How does this find an available port? Show me the actual implementation.

Here's how get-port finds an available port:

Core Strategy

The implementation has three main steps:

1. Test port availability [index.js:L47-L60]

const checkAvailablePort = options =>
	new Promise((resolve, reject) => {
		const server = net.createServer();
		server.unref();
		server.on('error', reject);

		server.listen(options, () => {
			const {port} = server.address();
			server.close(() => {
				resolve(port);
			});
		});
	});

This creates a temporary TCP server and tries to listen on the requested port. If it succeeds, the port is available.

2. Iterate through preferred ports [index.js:L81-L88]

const portCheckSequence = function * (ports) {
	if (ports) {
		yield * ports;
	}

	yield 0; // Fall back to 0 if anything else failed
};

If you specify preferred ports, they're checked in order. If none work, port 0 is yielded, which tells the OS to assign a random available port.

3. Main loop with host checking [index.js:L89-L168]

The main getPorts function iterates through the port sequence. For each port, it:

• Skips excluded ports
• Checks availability via getAvailablePort, which validates the port across all local hosts (unless a specific host is given) [index.js:L61-L78]
• Checks if the port is already locked by the current process [index.js:L79-L80]
• If using reserve: true, adds it to reservedPorts (permanent for the process lifetime); otherwise adds it to lockedPorts.young (released after 15-30 seconds)
• Returns the first available port or throws an error if none found

FOLLOW_UPS:

• What does the reserve option actually do?
• How are locked ports tracked over time?
• What happens if all preferred ports fail?