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
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