java-script arrow function returns (x++,x)

x is a variable inside the outer (x =>) function, therefore all inner functions (() => (x++, x)) share the same variable. x++ post increments that variable whenever the inner function executes. The comma operator (..., x) evaluates to the last comma seperated expression, x in this case.

It is maybe easier to understand without the comma operator:

 const counter = x => () => x = x + 1;