How to correctly configure server and browser to avoid cors errors? Fetch API + Node.js

Question

I was trying to make a simple api call from index.html but I kept getting an error no matter what I did.

From my understanding, the cors errors occur because I am making

Answer 1

Edit: OP has changed the question based on the answer - but for no avail.

You don't have to set CORS headers manually if you use the cors library.

var cors = require('cors');


app.options('*', cors()); // Enable preflight by using this middle ware before any other route. 
app.use(cors());

And if the CORS should be enabled for a white list of domains, use the following options:

var whitelist = ['http://example1.com', 'http://example2.com']
var corsOptions = {
  origin: function (origin, callback) {
    if (whitelist.indexOf(origin) !== -1) {
      callback(null, true)
    } else {
      callback(new Error('Not allowed by CORS'))
    }
  }
}