A very old thread in stackoverflow to add some redundant info to the already existing answers - https://stackoverflow.com/questions/29927278/array-prototype-map-and-array-prototype-foreach

Why should you be worried? No one is stopping you from learning them all, are they? I'm a (Core java and JEEE) Java Developer by my job profile, but I work on Android, Node.js and some of the UI frameworks - ExtJs, AngularJs, React and even Haskell. In short, learn whichever you like.

Koa and Hapi are still maturing. Probably will need some time. And a lot of the popular packages are built dependent on Express. Thus, at this point of time, I'll always go for Express.