Question 1

Why does my Node process fail with EADDRINUSE address already in use?

Accepted Answer

The error means something is already listening on the port your process tried to bind. The operating system allows only one listener per port and address combination, so the second attempt is refused. The question is always which process holds it, and there are a few recurring answers.

The most common is a previous run of the same application that did not shut down. This happens when a process was suspended rather than terminated, when a watcher or debugger left an orphan behind after a crash, or when a terminal was closed without the process receiving a signal. The old process is still running and still holding the port even though nothing appears to be attached to it.

On Linux and macOS you can identify the owner by listing open files filtered to that port, which reports the process ID and the command name. Windows offers the equivalent through its network statistics tool. Confirm what the process actually is before terminating it, since the port may legitimately belong to something else, particularly on common ports.

A container publishing the same host port produces the identical error, and is easy to overlook because nothing on the host itself appears to be listening. Listing running containers and their port mappings will show it.

Two subtler causes are worth knowing. Calling listen more than once in a single application, often after a refactor duplicated the startup path, fails the second time against your own process. Similarly, forking workers that each bind the same port without using a mechanism designed for sharing will fail for all but the first.

The durable fix for the orphan case is graceful shutdown. Handling termination signals and closing the server explicitly ensures the port is released when the process is asked to stop, which removes most of these incidents rather than requiring you to hunt down a stale process each time.

Question 2

Why does my Docker container exit immediately after starting?

Accepted Answer

A container lives exactly as long as its main process. When the process started by the image's CMD or ENTRYPOINT finishes, the container stops, regardless of anything else running inside it. So an

Question 3

Why can my Docker container not connect to a service on localhost?

Accepted Answer

Inside a container, localhost means the container itself, not the machine running it. Each container has its own network namespace and its own loopback interface, so a connection to localhost from

Question 4

Why does my Node app crash with an unhandled promise rejection?

Accepted Answer

A promise that rejects with nothing attached to handle it produces an unhandled rejection, and modern Node treats that as fatal by default. The process exits. This changed from earlier versions where

Question 5

Why does my API return 502 Bad Gateway?

Accepted Answer

A 502 is reported by a proxy or load balancer, not by your application. It means the intermediary tried to forward the request to an upstream server and received an invalid response, or none at all.

Question 6

Why is my Node.js application using so much memory?

Accepted Answer

High memory in a long-running process is either legitimate caching, a genuine leak, or garbage collection that has not needed to run yet. Distinguishing them is the first task, because the response

Question 7

What is the difference between a process and a thread?

Accepted Answer

A process is an independent running program with its own memory space. A thread is a unit of execution inside a process, sharing that process's memory with its sibling threads. What a process owns:

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