@sveltejs/kit').Config} */
const config = {
  preprocess: [
    // Preprocessors are run in sequence.
    // If using TypeScript, the code must be transpiled first.
    vitePreprocess(),
    optimizeImports(),
  ],
  kit: {
    adapter: adapter(),
  },
};

export default config;

Vite

See examples/vite.

// vite.config.js
import { svelte } from "@sveltejs/vite-plugin-svelte";
import { vitePreprocess } from "@sveltejs/vite-plugin-svelte";
import { optimizeImports } from "carbon-preprocess-svelte";

/** @type {import('vite').UserConfig} */
export default {
  plugins: [
    svelte({
      preprocess: [
        // Preprocessors are run in sequence.
        // If using TypeScript, the code must be transpiled first.
        vitePreprocess(),
        optimizeImports(),
      ],
    }),
  ],
};

Rollup

This code is abridged; see examples/rollup for a full set-up.

// rollup.config.js
import svelte from "rollup-plugin-svelte";
import { optimizeImports } from "carbon-preprocess-svelte";

export default {
  plugins: [
    svelte({
      preprocess: [optimizeImports()],
    }),
  ],
};

Webpack

This code is abridged; see examples/webpack for a full set-up.

// webpack.config.mjs
import { optimizeImports } from "carbon-preprocess-svelte";

export default {
  module: {
    rules: [
      {
        test: /\.svelte$/,
        use: {
          loader: "svelte-loader",
          options: {
            hotReload: !PROD,
            preprocess: [optimizeImports()],
            compilerOptions: { dev: !PROD },
          },
        },
      },
    ],
  },
};

optimizeCss

optimizeCss is a Vite plugin that removes unused Carbon styles at build time. The plugin is compatible with Rollup (Vite extends the Rollup plugin API).

The plugin uses apply: "build" and enforce: "post", so it runs only on production builds and after other plugins.

1. During transform, it collects absolute paths of imported carbon-components-svelte sources.
2. During generateBundle, for each emitted CSS file it builds an allowlist of every bx-- class tied to those components via an internal index, plus global selectors like .bx--body.
3. A PostCSS plugin removes rules whose selectors are only Carbon (bx--) classes outside that allowlist. BEM-style variants are kept when they match a needed base class; selectors without that prefix are left unchanged.
4. Empty rules are discarded, and the CSS bundles are optimized.

flowchart TB
  subgraph scan["Module scan"]
    T[transform hook] --> S["Collect imported Carbon<br/>component paths"]
  end
  subgraph emit["Bundle phase"]
    S --> G[generateBundle]
    G --> A["Allowlist bx-- selectors<br/>(index + .bx--body)"]
    A --> P[Prune unused Carbon styles with PostCSS]
    P --> R[Optimize CSS assets]
  end

  class T,G hook
  class S,A data
  class P,R css

$ vite build

Optimized index-CU4gbKFa.css
- Before: 606.26 kB
+ After:   53.22 kB (-91.22%)

dist/index.html                  0.34 kB │ gzip:  0.24 kB
dist/assets/index-CU4gbKFa.css  53.22 kB │ gzip:  6.91 kB
dist/assets/index-Ceijs3eO.js   53.65 kB │ gzip: 15.88 kB

[!NOTE] This is a plugin and not a Svelte preprocessor. It should be added to the list of vite.plugins. For Vite set-ups, this plugin is not run during development and is only executed when building the app (i.e., vite build). For Rollup and Webpack, you should conditionally apply the plugin to only execute when building for production.

SvelteKit

See examples/sveltekit.

// vite.config.js
import { sveltekit } from "@sveltejs/kit/vite";
import { optimizeCss } from "carbon-preprocess-svelte";
import { defineConfig } from "vite";

export default defineConfig({
  plugins: [sveltekit(), optimizeCss()],
});

Vite

See examples/vite.

// vite.config.js
import { svelte } from "@sveltejs/vite-plugin-svelte";
import { optimizeCss } from "carbon-preprocess-svelte";

/** @type {import('vite').UserConfig} */
export default {
  plugins: [svelte(), optimizeCss()],
};

Rollup

This code is abridged; see examples/rollup for a full set-up.

// rollup.config.js
import svelte from "rollup-plugin-svelte";
import { optimizeCss } from "carbon-preprocess-svelte";

const production = !process.env.ROLLUP_WATCH;

export default {
  plugins: [
    svelte({
      preprocess: [optimizeImports()],
    }),

    // Only apply the plugin when building for production.
    production && optimizeCss(),
  ],
};

optimizeCss API

optimizeCss({
  /**
   * Set to `true` to suppress the size difference
   * logging between original and optimized CSS.
   * @default false
   */
  silent: true,

  /**
   * By default, pre-compiled Carbon StyleSheets ship `@font-face` rules
   * for all available IBM Plex fonts, many of which are not actually
   * used in Carbon Svelte components.
   *
   * The default behavior is to preserve the following IBM Plex fonts:
   * - IBM Plex Sans (300/400/600-weight and normal-font-style rules)
   * - IBM Plex Mono (400-weight and normal-font-style rules)
   *
   * Set to `true` to disable this behavior and
   * retain *all* IBM Plex `@font-face` rules.
   * @default false
   */
  preserveAllIBMFonts: true,

  /**
   * Experimental. Enables stricter CSS tree-shaking that can drastically
   * reduce output size compared to the default baseline, depending on which
   * Carbon components you import. Small bundles that only use a handful of
   * components tend to see the largest gains.
   *
   * Compared to the default matcher, `strict`:
   * - Prunes individual selectors from comma-separated lists instead of
   *   keeping the entire rule when any selector matches
   * - Requires every Carbon class in a compound selector to match when only
   *   shared modifiers (e.g. `.bx--skeleton`) hit, so importing Button no
   *   longer pulls in Tabs skeleton styles
   * - Drops flatpickr and legacy single-hyphen `bx-` rules unless DatePicker
   *   (or similar) is in the bundle
   * - Uses parenthesis-aware selector parsing for `:is()` and similar
   *
   * @default false
   */
  experimental: {
    strict: true,
  },
});

OptimizeCssPlugin

For Webpack users, OptimizeCssPlugin is a drop-in replacement for optimizeCss. The plugin API is identical to that of optimizeCss. Similarly, the plugin only runs in production mode.

This code is abridged; see examples/webpack or examples/webpack@svelte-5 for a full set-up.

// webpack.config.mjs
import { OptimizeCssPlugin } from "carbon-preprocess-svelte";

export default {
  plugins: [new OptimizeCssPlugin()],
};

Examples

Refer to examples for common set-ups.

Contributing

Refer to the contributing guidelines.

License

Apache 2.0