87 lines
3.5 KiB
Markdown
87 lines
3.5 KiB
Markdown
---
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description: Extend the default document markup added by Next.js.
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---
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# Custom `Document`
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A custom `Document` is commonly used to augment your application's `<html>` and `<body>` tags. This is necessary because Next.js pages skip the definition of the surrounding document's markup.
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A custom `Document` can also include `getInitialProps` for expressing asynchronous server-rendering data requirements.
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To override the default `Document`, create the file `./pages/_document.js` and extend the `Document` class as shown below:
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```jsx
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import Document, { Html, Head, Main, NextScript } from 'next/document'
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class MyDocument extends Document {
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static async getInitialProps(ctx) {
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const initialProps = await Document.getInitialProps(ctx)
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return { ...initialProps }
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}
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render() {
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return (
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<Html>
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<Head />
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<body>
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<Main />
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<NextScript />
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</body>
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</Html>
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)
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}
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}
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export default MyDocument
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```
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`<Html>`, `<Head />`, `<Main />` and `<NextScript />` are required for the page to be properly rendered.
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Custom attributes are allowed as props, like `lang`:
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```jsx
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<Html lang="en">
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```
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The `ctx` object is equivalent to the one received in [`getInitialProps`](/docs/api-reference/data-fetching/getInitialProps.md#context-object), with one addition:
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- `renderPage`: `Function` - a callback that runs the actual React rendering logic (synchronously). It's useful to decorate this function in order to support server-rendering wrappers like Aphrodite's [`renderStatic`](https://github.com/Khan/aphrodite#server-side-rendering)
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## Caveats
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- `Document` is only rendered in the server, event handlers like `onClick` won't work
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- React components outside of `<Main />` will not be initialized by the browser. Do _not_ add application logic here. If you need shared components in all your pages (like a menu or a toolbar), take a look at the [`App`](/docs/advanced-features/custom-app.md) component instead
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- `Document`'s `getInitialProps` function is not called during client-side transitions, nor when a page is [statically optimized](/docs/advanced-features/automatic-static-optimization.md)
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- Make sure to check if `ctx.req` / `ctx.res` are defined in `getInitialProps`. Those variables will be `undefined` when a page is being statically exported by [Automatic Static Optimization](/docs/advanced-features/automatic-static-optimization.md) or by [`next export`](/docs/advanced-features/static-html-export.md)
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- Common errors include adding a `<title>` in the `<Head />` tag or using `styled-jsx`. These should be avoided in `pages/_document.js` as they lead to unexpected behavior
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## Customizing `renderPage`
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> It should be noted that the only reason you should be customizing `renderPage` is for usage with **css-in-js** libraries that need to wrap the application to properly work with server-side rendering.
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It takes as argument an options object for further customization:
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```jsx
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import Document from 'next/document'
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class MyDocument extends Document {
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static async getInitialProps(ctx) {
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const originalRenderPage = ctx.renderPage
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ctx.renderPage = () =>
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originalRenderPage({
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// useful for wrapping the whole react tree
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enhanceApp: (App) => App,
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// useful for wrapping in a per-page basis
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enhanceComponent: (Component) => Component,
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})
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// Run the parent `getInitialProps`, it now includes the custom `renderPage`
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const initialProps = await Document.getInitialProps(ctx)
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return initialProps
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}
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}
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export default MyDocument
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```
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