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feat: support experimental.async and dynamicCompileOptions for Svelte modules #1176

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Jul 15, 2025
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5 changes: 5 additions & 0 deletions .changeset/huge-ideas-nail.md
Original file line number Diff line number Diff line change
@@ -0,0 +1,5 @@
---
'@sveltejs/vite-plugin-svelte': minor
---

feat: add support for the new experimental.async option and apply dynamicCompileOptions when compiling Svelte modules
16 changes: 14 additions & 2 deletions .github/workflows/ci.yml
Original file line number Diff line number Diff line change
Expand Up @@ -49,6 +49,8 @@ jobs:
cache-dependency-path: '**/pnpm-lock.yaml'
- name: install
run: pnpm install --frozen-lockfile --prefer-offline --ignore-scripts
- name: sync
run: pnpm -r sync # required to ensure sveltekit test project have tsconfig.json which may be required by the checks below
- name: format
run: pnpm check:format
- name: lint
Expand Down Expand Up @@ -77,21 +79,26 @@ jobs:
node: [22]
os: [ubuntu-latest, macos-latest, windows-latest]
vite: ['current']
svelte: ['current']
include:
- node: 20
os: ubuntu-latest
vite: 'current'
svelte: 'current'
- node: 24
os: ubuntu-latest
vite: 'current'
# baseline test lowest vite and node version
svelte: 'current'
# baseline test lowest svelte, vite and node version
- node: 20.19
os: ubuntu-latest
vite: 'baseline'
svelte: 'baseline'
# future test with rolldown-vite
- node: 24
os: ubuntu-latest
vite: 'rolldown-vite'
svelte: 'current'
steps:
- uses: actions/checkout@v4
- uses: actions/setup-node@v4
Expand All @@ -115,6 +122,11 @@ jobs:
run: |
pnpm update -r --no-save [email protected]
pnpm ls vite
- name: downgrade svelte to baseline
if: matrix.svelte == 'baseline'
run: |
pnpm update -r --no-save [email protected]
pnpm ls svelte
- name: update vite to rolldown-vite
if: matrix.vite == 'rolldown-vite'
run: |
Expand All @@ -134,7 +146,7 @@ jobs:
- uses: actions/upload-artifact@v4
if: failure()
with:
name: test-failure-${{github.run_id}}-${{ matrix.os }}-${{ matrix.node }}-${{ matrix.vite }}
name: test-failure-${{github.run_id}}-os_${{ matrix.os }}-node_${{ matrix.node }}-vite_${{ matrix.vite }}-svelte_${{matrix.svelte}}
path: |
test-temp.tar
pnpm-debug.log
4 changes: 2 additions & 2 deletions package.json
Original file line number Diff line number Diff line change
Expand Up @@ -28,7 +28,7 @@
"@eslint/js": "^9.31.0",
"@stylistic/eslint-plugin-js": "^4.4.1",
"@sveltejs/eslint-config": "^8.2.0",
"@sveltejs/kit": "^2.22.5",
"@sveltejs/kit": "^2.23.0",
"@svitejs/changesets-changelog-github-compact": "^1.2.0",
"@types/fs-extra": "^11.0.4",
"@types/node": "^22.16.3",
Expand All @@ -51,7 +51,7 @@
"prettier": "^3.6.2",
"prettier-plugin-svelte": "^3.4.0",
"publint": "^0.3.12",
"svelte": "^5.35.6",
"svelte": "^5.36.1",
"svelte-eslint-parser": "^1.2.0",
"typescript": "^5.8.3",
"typescript-eslint": "^8.36.0",
Expand Down
Original file line number Diff line number Diff line change
Expand Up @@ -15,6 +15,6 @@
},
"type": "module",
"dependencies": {
"svelte": "^5.35.6"
"svelte": "^5.0.0"
}
}
2 changes: 1 addition & 1 deletion packages/e2e-tests/autoprefixer-browerslist/package.json
Original file line number Diff line number Diff line change
Expand Up @@ -15,7 +15,7 @@
"autoprefixer": "^10.4.21",
"postcss": "^8.5.6",
"postcss-load-config": "^6.0.1",
"svelte": "^5.35.6",
"svelte": "^5.36.1",
"svelte-preprocess": "^6.0.3",
"vite": "^7.0.4"
},
Expand Down
2 changes: 1 addition & 1 deletion packages/e2e-tests/configfile-custom/package.json
Original file line number Diff line number Diff line change
Expand Up @@ -12,7 +12,7 @@
},
"devDependencies": {
"@sveltejs/vite-plugin-svelte": "workspace:^",
"svelte": "^5.35.6",
"svelte": "^5.36.1",
"vite": "^7.0.4"
},
"type": "module"
Expand Down
2 changes: 1 addition & 1 deletion packages/e2e-tests/configfile-esm/package.json
Original file line number Diff line number Diff line change
Expand Up @@ -12,7 +12,7 @@
},
"devDependencies": {
"@sveltejs/vite-plugin-svelte": "workspace:^",
"svelte": "^5.35.6",
"svelte": "^5.36.1",
"svelte-preprocess": "^6.0.3",
"vite": "^7.0.4"
},
Expand Down
2 changes: 1 addition & 1 deletion packages/e2e-tests/css-dev-sourcemap/package.json
Original file line number Diff line number Diff line change
Expand Up @@ -11,7 +11,7 @@
"devDependencies": {
"@sveltejs/vite-plugin-svelte": "workspace:^",
"sass": "^1.89.2",
"svelte": "^5.35.6",
"svelte": "^5.36.1",
"vite": "^7.0.4"
}
}
71 changes: 41 additions & 30 deletions packages/e2e-tests/css-treeshake/__tests__/css-treeshake.spec.ts
Original file line number Diff line number Diff line change
@@ -1,37 +1,48 @@
import { browserLogs, findAssetFile, getColor, getEl, getText, isBuild } from '~utils';
import {
browserLogs,
findAssetFile,
getColor,
getEl,
getText,
IS_SVELTE_BASELINE,
isBuild
} from '~utils';
import { expect } from 'vitest';

test('should not have failed requests', async () => {
browserLogs.forEach((msg) => {
expect(msg).not.toMatch('404');
// support for treeshaking css was added after 5.0.0
describe.skipIf(IS_SVELTE_BASELINE)('css-treeshake', () => {
test('should not have failed requests', async () => {
browserLogs.forEach((msg) => {
expect(msg).not.toMatch('404');
});
});
});

test('should apply css from used components', async () => {
expect(await getText('#app')).toBe('App');
expect(await getColor('#app')).toBe('blue');
expect(await getText('#a')).toBe('A');
expect(await getColor('#a')).toBe('red');
});

test('should apply css from unused components that contain global styles', async () => {
expect(await getEl('head style[src]'));
expect(await getColor('#test')).toBe('green'); // from B.svelte
});

test('should not render unused components', async () => {
expect(await getEl('#b')).toBeNull();
expect(await getEl('#c')).toBeNull();
});
test('should apply css from used components', async () => {
expect(await getText('#app')).toBe('App');
expect(await getColor('#app')).toBe('blue');
expect(await getText('#a')).toBe('A');
expect(await getColor('#a')).toBe('red');
});

if (isBuild) {
test('should include unscoped global styles from unused components', async () => {
const cssOutput = findAssetFile(/index-.*\.css/);
expect(cssOutput).toContain('#test{color:green}'); // from B.svelte
test('should apply css from unused components that contain global styles', async () => {
expect(await getEl('head style[src]'));
expect(await getColor('#test')).toBe('green'); // from B.svelte
});
test('should not include scoped styles from unused components', async () => {
const cssOutput = findAssetFile(/index-.*\.css/);
// from C.svelte
expect(cssOutput).not.toContain('.unused');

test('should not render unused components', async () => {
expect(await getEl('#b')).toBeNull();
expect(await getEl('#c')).toBeNull();
});
}

if (isBuild) {
test('should include unscoped global styles from unused components', async () => {
const cssOutput = findAssetFile(/index-.*\.css/);
expect(cssOutput).toContain('#test{color:green}'); // from B.svelte
});
test('should not include scoped styles from unused components', async () => {
const cssOutput = findAssetFile(/index-.*\.css/);
// from C.svelte
expect(cssOutput).not.toContain('.unused');
});
}
});
2 changes: 1 addition & 1 deletion packages/e2e-tests/css-treeshake/package.json
Original file line number Diff line number Diff line change
Expand Up @@ -11,7 +11,7 @@
"devDependencies": {
"@sveltejs/vite-plugin-svelte": "workspace:^",
"sass": "^1.89.2",
"svelte": "^5.35.6",
"svelte": "^5.36.1",
"vite": "^7.0.4"
}
}
2 changes: 1 addition & 1 deletion packages/e2e-tests/custom-extensions/package.json
Original file line number Diff line number Diff line change
Expand Up @@ -9,7 +9,7 @@
},
"devDependencies": {
"@sveltejs/vite-plugin-svelte": "workspace:^",
"svelte": "^5.35.6",
"svelte": "^5.36.1",
"vite": "^7.0.4"
},
"type": "module"
Expand Down
2 changes: 1 addition & 1 deletion packages/e2e-tests/dynamic-compile-options/package.json
Original file line number Diff line number Diff line change
Expand Up @@ -9,7 +9,7 @@
},
"devDependencies": {
"@sveltejs/vite-plugin-svelte": "workspace:^",
"svelte": "^5.35.6",
"svelte": "^5.36.1",
"vite": "^7.0.4"
},
"type": "module"
Expand Down
11 changes: 11 additions & 0 deletions packages/e2e-tests/e2e-server.js
Original file line number Diff line number Diff line change
Expand Up @@ -4,6 +4,7 @@ import treeKill from 'tree-kill';
import fs from 'node:fs';
import path from 'node:path';
import process from 'node:process';
import { rootDir } from 'vitest/node';
const isWin = process.platform === 'win32';

async function startedOnPort(serverProcess, port, timeout) {
Expand Down Expand Up @@ -77,6 +78,16 @@ export async function serve(root, isBuild, port) {
}
};

const pkg = JSON.parse(fs.readFileSync(path.join(rootDir, 'package.json'), 'utf-8'));
if (pkg.scripts?.sync) {
try {
await execa('pnpm', ['sync']);
} catch (e) {
console.error(`Failed to run sync script in ${rootDir}`);
throw e;
}
}

if (isBuild) {
let buildResult;
let hasErr = false;
Expand Down
2 changes: 1 addition & 1 deletion packages/e2e-tests/env/package.json
Original file line number Diff line number Diff line change
Expand Up @@ -9,7 +9,7 @@
},
"devDependencies": {
"@sveltejs/vite-plugin-svelte": "workspace:^",
"svelte": "^5.35.6",
"svelte": "^5.36.1",
"vite": "^7.0.4"
},
"type": "module"
Expand Down
2 changes: 1 addition & 1 deletion packages/e2e-tests/hmr/package.json
Original file line number Diff line number Diff line change
Expand Up @@ -14,7 +14,7 @@
"@sveltejs/vite-plugin-svelte": "workspace:^",
"e2e-test-dep-vite-plugins": "file:../_test_dependencies/vite-plugins",
"node-fetch": "^3.3.2",
"svelte": "^5.35.6",
"svelte": "^5.36.1",
"vite": "^7.0.4"
},
"type": "module"
Expand Down
Original file line number Diff line number Diff line change
@@ -1,10 +1,14 @@
<script lang="ts">
export let name: string;
let clicks = 0;
interface Props {
name: string;
}

let { name }: Props = $props();
let clicks = $state(0);
</script>

<button
on:click={() => {
onclick={() => {
clicks++;
}}>{name} clicks: {clicks}</button
>
Expand Down
Original file line number Diff line number Diff line change
@@ -1,10 +1,14 @@
<script lang="ts">
export let name: string;
let clicks = 0;
interface Props {
name: string;
}

let { name }: Props = $props();
let clicks = $state(0);
</script>

<button
on:click={() => {
onclick={() => {
clicks++;
}}>{name} clicks: {clicks}</button
>
Expand Down
2 changes: 1 addition & 1 deletion packages/e2e-tests/import-queries/package.json
Original file line number Diff line number Diff line change
Expand Up @@ -11,7 +11,7 @@
"devDependencies": {
"@sveltejs/vite-plugin-svelte": "workspace:^",
"sass": "^1.89.2",
"svelte": "^5.35.6",
"svelte": "^5.36.1",
"vite": "^7.0.4"
}
}
10 changes: 7 additions & 3 deletions packages/e2e-tests/import-queries/src/Dummy.svelte
Original file line number Diff line number Diff line change
@@ -1,10 +1,14 @@
<script lang="ts">
export let name: string;
let clicks = 0;
interface Props {
name: string;
}

let { name }: Props = $props();
let clicks = $state(0);
</script>

<button
on:click={() => {
onclick={() => {
clicks++;
}}>{name} clicks: {clicks}</button
>
Expand Down
2 changes: 1 addition & 1 deletion packages/e2e-tests/inspector-kit/package.json
Original file line number Diff line number Diff line change
Expand Up @@ -10,7 +10,7 @@
"devDependencies": {
"@sveltejs/kit": "^2.22.5",
"@sveltejs/vite-plugin-svelte": "workspace:^",
"svelte": "^5.35.6",
"svelte": "^5.36.1",
"vite": "^7.0.4"
},
"type": "module"
Expand Down
2 changes: 1 addition & 1 deletion packages/e2e-tests/inspector-vite/package.json
Original file line number Diff line number Diff line change
Expand Up @@ -10,7 +10,7 @@
},
"devDependencies": {
"@sveltejs/vite-plugin-svelte": "workspace:^",
"svelte": "^5.35.6",
"svelte": "^5.36.1",
"vite": "^7.0.4"
}
}
8 changes: 8 additions & 0 deletions packages/e2e-tests/kit-async/.gitignore
Original file line number Diff line number Diff line change
@@ -0,0 +1,8 @@
.DS_Store
node_modules
/build
/.svelte-kit
#.env
.env.*
!.env.example
logs
12 changes: 12 additions & 0 deletions packages/e2e-tests/kit-async/__tests__/kit-async.spec.ts
Original file line number Diff line number Diff line change
@@ -0,0 +1,12 @@
import { describe, expect, it } from 'vitest';
import { getText, IS_SVELTE_BASELINE, sleep } from '~utils';

describe.skipIf(IS_SVELTE_BASELINE)('kit-async', async () => {
it('works', async () => {
expect(await getText('h1')).toBe('Hello async world!');
expect(await getText('span')).toBe('Wait for it ...');
// TODO ideally this would wait for the actual async settled but that requires a new util
await sleep(700); // the async derived resolves after 500ms
expect(await getText('span')).toBe('foo');
});
});
15 changes: 15 additions & 0 deletions packages/e2e-tests/kit-async/jsconfig.json
Original file line number Diff line number Diff line change
@@ -0,0 +1,15 @@
{
"extends": "./.svelte-kit/tsconfig.json",
"compilerOptions": {
"allowJs": true,
"checkJs": true,
"esModuleInterop": true,
"forceConsistentCasingInFileNames": true,
"resolveJsonModule": true,
"skipLibCheck": true,
"sourceMap": true,
"strict": true,
"moduleResolution": "bundler"
},
"exclude": ["src/routes/+page.svelte"]
}
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