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;}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;
const counter = x => () => ++x; yet…
x = x + 1 was easier to understand, would've chosen ++x if I'd write it for myself though
//const countFrom = x => () => (x++, x);
//Lets refactor the code a bit:
function countFrom(x){
return function(){
x++;
return x;
}
}
let a = countFrom(1)
//now a is the returned function at line 5, and x is stored as a local variable.
console.log('output:', a()) // output: 2 you call the function at line 5 so it adds 1 to x and returns it.as-console-wrapper {min-height: 100%!important; top: 0;}It's quite simple if you understand some aspects:
()=> (x++,x) (see what closure is)return x, that is => (x) is the same as => return x
countFromreceivesxas an argument and then returns a closure that when called increments x and returns its value.(x++, x)is just shorthand for{ x++; return x; }