java-script arrow function returns (x++,x) [duplicate]

Question

i don't know how the code:const countFrom = x => () => (x++, x); from here, works:

const countFrom = x => () => (x++, x);
let a = countFrom(1)

console.log('output:', a()) // output: 2
console.log('output:', a()) // output: 3
console.log('output:', a()) // output: 4
console.log('output:', a()) // output: 5

.as-console-wrapper {min-height: 100%!important; top: 0;}

Answer 1

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;