Lets start with the last part and write it more verbose
var arr = [4,5,6];
var value = arr[1];
Is the same as
[4,5,6][1]
and as arrays are zero based, that gives 5, so we really have
[1,2] + 5;
The first part is equal to
[1,2].toString(),
and that returns the string "1,2" because the array is converted to a string when trying to use it in an expression like that, as arrays can't be added to numbers, and then we have
"1,2" + 5
Concatenating strings with numbers gives strings, and adding the strings together we get
"1,25"
