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To aggregate modules means to import a number of related modules and export them together. This helps with code organisation and is sometimes known as a barrel module. There is a dedicated syntax for this in JavaScript.

For example, the default export of the SWR package is a React hook for data fetching. You can import it as follows:

import useSWR from "swr";

The hook expects a fetcher function as its second argument, but the SWR package doesn’t actually provide one. You have to provide your own. In the Quick Start guide, they suggest a simple wrapper around the Fetch API:

const fetcher = (...args) => fetch(...args).then(res => res.json());

Whenever you use the hook, you need to provide the fetcher function:

import useSWR from "swr";

const fetcher = (...args) => fetch(...args).then(res => res.json());

export default function Profile() {
const { data, error, isLoading } = useSWR("/api/user", fetcher);
// Conditionally render the UI...
}

You’ll probably need to use the fetcher function across multiple files. You could move it into its own module, e.g. fetcher.js:

const fetcher = (...args) => fetch(...args).then(res => res.json());
export default fetcher;

But this isn’t ideal. The two functions are tightly coupled and intended to be used together. It doesn’t really make sense to import them from separate modules:

import useSWR from "swr";
import fetcher from "./fetcher.js";

To fix this problem, you could aggregate the modules, e.g. in a file called swr.js. Your first instinct might be to import the useSWR hook, export it as the default export, and export the fetcher function as a named export:

import useSWR from "swr";

export default useSWR;
export const fetcher = (...args) => fetch(...args).then(res => res.json());

This is fine, but you can do better! There’s a dedicated syntax for aggregation:

export { default } from "swr"; // Exports the default export of the SWR package
export const fetcher = (...args) => fetch(...args).then(res => res.json());

Now you can import the default and named exports from the same module. This makes it clear that they’re tightly coupled:

import useSWR, { fetcher } from "./swr.js";

Note that you’re importing from "./swr.js" here, not "swr". The former is the path to the swr.js module you created, relative to the current directory. The latter refers to the npm package in the node_modules/ directory.

For more on this topic, check out JavaScript modules > Aggregating modules in the MDN Web Docs.