I can\'t wrap my mind around this quirk.
[1,2,3,4,5,6][1,2,3]; // 4
[1,2,3,4,5,6][1,2]; // 3
I know [1,2,3] + [1,2] = \"1,2,31,2\"
Because (1,2) == 2
. You've stumbled across the comma operator (or simpler explanation here).
Unless commas appear in a declaration list, parameter list, object or array literal, they act like any other binary operator. x, y
evaluates x
, then evaluates y
and yields that as the result.
[1,2,3,4,5,6][1,2,3];
Here the second box i.e. [1,2,3]
becomes [3]
i.e. the last item so the result will be 4
for example if you keep [1,2,3,4,5,6]
in an array
var arr=[1,2,3,4,5,6];
arr[3]; // as [1,2,3] in the place of index is equal to [3]
similarly
*var arr2=[1,2,3,4,5,6];
// arr[1,2] or arr[2] will give 3*
But when you place a + operator in between then the second square bracket is not for mentioning index. It is rather another array That's why you get
[1,2,3] + [1,2] = 1,2,31,2
i.e.
var arr_1=[1,2,3];
var arr_2=[1,2];
arr_1 + arr_2; // i.e. 1,2,31,2
Basically in the first case it is used as index of array and in the second case it is itself an array.
[1,2,3,4,5,6][1,2,3];
^ ^
| |
array + — array subscript access operation,
where index is `1,2,3`,
which is an expression that evaluates to `3`.
The second [...]
cannot be an array, so it’s an array subscript operation. And the contents of a subscript operation are not a delimited list of operands, but a single expression.
Read more about the comma operator here.