Why total 9

Question

var x =1;
var arr=[2,3,7];
arr[x]=arr[x+1];
alert(arr[0]+arr[1])

Answer 1

arr[1] = arr[1+1]
so arr[1] which was 3 becomes arr[2] which is 7
arr[0] which is 2 plus arr[1] which is 7 equals 9

Answer 2

thx so much I understand all