Doea code size really matter to node.js ? And how common was commonjs (no pun intended) on the client before ES6 ? Also doesn't commonjs bundling add a significant overhead when talking 5 line function modules ?
Quite common actually (see Browserify)! In fact, the increasingly widespread use of npm and CommonJS on the client is one of the factors that motivated the npm team to transition to flat, auto-deduplicated modules in version 3.
There is little reason to bundle node.js code. It's an optimization, and a dubious one. In my experience, speed of execution isn't impacted at all. I haven't tested the memory footprint, but it seems vanishingly unlikely that dead code elimination would have any substantial effect.
There's probably not any overhead in bundling, though. Not in speed or memory, at least. The overhead is in the complexity: the programmer now has one more place to check for faults, and debugging stack traces now point to a bundled version of code instead of the original sources.
The case where none of this is true is when require() ends up being called thousands of times. Startup time will be correspondingly slow, and bundling is the cure. But that should only be done as a last resort, not a preemptive measure.
