Express foundations
Configure without global mystery
Read, validate, and pass configuration deliberately instead of reaching into process.env everywhere.
9 minute lesson
The session secret is required, the port has a safe local default, and environment-specific trust proxy behavior is explicit. That contrast gives us something useful to test instead of a rule to memorize.
Configuration is input to the application and should have a schema, defaults, and an obvious production failure mode. The useful target is not encyclopedic coverage. Make one deliberate choice, observe its consequences, and know which requirement would make you revise it.
The happy path can hide a bad design: you can scatter environment lookups through handlers and discover invalid values on the first user request and still get one successful demo. Push past the demo. validate configuration once at startup and inject the resulting values where they are needed, then test the condition most likely to prove the choice wrong.
The module is moving toward one outcome: Start a small Express 5 application whose configuration and request path can be explained from the first byte. Save the before-and-after evidence now; it will make the final project review much more honest.
Run the server with a missing secret, an invalid port, and a complete local configuration; capture each outcome. Save the evidence, then explain which requirement would force you to choose a different design.
Lesson completed