Vitaly Gordon explains how the little known comma operator makes it easy to add some debugging statements in short hand array functions.

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)

https://medium.com/@xpl/javascript-did-you-know-about-the-comma-operator-ff9b511cc33