Why Does Node.js Throw 'Error: listen EADDRINUSE' When Using XMLHttpRequest?

Addressing Error: listen EADDRINUSE in Node.js

The "Error: listen EADDRINUSE" issue arises when Node.js encounters an attempt to listen on an already occupied port. Node.js is a runtime environment for executing JavaScript applications.

In the provided scenario, a server is listening on port 80 while simultaneously attempting to use XMLHttpRequest for a request. Browsers can handle concurrent connections, but the Node.js server faces an issue due to the port's unavailability.

The server code, as mentioned:

net.createServer(function (socket) {
    socket.name = socket.remoteAddress + ":" + socket.remotePort;
    console.log('connection request from: ' + socket.remoteAddress);
    socket.destroy();
}).listen(options.port);

The server is essentially listening for incoming connections but immediately destroying them. This prevents any actual requests from being processed.

The request code, as provided:

var xhr = new XMLHttpRequest();

xhr.onreadystatechange = function() {
    sys.puts("State: " + this.readyState);

    if (this.readyState == 4) {
        sys.puts("Complete.\nBody length: " + this.responseText.length);
        sys.puts("Body:\n" + this.responseText);
    }
};

xhr.open("GET", "http://mywebsite.com");
xhr.send();

The request is attempting to make an HTTP GET request to a remote website while the Node.js server is running on port 80.

To resolve the "Error: listen EADDRINUSE", the server should either:

• Use a Different Port: The server can listen on a different port, such as 8080, allowing the XMLHttpRequest request to proceed on port 80.
• Stop the Server: If the server needs to be running on port 80, it should be stopped before executing the XMLHttpRequest request. A common method is using killall -9 node to terminate the Node.js process.
• Use a Proxy Server: A proxy server can be used to handle incoming connections and forward requests to the Node.js server. This allows multiple applications to use the same port.

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