Why there is still a cross oorigin error after setting the Access-Control-Allow-Origin in server side?

Question

I just run a CORS cross origin demo.The demo is running with node.js. Here is the index.html:

<button>click to cross origin using CROS</button>
<p>hello world 😘</p>
<script>
    var btn = document.getElementsByTagName('button')[0];
    var text = document.getElementsByTagName('p')[0];
    btn.addEventListener('click', function () {
        var xhr = new XMLHttpRequest();
        var url = 'http://localhost:3001';    
        xhr.open('PUT',url,true);                
        xhr.send();                       
        xhr.onreadystatechange = () => {     
            if (xhr.readyState === XMLHttpRequest.DONE && xhr.status === 200) {  
                text.innerHTML = xhr.response;
            }
        }
    })
</script>

Here is the serverRes.js:

var express = require('express');
var app = express();
var responsePort = 3001;  
app.get('/', (req, res) => {
    res.set('Access-Control-Allow-Origin', 'http://localhost:3000'); 
    res.send("Hello world from CROS.😡");  
});

app.listen(responsePort, function () {
    console.log('cros_responser is listening on port '+ responsePort);
});

You can see that I have set the Access-Control-Allow-Origin with http://localhost:3000 ,so the response to the preflighted request should actually pass the access control check,which means the request will succeed anyway.But when I go to the port 3000,what I get is:

But why?Why there is still a cross oorigin error after setting the Access-Control-Allow-Origin in server side? Also,I have tried writing:

app.all('/', function (req, res, next) {
    res.header('Access-Control-Allow-Origin', 'http://localhost:3000');
    res.header('Access-Control-Allow-Methods', 'GET,PUT,POST,DELETE');
    res.send("Hello world from CROS.😡");   
    next(); // pass control to the next handler
});

according to Why doesn't adding CORS headers to an OPTIONS route allow browsers to access my API? . But the error still exists.

Answer 1

The code you posted should work.

var express = require('express');
var app = express();
var responsePort = 3001;  


app.all("/", function(req, res, next){
  res.header('Access-Control-Allow-Origin', 'http://localhost:3000');
  res.header('Access-Control-Allow-Methods', 'GET,PUT,POST,DELETE');
  next();
});


app.get('/', (req, res) => {
    res.set('Access-Control-Allow-Origin', 'http://localhost:3000'); 
    res.send("Hello world from CROS.😡");  
});

app.listen(responsePort, function () {
    console.log('cros_responser is listening on port '+ responsePort);
});

Try hitting this repl instead.

https://repl.it/@nithinthampi/WirelessGentleSigns

Answer 2

you also need to allow the http verbs with the header Access-Control-Allow-Methods :

res.header('Access-Control-Allow-Origin', 'http://localhost:3000');
res.header('Access-Control-Allow-Methods', 'GET,PUT,POST,DELETE,OPTIONS');

This answer has a more detailed description: Why doesn't adding CORS headers to an OPTIONS route allow browsers to access my API?