assignment operators

Question

in this code:

var x = 2
var y = 3
x +=y
console.log(x)

how JS knows that it should calculate x from 3-rd row of code?
and not the x from first row?

or here:
var x = 2
var y = 3
x -= y += 9
console.log(x)

why on earth the output is: -10???
x-=y that will be in my case: 2-3=-1
and y+=9 that will be 3+9=12

Answer 1

Yes, that's right. Variable <x> at line 3 and 4 are the same <x> that was initially defined at line 1. Javascript knows this by differentiating variable scope, a specification that defines where a certain variable is recognisable.

https://www.w3docs.com/learn-javascript/variable-scope.html

Answer 2

I didn't get your doubt about the first snippet. Can you elaborate further?

About second snippet, the += and -= operator associativity is right-to-left. So ...

x -= y += 9;

Will be processed from right to left, like so ...

x -= ( y += 9 );
x -= ( 3 + 9 );
x -= ( 12 ); // this is <x> (2) - (12) => -10

http://www.scriptingmaster.com/javascript/operator-precedence.asp

Answer 3

thank you :)
you gave me some hope to try further :)

my first doubt, regards to this: in this code, te X is used twice. In first row as var and in third (x+=y).

var x = 2
var y = 3
x +=y
console.log(x)

How JS knows that when I wrote console.log(x) it regards x from third row, and not x from first (when it is defined as variable). For me those are the same values...