HTML MASTER CLASS /// LEARN TAGS /// BUILD STRUCTURE /// SEMANTIC WEB /// HTML MASTER CLASS /// LEARN TAGS ///

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Pascual Vila

Frontend Instructor // Code Syllabus

Common Pitfalls & Errors

The Error //

Forgetting -p and then wondering why localhost:3000 refuses the connection

# Wrong: no port published, browser can never reach it
docker run my-node-app

# Correct: maps host port 3000 to the container's port 3000
docker run -p 3000:3000 my-node-app

The Solution //

A container's network is isolated from the host by default — the Node server inside genuinely is listening on port 3000, but nothing on the host machine is forwarded to it. You must explicitly map a host port to the container's port with -p [hostPort]:[containerPort], and the container port must match the port your app.listen() call actually uses.

The Error //

Accumulating dozens of stopped containers because `docker run` was used instead of `docker run --rm` for one-off tests

# Leaves a stopped container behind after Ctrl+C
docker run -p 3000:3000 my-node-app

# Auto-removes itself on exit — ideal for quick tests
docker run --rm -p 3000:3000 my-node-app

The Solution //

Every `docker run` (without -d and without stopping it) creates a new container instance that lingers in a stopped state after Ctrl+C, visible in `docker ps -a`, silently consuming disk space over weeks of development. For throwaway test runs, add --rm so the container is automatically deleted the moment it exits.