#Data Fetching

Modern.js provides out-of-the-box data fetching capabilities. Developers can use these APIs to fetch data in their projects. It's important to note that these APIs do not help the application make requests but assist developers in managing data better and improving project performance.

#What is Data Loader

Modern.js recommends managing routes using conventional routing. Each route component (layout.ts, page.ts, or $.tsx) can have a same-named .data file. These files can export a loader function, known as Data Loader, which executes before the corresponding route component renders to provide data for the component. Here is an example:

.
└── routes
    ├── layout.tsx
    └── user
        ├── layout.tsx
        ├── layout.data.ts
        ├── page.tsx
        └── page.data.ts

In the routes/user/page.data.ts file, you can export a Data Loader function:

export type ProfileData = {
  /*  some types */
};

export const loader = async (): Promise<ProfileData> => {
  const res = await fetch('https://api/user/profile');
  return await res.json();
};

In the route component, you can use the useLoaderData function to fetch data:

import { useLoaderData } from '@modern-js/runtime/router';
import type { ProfileData } from './page.data.ts';

export default function UserPage() {
  const profileData = useLoaderData() as ProfileData;
  return <div>{profileData}</div>;
}

In a CSR environment, the loader function executes on the browser side and can use browser APIs.

In an SSR environment, the loader function only executes on the server side for initial page loads and when navigating. Here it can call any Node.js APIs, and any dependencies or code used won't be included in the client-side bundle.

When navigating on the client side, based on conventional routing, Modern.js supports parallel execution (requests) of all loader functions. For example, when visiting /user/profile, the loader functions under /user and /user/profile will execute in parallel, solving the request-render waterfall issue and significantly improving page performance.

#loader Function

The loader function has two parameters used for getting route parameters and request information.

#params

params is the dynamic route segments when the route is a dynamic route, which passed as parameters to the loader function:

import { LoaderFunctionArgs } from '@modern-js/runtime/router';

// When visiting /user/123, the function parameter is `{ params: { id: '123' } }`
export const loader = async ({ params }: LoaderFunctionArgs) => {
  const { id } = params;
  const res = await fetch(`https://api/user/${id}`);
  return res.json();
};

#request

request is an instance of Fetch Request. A common use case is to get query parameters from request:

import { LoaderFunctionArgs } from '@modern-js/runtime/router';

export const loader = async ({ request }: LoaderFunctionArgs) => {
  const url = new URL(request.url);
  const userId = url.searchParams.get('id');
  return queryUser(userId);
};

#Return Value

The return value of the loader function must be one of two data structures: a serializable data object or an instance of Fetch Response:

const loader = async (): Promise<ProfileData> => {
  return {
    message: 'hello world',
  };
};
export default loader;

By default, the loader response's Content-type is application/json, and its status is 200. You can customize the Response to change these:

const loader = async (): Promise<ProfileData> => {
  const data = { message: 'hello world' };
  return new Response(JSON.stringify(data), {
    status: 200,
    headers: {
      'Content-Type': 'application/json; utf-8',
    },
  });
};

#Using Data Loader in Different Environments

The loader function may run on the server or client. When it runs on the server, it's called a Server Loader; when it runs on the client, it's called a Client Loader.

In CSR applications, the loader function runs on the client, hence it is a Client Loader by default.

In SSR applications, the loader function runs only on the server, hence it is a Server Loader by default. During SSR rendering, Modern.js will directly call the loader function on the server side. When navigating on the client side, Modern.js sends an HTTP request to the SSR service, also triggering the loader function on the server side.

We recommend using the fetch API in loader functions to make requests. Modern.js provides a default polyfill for the fetch API, allowing it to be used on the server side. This means you can fetch data in a consistent manner whether in CSR or SSR:

export async function loader() {
  const res = await fetch('URL_ADDRESS');
  const data = await res.json();
  return {
    message: data.message,
  };
}

#Error Handling

#Basic Usage

In a loader function, you can handle errors by using throw error or throw response. When an error is thrown in the loader function, Modern.js will stop executing the remaining code in the current loader and switch the front-end UI to the defined ErrorBoundary component:

// routes/user/profile/page.data.ts
export async function loader() {
  const res = await fetch('https://api/user/profile');
  if (!res.ok) {
    throw res;
  }
  return res.json();
}

// routes/user/profile/error.tsx
import { useRouteError } from '@modern-js/runtime/router';
const ErrorBoundary = () => {
  const error = useRouteError() as Response;
  return (
    <div>
      <h1>{error.status}</h1>
      <h2>{error.statusText}</h2>
    </div>
  );
};

export default ErrorBoundary;

#Modify HTTP Code

In SSR projects, you can control the page status code by throwing a response in the loader function and display the corresponding UI.

In the following example, the page's status code will match this response, and the page will display the ErrorBoundary UI:

// routes/user/profile/page.data.ts
export async function loader() {
  const user = await fetchUser();
  if (!user) {
    throw new Response('The user was not found', { status: 404 });
  }
  return user;
}

// routes/error.tsx
import { useRouteError } from '@modern-js/runtime/router';
const ErrorBoundary = () => {
  const error = useRouteError() as { data: string };
  return <div className="error">{error.data}</div>;
};

export default ErrorBoundary;

#Accessing Data from Upper Components

In many scenarios, child components need to access data from the upper component's loader. You can use the useRouteLoaderData function to easily get data from the upper component:

// routes/user/profile/page.tsx
import { useRouteLoaderData } from '@modern-js/runtime/router';

export function UserLayout() {
  // Get data returned by the `loader` in routes/user/layout.data.ts
  const data = useRouteLoaderData('user/layout');
  return (
    <div>
      <h1>{data.name}</h1>
      <h2>{data.age}</h2>
    </div>
  );
}

useRouteLoaderData accepts a parameter routeId. In conventional routing, Modern.js automatically generates the routeId, which is the path of the corresponding component relative to src/routes. For example, in the example above, if a child component wants to get data returned by the loader in routes/user/layout.tsx, the routeId value is user/layout.

In a multi-entry scenario, the routeId value needs to include the corresponding entry name, which is typically the directory name if not explicitly specified. For example, with the following directory structure:

.
└── src
    ├── entry1
    │     └── routes
    │           └── layout.tsx
    └── entry2
          └── routes
                └── layout.tsx

To get data returned by the loader in entry1/routes/layout.tsx, the routeId value would be entry1_layout.

#Loading UI (Experimental)

Create user/layout.data.ts and add the following code:

import { defer } from '@modern-js/runtime/router';

export const loader = () =>
  defer({
    userInfo: new Promise(resolve => {
      setTimeout(() => {
        resolve({
          age: 1,
          name: 'user layout',
        });
      }, 1000);
    }),
  });

Add the following code in user/layout.tsx:

import { Await, defer, useLoaderData, Outlet } from '@modern-js/runtime/router';

export default function UserLayout() {
  const { userInfo } = useLoaderData() as { userInfo: Promise<UserInfo> };
  return (
    <div>
      <React.Suspense fallback={<p>Loading...</p>}>
        <Await
          resolve={userInfo}
          children={userInfo => (
            <div>
              <span>{userInfo.name}</span>
              <span>{userInfo.age}</span>
              <Outlet />
            </div>
          )}
        ></Await>
      </React.Suspense>
    </div>
  );
}

#Data Caching

During route navigation, Modern.js will only load data for the parts of the route that change. For example, if the current route is a/b, and the Data Loader for the a path has already executed, then when transitioning from /a/b to /a/c, the Data Loader for the a path will not re-execute, but the Data Loader for the c path will execute and fetch the data.

This default optimization strategy avoids redundant data requests. However, you might wonder how to update the data for the a path's Data Loader?

In Modern.js, the Data Loader for a specific path will reload in the following scenarios:

1. After triggering a Data Action
2. When URL parameters change
3. When the user clicks a link that matches the current page URL
4. When the route component defines a shouldRevalidate function that returns true

#shouldRevalidate

In route components (layout.tsx, page.tsx, $.tsx), you can export a shouldRevalidate function. This function is triggered on each route change in the project and can control which route data to reload. If this function returns true, Modern.js will reload the corresponding route data.

import type { ShouldRevalidateFunction } from '@modern-js/runtime/router';
export const shouldRevalidate: ShouldRevalidateFunction = ({
  actionResult,
  currentParams,
  currentUrl,
  defaultShouldRevalidate,
  formAction,
  formData,
  formEncType,
  formMethod,
  nextParams,
  nextUrl,
}) => {
  return true;
};

#Incorrect Usages

1. The loader can only return serializable data. In an SSR environment, the return value of the loader function will be serialized as a JSON string and then deserialized as an object on the client side. Therefore, the loader function should not return non-serializable data such as functions.

// This won't work!
export default () => {
  return {
    user: {},
    method: () => {},
  };
};

2. Modern.js will call the loader function for you, and you should not call it yourself:

// This won't work!
export const loader = async () => {
  const res = fetch('https://api/user/profile');
  return res.json();
};

import { loader } from './page.data.ts';
export default function RouteComp() {
  const data = loader();
}

3. Do not import loader files from route components, and do not import variables from route components into loader files. If you need to share types, use import type.

// Not allowed
// routes/layout.tsx
import { useLoaderData } from '@modern-js/runtime/router';
import { ProfileData } from './page.data.ts'; // should use "import type" instead

export const fetch = wrapFetch(fetch);

export default function UserPage() {
  const profileData = useLoaderData() as ProfileData;
  return <div>{profileData}</div>;
}

// routes/layout.data.ts
import { fetch } from './layout.tsx'; // should not be imported from the route component
export type ProfileData = {
  /*  some types */
};

export const loader = async (): Promise<ProfileData> => {
  const res = await fetch('https://api/user/profile');
  return await res.json();
};

4. When running on the server, loader functions are packaged into a single bundle. Therefore, we do not recommend using __filename and __dirname in server code.

#Frequently Asked Questions

1. What is the relationship between loader and BFF functions?

In CSR projects, the loader executes on the client side and can directly call BFF functions to make API requests.

In SSR projects, each loader is also a server-side API. We recommend using loader instead of BFF functions with HTTP Method GET to avoid an extra layer of forwarding and execution.