How to Enable Cors Nodejs with Express

Why doesn't adding CORS headers to an OPTIONS route allow browsers to access my API?

To answer your main question, the CORS spec only requires the OPTIONS call to precede the POST or GET if the POST or GET has any non-simple content or headers in it.

Content-Types that require a CORS pre-flight request (the OPTIONS call) are any Content-Type except the following:

1. application/x-www-form-urlencoded
2. multipart/form-data
3. text/plain

Any other Content-Types apart from those listed above will trigger a pre-flight request.

As for Headers, any Request Headers apart from the following will trigger a pre-flight request:

1. Accept
2. Accept-Language
3. Content-Language
4. Content-Type
5. DPR
6. Save-Data
7. Viewport-Width
8. Width

Any other Request Headers will trigger the pre-flight request.

So, you could add a custom header such as: x-Trigger: CORS, and that should trigger the pre-flight request and hit the OPTIONS block.

See MDN Web API Reference - CORS Preflighted requests

How to enable CORS for nodejs Express server?

Your cors error message clearly says you need to use http when making the request

$.ajax({    type: "GET",    url: "http://localhost:3000/send",    beforeSend:function(){             $(".loading_msg").hide();        },    complete:function(){             $(".loading_msg").show();        }});

How to allow CORS in node.js?

Since you use nodejs

installing cors

npm install cors

After that

var express = require('express')
var cors = require('cors')
var app = express()

app.use(cors())

app.get('/products/:id', function (req, res, next) {
  res.json({msg: 'This is CORS-enabled for all origins!'})
})

app.listen(80, function () {
  console.log('CORS-enabled web server listening on port 80')
})

Then after applying "cors" middleware. You need to insert "http://" before "localhost: in your react app".

Example

axios.get(`http://localhost:3001/api/app`)
        .then(res => {
            console.log("res", res);
        })
        .catch(err => {
            console.log("AXIOS ERROR:", err);
        })

How to allow CORS with Node.js (without using Express)

This is because you are requesting a not-so-simple request, meaning it needs to handle preflight request which is made as an HTTP OPTIONS request (so be sure your server is able to respond to this method). The preflight request is a way of asking permissions for the actual request, before making the actual request. The server should inspect the two headers above to verify that both the HTTP method and the requested headers are valid and accepted.

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