Do not understand the function with Express and CORS [duplicate] - node.js

Question

This question already has an answer here:
NodeJS callback with null as first argument
(1 answer)
Closed 9 months ago.
This is a code I found in the internet using Express and CORS:
const express = require('express');
const app = express();
const cors = require('cors');
var corsOptionsDelegate = async (req, callback) => {
var corsOptions = { origin: false };
try {
...
corsOptions.origin = true;
} catch (err) {
console.log(err);
}
callback(null, corsOptions)
}
app.use(cors(corsOptionsDelegate));
I don't know how this function callback(null, corsOptions) process in this code.

Answer 1

This is just a standard callback design pattern. Here's an article you can read but also you should learn more about what design patterns are and how Node.js works. This is a very basic question you're asking here.

Answer 2

You actually do not need any function callback for simple servers.
There is a much simpler way.
const whiteList = [ "https://myRealBackendUrl-1", "https://myRealBackendUrl-2" ];
// you can also pass a string here instead here instead of array
const corsOptions = {
credentials: true,
origin: process.env.NODE_ENV !== production ? "http://localhost:3000" : whiteList
// if you are in a dev environment, you probably want something like localhost
// if you are in a production environment, for example heroku then your backend
// url will be something like http://example.herokuapp.com
// in that case `const whiteList = [ "http://example.herokuapp.com" ];`
};
app.use(cors(corsOptions));
The above code should be enough for the normal use case.
As for the callback function, it is if you want to run some function of your own.
var corsOptionsDelegate = async (req, callback) => {
var corsOptions = { origin: false };
try {
// you can do some dynamic check here
// For example: check database for some conditions then allow access
if( myDatabaseSays == true ) corsOptions.origin = true;
else corsOptions.origin = false;
} catch (err) {
console.log(err);
// corsOptions.origin = false;
}
callback(null, corsOptions) // chain it
}
Anyway read the docs properly for more info
[1]: https://expressjs.com/en/resources/middleware/cors.html