From a local error to a working release
Localhost Help
Diagnose local development issues.Understand your ports.Move from localhost to production.
Start with the symptom
A port number alone does not identify the application. Check the listener and its process, then test the HTTP response. When the service runs in a container, check the host-to-container mapping too.
What is failing?
Choose occupied port, unreachable service, Docker mapping, CORS, or framework configuration.
Connection errorslocalhost:3000 not working
Investigate refused connections, EADDRINUSE, wrong routes, and bind addresses.
Browser errorsCORS Header Analyzer
Compare the page origin, API URL, credentials, and response headers.
Tools for the next check
These browser tools process what you enter locally. Generate a command, inspect its output, and compare the result with your Docker or framework settings.
Port Command Generator
Choose an operating system, port, and check; copy the command to your terminal.
MapDocker Port Mapper
See how Compose publishes container ports on the host and flags conflicts.
ReviewFramework Config Linter
Inspect pasted development configuration for bind, port, and override problems.
Make the production handoff
Localhost points back to the machine making the request. A production release needs a reachable host, an appropriate bind address, environment variables, routing, and a repeatable health check. Choose the stack you are deploying:
Next.js
Check the build, runtime configuration, and public versus server-only variables.
BackendNode.js
Bind the service correctly, manage secrets, and verify a health endpoint.
ContainersDocker
Separate container ports from published host ports and test the image.
CMSHeadless WordPress
Verify the public API, CORS policy, HTTPS, and preview flow.
Looking for localhost:3000?
The original localhost:3000 guide still explains the URL, routes, listener checks, and common framework defaults. For other numbers, use the ports directory.