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docs: clarify dynamic new URL(foo, import.meta.url) behavior #1707

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Nov 6, 2024
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docs: clarify dynamic new URL(foo, import.meta.url) behavior #1707

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20 changes: 20 additions & 0 deletions guide/assets.md
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Original file line number Diff line number Diff line change
Expand Up @@ -135,6 +135,7 @@ document.getElementById('hero-img').src = imgUrl

```js
function getImageUrl(name) {
// サブディレクトリー内のファイルは含まれないことに注意してください
return new URL(`./dir/${name}.png`, import.meta.url).href
}
```
Expand All @@ -146,6 +147,25 @@ function getImageUrl(name) {
const imgUrl = new URL(imagePath, import.meta.url).href
```

::: details 動作の仕組み

Vite は `getImageUrl` 関数を次のように変換します:

```js
import __img0png from './dir/img0.png'
import __img1png from './dir/img1.png'

function getImageUrl(name) {
const modules = {
'./dir/img0.png': __img0png,
'./dir/img1.png': __img1png,
}
return new URL(modules[`./dir/${name}.png`], import.meta.url).href
}
```

:::

::: warning SSR では動作しません
ブラウザーと Node.js で `import.meta.url` のセマンティクスが異なるため、 このパターンは Vite をサーバーサイドレンダリングで使用している場合には動作しません。サーバーバンドルは事前にクライアントホストの URL を決定することもできません。
:::