Having issues getting refresh_token in browser. Can't read XML Response - reactjs

Question

I’m mainly a front end guy, and I’m having some issues getting data back from an api call. I think I just don’t understand something properly, so it’s proving to be massively frustrating. I wonder if anyone can offer some input.
I make the call like this:
var config = {
headers: {
'Access-Control-Allow-Origin': '*',
'Content-Type': 'application/xml',
},
proxy: {
host: '127.0.0.1',
port: 3000
}
};
axios
.post(
`https://login.salesforce.com/services/oauth2/token?grant_type=authorization_code&redirect_uri=http://localhost:3000/callback/&client_id=CLIENT_ID&client_secret=CLIENT_SECRET&code=${authCode}`,
config
)
.then(res => {
console.log(`statusCode: ${res.statusCode}`);
console.log('Result:', res);
})
.catch(error => {
console.error('Error:', error);
});
Then the response I get is this:
Access to XMLHttpRequest at 'https://login.salesforce.com/services/oauth2/token?
grant_type=authorization_code&redirect_uri=http://localhost:3000/callback/&client_id=CLINET_ID&client_secret=CLIENT_SECRET&code=AUTH_CODE'
from origin 'http://localhost:3000' has been blocked by CORS policy: Response to preflight request doesn't pass access control check: No
'Access-Control-Allow-Origin' header is present on the requested resource.
and this:
POST https://login.salesforce.com/services/oauth2/token?grant_type=authorization_code&redirect_uri=https://localhost:3000/callback/&client_id=3[ID]&client_secret=[SECRET]&code=[CODE] 400 (Bad Request)
But if I put this link in my browser: https://login.salesforce.com/services/oauth2/token?grant_type=authorization_code&redirect_uri=http://localhost:3000/callback/&client_id=CLINET_ID&client_secret=CLIENT_SECRET&code=AUTH_CODE I can see it’s an XML file with all the info I need. So why can’t I just get that info in my response?
This is what I see when I paste the link in my browser:
<OAuth>
<access_token>
{Token}
</access_token>
<refresh_token>
{Token}
</refresh_token>
<signature>{Signature}</signature>
<scope>refresh_token api full</scope>
<id_token>
{Token}
</id_token>
<instance_url>https://um5.salesforce.com</instance_url>
<id>
{ID}</id>
<token_type>Bearer</token_type>
<issued_at>1558167449074</issued_at>
</OAuth>
I've obviously taken out the actual tokens. This is the info I need.
Please can someone assist me in actually accessing that XML info on the front end? Id I do the request on Postman, I get exactly what I need.

Answer 1

I fixed it by using this code on a Node server and seemed to be getting the response I needed:
var request = require('request');
var options = {
url: `https://login.salesforce.com/services/oauth2/token?grant_type=authorization_code&redirect_uri=http://localhost:3000/callback/&client_id=CLIENT_ID.&client_secret=CLIENT_SECRET&code=${token}`
};
function callback(error, response, body) {
if (!error) {
console.log('Body: ', body);
} else {
console.log('Error: ', error);
}
}
I hope this helps someone. Salesforce documentation is very disjointed and complicated.