Svelte

Svelte 5 + Astro Guide (v5.53+)

Runes API

$state

<script>
  let count = $state(0);
  let items = $state([{ text: "hello" }]); // deep reactive proxy
</script>
<button => count++}>{count}</button>

$state.raw() — 無深層 proxy，需整個重新賦值：

let data = $state.raw({ big: "object" });
data = { big: "new" }; // 觸發更新

$state.snapshot() — 提取 plain object（給外部 lib 用）。

$derived

let doubled = $derived(count * 2);
let total = $derived.by(() => items.reduce((s, i) => s + i.value, 0));

$effect

$effect(() => { console.log(count); }); // 自動追蹤依賴
$effect.pre(() => { /* beforeUpdate */ });

$props

<script lang="ts">
  let { name, age = 25, onchange }: {
    name: string;
    age?: number;
    onchange?: (val: string) => void;
  } = $props();
</script>

事件處理（Svelte 5）

不用 on: — 直接用 HTML 事件屬性：

<button
<button => { e.preventDefault(); handler(e); }}>Click</button>

元件事件 = callback props：

<!-- Parent -->
<Child => console.log(msg)} />
<!-- Child -->
<script> let { onmessage } = $props(); </script>
<button => onmessage?.("hello")}>Send</button>

Snippets（取代 Slots）

<!-- Parent -->
<Card>
  {#snippet header()}<h2>Title</h2>{/snippet}
  <p>Body (children)</p>
</Card>

<!-- Card.svelte -->
<script> let { header, children } = $props(); </script>
{@render header?.()}
{@render children()}

Astro 整合

// astro.config.mjs
import svelte from "@astrojs/svelte";
export default defineConfig({ integrations: [svelte()] });

<Counter client:load />      <!-- 立即 hydrate -->
<Chart client:visible />     <!-- 進入視窗才 hydrate -->
<Widget client:idle />       <!-- 瀏覽器 idle 時 hydrate -->
<App client:only="svelte" /> <!-- 純 client-side -->

不加 client:* = 靜態 HTML，零 JS。

傳資料給 Svelte

<Chart client:visible items={data} title="Sales" />

只接受可序列化型別。不能傳 function。

跨 island 共享狀態（Nanostores）

npm install nanostores @nanostores/svelte

// stores/cart.ts
import { atom } from "nanostores";
export const isOpen = atom(false);

<script>
  import { isOpen } from "../stores/cart";
</script>
<button => isOpen.set(true)}>Open ({$isOpen})</button>

測試（Vitest + Testing Library）

npm install -D @testing-library/svelte @testing-library/jest-dom @testing-library/user-event

vitest.config.ts

import { svelte } from "@sveltejs/vite-plugin-svelte";
import { svelteTesting } from "@testing-library/svelte/vite";
import { defineConfig } from "vitest/config";

export default defineConfig({
  plugins: [svelte(), svelteTesting()],
  test: {
    environment: "jsdom",
    setupFiles: ["./vitest-setup.ts"],
  },
});

svelteTesting() 自動做兩件事：browser resolve condition + afterEach cleanup。可個別關閉：

svelteTesting({ autoCleanup: false, resolveBrowser: false })

vitest-setup.ts

import "@testing-library/jest-dom/vitest";

tsconfig.json 加上：

{ "compilerOptions": { "types": ["@testing-library/jest-dom"] } }

基本模式：render + props + user interaction

import { render, screen } from "@testing-library/svelte";
import userEvent from "@testing-library/user-event";
import Counter from "./Counter.svelte";

test("increments on click", async () => {
  const user = userEvent.setup();
  render(Counter, { initial: 5 });
  expect(screen.getByRole("button")).toHaveTextContent("5");
  await user.click(screen.getByRole("button"));
  expect(screen.getByRole("button")).toHaveTextContent("6");
});

props 可直接攤平傳入，也可用 { props: { ... } } 形式：

render(MyComponent, { name: "Alice" });
// 等同
render(MyComponent, { props: { name: "Alice" } });

更新 props（rerender）

test("reacts to prop changes", async () => {
  const { rerender } = render(Greeting, { name: "Alice" });
  expect(screen.getByText("Hello Alice")).toBeInTheDocument();
  await rerender({ name: "Bob" });
  expect(screen.getByText("Hello Bob")).toBeInTheDocument();
});

rerender() 內部用 Object.assign 更新 $state 化的 props，再 await tick()。

測試 $state + $derived

元件內的 $state / $derived 不需特殊處理 — 透過 DOM 驗證即可：

<!-- PriceCalc.svelte -->
<script lang="ts">
  let { price, taxRate = 0.1 }: { price: number; taxRate?: number } = $props();
  let quantity = $state(1);
  let total = $derived(price * quantity * (1 + taxRate));
</script>
<input type="number" aria-label="quantity" bind:value={quantity} />
<p data-testid="total">{total}</p>

test("derived total updates when quantity changes", async () => {
  const user = userEvent.setup();
  render(PriceCalc, { price: 100 });

  expect(screen.getByTestId("total")).toHaveTextContent("110"); // 100*1*1.1

  const input = screen.getByLabelText("quantity");
  await user.clear(input);
  await user.type(input, "3");

  expect(screen.getByTestId("total")).toHaveTextContent("330"); // 100*3*1.1
});

測試 $effect（async data loading）

$effect 中的非同步操作需搭配 waitFor 或 findBy*：

<!-- UserProfile.svelte -->
<script lang="ts">
  import { fetchUser } from "./api";
  let { userId }: { userId: string } = $props();
  let user = $state<{ name: string } | null>(null);
  let loading = $state(true);

  $effect(() => {
    loading = true;
    fetchUser(userId).then((u) => {
      user = u;
      loading = false;
    });
  });
</script>
{#if loading}<p>Loading...</p>{/if}
{#if user}<p>{user.name}</p>{/if}

import { render, screen, waitFor } from "@testing-library/svelte";
import { vi } from "vitest";

// Mock 模組
vi.mock("./api", () => ({
  fetchUser: vi.fn(),
}));

import { fetchUser } from "./api";
import UserProfile from "./UserProfile.svelte";

test("loads and displays user", async () => {
  vi.mocked(fetchUser).mockResolvedValue({ name: "Alice" });

  render(UserProfile, { userId: "1" });
  expect(screen.getByText("Loading...")).toBeInTheDocument();

  // findBy* 自帶 waitFor，等待元素出現
  expect(await screen.findByText("Alice")).toBeInTheDocument();
});

Mock imports（vi.mock）

// Mock 整個模組
vi.mock("./api", () => ({
  fetchUsers: vi.fn().mockResolvedValue([]),
}));

// Mock default export（如 Svelte component）
vi.mock("./HeavyChart.svelte", () => ({
  default: { /* mock component */ },
}));

// Mock 部分匯出，保留其餘
vi.mock("./utils", async (importOriginal) => {
  const actual = await importOriginal<typeof import("./utils")>();
  return { ...actual, expensiveCalc: vi.fn().mockReturnValue(42) };
});

每次測試後記得清理：

afterEach(() => {
  vi.restoreAllMocks();
});

測試 {#each} 迴圈 + 過濾列表（保留原始排名）

常見場景：列表有 filter 但顯示的 rank 需保持原始位置。

<!-- Leaderboard.svelte -->
<script lang="ts">
  let { users }: { users: { rank: number; name: string; score: number }[] } = $props();
  let filter = $state("");
  let filtered = $derived(
    users.filter((u) => u.name.toLowerCase().includes(filter.toLowerCase()))
  );
</script>
<input aria-label="filter" bind:value={filter} />
<table>
  <tbody>
    {#each filtered as user (user.rank)}
      <tr><td>{user.rank}</td><td>{user.name}</td><td>{user.score}</td></tr>
    {/each}
  </tbody>
</table>

test("filtering preserves original rank", async () => {
  const user = userEvent.setup();
  const users = [
    { rank: 1, name: "Alice", score: 100 },
    { rank: 2, name: "Bob", score: 90 },
    { rank: 3, name: "Alicia", score: 80 },
  ];
  render(Leaderboard, { users });

  // 初始 3 列
  expect(screen.getAllByRole("row")).toHaveLength(3);

  // 輸入 filter
  await user.type(screen.getByLabelText("filter"), "Ali");

  // 剩 2 列，但 rank 保持原始值
  const rows = screen.getAllByRole("row");
  expect(rows).toHaveLength(2);
  expect(rows[0]).toHaveTextContent("1"); // Alice rank=1
  expect(rows[1]).toHaveTextContent("3"); // Alicia rank=3
});

測試 select 下拉選單

test("select changes category", async () => {
  const user = userEvent.setup();
  render(FilterPanel);

  await user.selectOptions(screen.getByRole("combobox"), "technology");
  expect(screen.getByRole("combobox")).toHaveValue("technology");
});

測試 callback props（Svelte 5 事件模式）

test("calls onsubmit with form data", async () => {
  const handleSubmit = vi.fn();
  const user = userEvent.setup();

  render(LoginForm, { onsubmit: handleSubmit });
  await user.type(screen.getByLabelText("Email"), "a@b.com");
  await user.click(screen.getByRole("button", { name: "Submit" }));

  expect(handleSubmit).toHaveBeenCalledWith(
    expect.objectContaining({ email: "a@b.com" })
  );
});

測試 context

import { render } from "@testing-library/svelte";

render(MyComponent, {
  props: { name: "test" },
  context: new Map([["theme", "dark"]]),
});

低階 mount + flushSync

不用 Testing Library 時，直接用 Svelte API：

import { mount, unmount, flushSync } from "svelte";

test("low level mount", () => {
  const target = document.createElement("div");
  document.body.appendChild(target);

  const component = mount(Counter, { target, props: { initial: 0 } });
  target.querySelector("button")!.click();
  flushSync(); // 強制同步更新 DOM
  expect(target.querySelector("button")!.textContent).toBe("1");

  unmount(component);
  target.remove();
});

測試 runes 邏輯（不渲染元件）

檔名必須是 .svelte.test.ts：

// counter.svelte.test.ts
import { flushSync } from "svelte";

test("reactive state without component", () => {
  let count = $state(0);
  let doubled = $derived(count * 2);

  expect(doubled).toBe(0);
  count = 5;
  // $derived 在同步 context 中自動更新
  expect(doubled).toBe(10);
});

test("effect runs on state change", () => {
  const log: number[] = [];
  const cleanup = $effect.root(() => {
    let count = $state(0);

    $effect(() => {
      log.push(count);
    });

    flushSync(); // 觸發 pending effects
    expect(log).toEqual([0]);

    count = 1;
    flushSync();
    expect(log).toEqual([0, 1]);
  });
  cleanup(); // 清除 effect root
});

重要注意事項

1. 使用 runes 的測試檔必須含 .svelte（如 .svelte.ts、.svelte.test.ts），否則 $state / $derived 等 runes 不會被編譯
2. 用 svelteTesting() plugin — 自動處理 browser resolve + cleanup
3. 低階 mount() 測試需要 flushSync() — Testing Library 的 fireEvent / act 已自動處理
4. 解構會斷開 reactivity — 用 getter 或直接存取
5. Snippets 不能從測試傳入 — 建立 wrapper component
6. $effect 中的非同步操作（await / setTimeout 之後）不會被追蹤依賴 — 用 waitFor 或 findBy* 等待 DOM 更新
7. 外部 .svelte.ts 模組的狀態變更需要 flushSync() 才能觸發 DOM 更新，Testing Library 的 auto-retry 只對元件內部 reactivity 有效
8. flushSync() 不能在 effect 執行期間呼叫 — 只能在 state change 後、effect 外部使用
9. Mock Svelte component 的 default export 要包在 { default: ... } 中
10. 優先用 screen.getByRole / getByLabelText 等語意查詢，避免用 container querySelector（無 auto-retry 且容易因結構變動而壞掉）