Why does [[]][0]++ work but []++ throws run-time exception?

Question

Why does the first line work while the second line throws run-time exception?

The first line:

[[]][0]++; //this line works fine 

The second line:

[]++; //this lines throws exception 

Answer 1

[[]][0]++ 

is equivalent to

var tmp = [[]]; tmp[0] = tmp[0]+1; 

tmp[0] is an empty array, which is cast to the number 0, which increments to 1.

This only works because <array>[<index>]++ looks valid. It takes some type juggling, but it gets there.

But []++ is outright invalid. There's no way to make it make sense.

[] = []+1; 

The left-hand side here is indeed invalid. You can't assign to an empty array.