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feat: scope css to js module to allow treeshaking it (requires vite 6.2) (#1092)
* feat: scope css to js module to allow treeshaking it (requires vite 6.2) * chore: improve condition and add comment * refactor: replace zimmerframe+walk with regex test in preprocessor * chore: remove extraneous check * refactor: use svelte 5.26 hasGlobal flag instead of custom impl * chore: revert code change from previous impl * chore: update changeset
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‎.changeset/plenty-eyes-talk.md

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---
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'@sveltejs/vite-plugin-svelte': minor
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---
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scope css to js module to enable treeshaking scoped css from unused components. Requires vite 6.2 and svelte 5.26

‎packages/e2e-tests/css-treeshake/__tests__/css-treeshake.spec.ts

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import { browserLogs, findAssetFile, getColor, getEl, getText, isBuild } from '~utils';
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import { expect } from 'vitest';
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test('should not have failed requests', async () => {
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browserLogs.forEach((msg) => {
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expect(msg).not.toMatch('404');
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});
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});
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test('should apply css from used components', async () => {
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expect(await getText('#app')).toBe('App');
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expect(await getColor('#app')).toBe('blue');
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expect(await getText('#a')).toBe('A');
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expect(await getColor('#a')).toBe('red');
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});
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test('should apply css from unused components that contain global styles', async () => {
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expect(await getEl('head style[src]'));
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expect(await getColor('#test')).toBe('green'); // from B.svelte
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});
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test('should not render unused components', async () => {
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expect(await getEl('#b')).toBeNull();
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expect(await getEl('#c')).toBeNull();
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});
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if (isBuild) {
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test('should include unscoped global styles from unused components', async () => {
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const cssOutput = findAssetFile(/index-.*\.css/);
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expect(cssOutput).toContain('#test{color:green}'); // from B.svelte
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});
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test('should not include scoped styles from unused components', async () => {
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const cssOutput = findAssetFile(/index-.*\.css/);
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// from C.svelte
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expect(cssOutput).not.toContain('.unused');
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});
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}

‎packages/e2e-tests/css-treeshake/index.html

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<!doctype html>
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<html lang="en">
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<head>
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<meta charset="utf-8" />
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<meta name="viewport" content="width=device-width,initial-scale=1" />
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<title>Svelte app</title>
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<script type="module" src="/src/main.js"></script>
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</head>
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<body></body>
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</html>

‎packages/e2e-tests/css-treeshake/package.json

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{
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"name": "e2e-tests-css-treeshake",
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"private": true,
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"version": "0.0.0",
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"type": "module",
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"scripts": {
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"dev": "vite",
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"build": "vite build",
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"preview": "vite preview"
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},
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"devDependencies": {
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"@sveltejs/vite-plugin-svelte": "workspace:^",
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"sass": "^1.85.1",
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"svelte": "^5.20.5",
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"vite": "^6.2.0"
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}
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}

‎packages/e2e-tests/css-treeshake/src/A.svelte

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<h1 id="a">A</h1>
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<style>
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h1 {
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color: red;
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}
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</style>

‎packages/e2e-tests/css-treeshake/src/App.svelte

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<script>
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import { A } from './barrel.js';
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</script>
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<div id="test">test</div>
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<h1 id="app">App</h1>
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<A />
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<style>
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#app {
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color: blue;
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}
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</style>

‎packages/e2e-tests/css-treeshake/src/B.svelte

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<h1 id="b">B</h1>
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<style>
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h1 {
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color: green;
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}
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:global(#test) {
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color: green;
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}
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</style>

‎packages/e2e-tests/css-treeshake/src/C.svelte

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<h1 id="c" class="unused"><strong>C</strong></h1>
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<style>
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.unused {
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color: magenta;
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}
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h1 :global {
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background: blue;
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}
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h1 :global(strong) {
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color: magenta;
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}
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</style>

‎packages/e2e-tests/css-treeshake/src/barrel.js

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export { default as A } from './A.svelte';
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// B and C are unused, their css should not be included
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export { default as B } from './B.svelte';
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export { default as C } from './C.svelte';

‎packages/e2e-tests/css-treeshake/src/main.js

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import App from './App.svelte';
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import { mount } from 'svelte';
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mount(App, { target: document.body });

‎packages/e2e-tests/css-treeshake/src/vite-env.d.ts

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/// <reference types="svelte" />
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/// <reference types="vite/client" />

‎packages/e2e-tests/css-treeshake/svelte.config.js

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import { vitePreprocess } from '@sveltejs/vite-plugin-svelte';
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export default {
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preprocess: [vitePreprocess()]
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};

‎packages/e2e-tests/css-treeshake/vite.config.js

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import { defineConfig } from 'vite';
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import { svelte } from '@sveltejs/vite-plugin-svelte';
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import { env } from 'node:process';
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// https://vitejs.dev/config/
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export default defineConfig({
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plugins: [svelte()]
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});

‎packages/vite-plugin-svelte/src/index.js

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};
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} else {
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if (query.svelte && query.type === 'style') {
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const css = cache.getCSS(svelteRequest);
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if (css) {
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const cachedCss = cache.getCSS(svelteRequest);
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if (cachedCss) {
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const { hasGlobal, ...css } = cachedCss;
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if (hasGlobal === false) {
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// hasGlobal was added in svelte 5.26.0, so make sure it is boolean false
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css.meta ??= {};
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css.meta.vite ??= {};
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css.meta.vite.cssScopeTo = [svelteRequest.filename, 'default'];
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}
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return css;
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}
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}

‎packages/vite-plugin-svelte/src/types/compile.d.ts

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import type { Processed, CompileResult } from 'svelte/compiler';
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import type { SvelteRequest } from './id.d.ts';
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import type { ResolvedOptions } from './options.d.ts';
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import type { CustomPluginOptionsVite } from 'vite';
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export type CompileSvelte = (
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svelteRequest: SvelteRequest,
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code: string;
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map?: any;
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dependencies?: any[];
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hasGlobal?: boolean;
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meta?: {
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vite?: CustomPluginOptionsVite;
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};
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}
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export interface CompileData {

‎packages/vite-plugin-svelte/src/utils/compile.js

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import * as svelte from 'svelte/compiler';
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import { safeBase64Hash } from './hash.js';
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import { log } from './log.js';
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let compiled;
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try {
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compiled = svelte.compile(finalCode, { ...finalCompileOptions, filename });
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// patch output with partial accept until svelte does it
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// TODO remove later
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if (

‎pnpm-lock.yaml

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