Popular Stackoverflow questions

To help inform writing new documentation "guides," let's examine some of the most popular Express questions asked on Stackoverflow.

NOTE: Must make sure the information is still relevant for Express 4.x/5.x.

• Most frequent questions
• Most-voted questions

There are lots of good questions there; here are a few that are in both lists:

• How to get GET (query string) variables in Express.js on Node.js?
  Also How to access the GET parameters in Express.js or Node.js?

• How to structure an application?

  • Related: Breaking up the app.js file

• How to get POST a query

• How to allow CORS?

• Node.js Error: Can't set headers after they are sent

• Error handling - This topic comes up a lot in various guises. Link is to a Joyent doc. Can we extract the stuff that's most relevant to Express?