Import module with folder and passing data to module in nodejs - node.js

Question

I Found The Tutorial about
Designing a clean REST API with Node.js (Express + Mongo)
project in github.
but the problem is i didn't get the concept of routing in one part.
the misundrestanding part is how is it possible to pass httpRequest data to handle method within contact-endpoint module?
because handle method is in here export default function makeContactsEndpointHandler({ contactList }) {
return async function handle(httpRequest) {
this is the index of project:
import handleContactsRequest from "./contacts";
import adaptRequest from "./helpers/adapt-request";
app.all("/contacts", contactsController);
app.get("/contacts/:id", contactsController);
function contactsController(req, res) {
const httpRequest = adaptRequest(req);
handleContactsRequest(httpRequest)
.then(({ headers, statusCode, data }) =>
res.set(headers).status(statusCode).send(data)
)
.catch((e) => res.status(500).end());
}
this is the adaptRequest:
export default function adaptRequest (req = {}) {
return Object.freeze({
path: req.path,
method: req.method,
pathParams: req.params,
queryParams: req.query,
body: req.body
})
}
this is the handleContactsRequest module:
import makeDb from "../db";
import makeContactList from "./contact-list";
import makeContactsEndpointHandler from "./contacts-endpoint";
const database = makeDb();
const contactList = makeContactList({ database });
const contactsEndpointHandler = makeContactsEndpointHandler({ contactList });
export default contactsEndpointHandler;
this is part of contact-endpoint module:
export default function makeContactsEndpointHandler({ contactList }) {
return async function handle(httpRequest) {
switch (httpRequest.method) {
case "POST":
return postContact(httpRequest);
case "GET":
return getContacts(httpRequest);
default:
return makeHttpError({
statusCode: 405,
errorMessage: `${httpRequest.method} method not allowed.`,
});
}
}

Answer 1

makeContactsEndpointHandler is a function that returns a function (async handle(xxx)).
In handleContactsRequest, we export the result of the call: makeContactsEndpointHandler({ contactList }). Which is therefore the function async handle(xxx) itself.
So, in index, when we call handleContactsRequest with the constant httpRequest as argument, we're actually calling that handle(xxx) function. (I wrote xxx as parameter name to highlight the difference between the two httpRequest declarations.)