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website/docs/advanced/client.mdx
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Sébastien Lorber
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---
description: How the Docusaurus client is structured
---

# Client architecture

## Theme aliases {/* #theme-aliases */}

A theme works by exporting a set of components, e.g. `Navbar`, `Layout`, `Footer`, to render the data passed down from plugins. Docusaurus and users use these components by importing them using the `@theme` webpack alias:

```js
import Navbar from '@theme/Navbar';
```

The alias `@theme` can refer to a few directories, in the following priority:

1. A user's `website/src/theme` directory, which is a special directory that has the higher precedence.
2. A Docusaurus theme package's `theme` directory.
3. Fallback components provided by Docusaurus core (usually not needed).

This is called a _layered architecture_: a higher-priority layer providing the component would shadow a lower-priority layer, making swizzling possible. Given the following structure:

```
website
├── node_modules
│   └── @docusaurus/theme-classic
│       └── theme
│           └── Navbar.js
└── src
    └── theme
        └── Navbar.js
```

`website/src/theme/Navbar.js` takes precedence whenever `@theme/Navbar` is imported. This behavior is called component swizzling. If you are familiar with Objective C where a function's implementation can be swapped during runtime, it's the exact same concept here with changing the target `@theme/Navbar` is pointing to!

We already talked about how the "userland theme" in `src/theme` can re-use a theme component through the [`@theme-original`](../swizzling.mdx#wrapping) alias. One theme package can also wrap a component from another theme, by importing the component from the initial theme, using the `@theme-init` import.

Here's an example of using this feature to enhance the default theme `CodeBlock` component with a `react-live` playground feature.

```js
import InitialCodeBlock from '@theme-init/CodeBlock';
import React from 'react';

export default function CodeBlock(props) {
  return props.live ? (
    <ReactLivePlayground {...props} />
  ) : (
    <InitialCodeBlock {...props} />
  );
}
```

Check the code of `@docusaurus/theme-live-codeblock` for details.

:::warning

Unless you want to publish a re-usable "theme enhancer" (like `@docusaurus/theme-live-codeblock`), you likely don't need `@theme-init`.

:::

It can be quite hard to wrap your mind around these aliases. Let's imagine the following case with a super convoluted setup with three themes/plugins and the site itself all trying to define the same component. Internally, Docusaurus loads these themes as a "stack".

```text
+-------------------------------------------------+
|        `website/src/theme/CodeBlock.js`         | <-- `@theme/CodeBlock` always points to the top
+-------------------------------------------------+
| `theme-live-codeblock/theme/CodeBlock/index.js` | <-- `@theme-original/CodeBlock` points to the topmost non-swizzled component
+-------------------------------------------------+
|  `plugin-awesome-codeblock/theme/CodeBlock.js`  |
+-------------------------------------------------+
|     `theme-classic/theme/CodeBlock/index.js`    | <-- `@theme-init/CodeBlock` always points to the bottom
+-------------------------------------------------+
```

The components in this "stack" are pushed in the order of `preset plugins > preset themes > plugins > themes > site`, so the swizzled component in `website/src/theme` always comes out on top because it's loaded last.

`@theme/*` always points to the topmost component—when `CodeBlock` is swizzled, all other components requesting `@theme/CodeBlock` receive the swizzled version.

`@theme-original/*` always points to the topmost non-swizzled component. That's why you can import `@theme-original/CodeBlock` in the swizzled component—it points to the next one in the "component stack", a theme-provided one. Plugin authors should not try to use this because your component could be the topmost component and cause a self-import.

`@theme-init/*` always points to the bottommost component—usually, this comes from the theme or plugin that first provides this component. Individual plugins / themes trying to enhance code block can safely use `@theme-init/CodeBlock` to get its basic version. Site creators should generally not use this because you likely want to enhance the _topmost_ instead of the _bottommost_ component. It's also possible that the `@theme-init/CodeBlock` alias does not exist at all—Docusaurus only creates it when it points to a different one from `@theme-original/CodeBlock`, i.e. when it's provided by more than one theme. We don't waste aliases!

## Client modules {/* #client-modules */}

Client modules are part of your site's bundle, just like theme components. However, they are usually side-effect-ful. Client modules are anything that can be `import`ed by Webpack—CSS, JS, etc. JS scripts usually work on the global context, like registering event listeners, creating global variables...

These modules are imported globally before React even renders the initial UI.

```js title="@docusaurus/core/App.tsx"
// How it works under the hood
import '@generated/client-modules';
```

Plugins and sites can both declare client modules, through [`getClientModules`](../api/plugin-methods/lifecycle-apis.mdx#getClientModules) and [`siteConfig.clientModules`](../api/docusaurus.config.js.mdx#clientModules), respectively.

Client modules are called during server-side rendering as well, so remember to check the [execution environment](./ssg.mdx#escape-hatches) before accessing client-side globals.

```js title="mySiteGlobalJs.js"
import ExecutionEnvironment from '@docusaurus/ExecutionEnvironment';

if (ExecutionEnvironment.canUseDOM) {
  // As soon as the site loads in the browser, register a global event listener
  window.addEventListener('keydown', (e) => {
    if (e.code === 'Period') {
      location.assign(location.href.replace('.com', '.dev'));
    }
  });
}
```

CSS stylesheets imported as client modules are [global](../styling-layout.mdx#global-styles).

```css title="mySiteGlobalCss.css"
/* This stylesheet is global. */
.globalSelector {
  color: red;
}
```

### Client module lifecycles {/* #client-module-lifecycles */}

Besides introducing side-effects, client modules can optionally export two lifecycle functions: `onRouteUpdate` and `onRouteDidUpdate`.

Because Docusaurus builds a single-page application, `script` tags will only be executed the first time the page loads, but will not re-execute on page transitions. These lifecycles are useful if you have some imperative JS logic that should execute every time a new page has loaded, e.g., to manipulate DOM elements, to send analytics data, etc.

For every route transition, there will be several important timings:

1. The user clicks a link, which causes the router to change its current location.
2. Docusaurus preloads the next route's assets, while keeping displaying the current page's content.
3. The next route's assets have loaded.
4. The new location's route component gets rendered to DOM.

`onRouteUpdate` will be called at event (2), and `onRouteDidUpdate` will be called at (4). They both receive the current location and the previous location (which can be `null`, if this is the first screen).

`onRouteUpdate` can optionally return a "cleanup" callback, which will be called at (3). For example, if you want to display a progress bar, you can start a timeout in `onRouteUpdate`, and clear the timeout in the callback. (The classic theme already provides an `nprogress` integration this way.)

Note that the new page's DOM is only available during event (4). If you need to manipulate the new page's DOM, you'll likely want to use `onRouteDidUpdate`, which will be fired as soon as the DOM on the new page has mounted.

```js title="myClientModule.js"
export function onRouteDidUpdate({location, previousLocation}) {
  // Don't execute if we are still on the same page; the lifecycle may be fired
  // because the hash changes (e.g. when navigating between headings)
  if (location.pathname !== previousLocation?.pathname) {
    const title = document.getElementsByTagName('h1')[0];
    if (title) {
      title.innerText += '❤️';
    }
  }
}

export function onRouteUpdate({location, previousLocation}) {
  if (location.pathname !== previousLocation?.pathname) {
    const progressBarTimeout = window.setTimeout(() => {
      nprogress.start();
    }, delay);
    return () => window.clearTimeout(progressBarTimeout);
  }
  return undefined;
}
```

Or, if you are using TypeScript and you want to leverage contextual typing:

```ts title="myClientModule.ts"
import type {ClientModule} from '@docusaurus/types';

const module: ClientModule = {
  onRouteUpdate({location, previousLocation}) {
    // ...
  },
  onRouteDidUpdate({location, previousLocation}) {
    // ...
  },
};
export default module;
```

Both lifecycles will fire on first render, but they will not fire on server-side, so you can safely access browser globals in them.

:::tip Prefer using React

Client module lifecycles are purely imperative, and you can't use React hooks or access React contexts within them. If your operations are state-driven or involve complicated DOM manipulations, you should consider [swizzling components](../swizzling.mdx) instead.

:::