return (a, b, …, z)

It will return z , evaluating a, b, ... first. At first it seems not very useful, but consider this example:

const fun = x => x + 1

What if you want to add a console.log call before x + 1 ? With comma operator, adding side-effects to functional expressions is simple:

const fun = x => (console.log (x), x + 1)

And in comparison with…

const fun = x => { console.log (x); return x + 1 }

…it requires fewer modifications.

P.S. you could actually create a wrapper over console.log that returns its first argument:

const fun = x => log (x) + 1

There’s even a library that does that (and much more): Ololog