To start, you need to know that JavaScript has type coercion. Numbers can become strings, strings can become numbers, and arrays can become strings.

Thus, for example: [] in a string context becomes "". "" in a number context becomes 0. Thus if we were to do: "" - 1 we'd get the number -1. Additionally if we were to do [] - 1 we'd get -1. ++ is also able to coerce a string into a number.

If ++ is able to turn a string into a number then you're probably wondering why they just don't do ++[] and be done with it (to get 1, that is). The problem is that ++ is attempting to assign that resulting value back somewhere and since [] doesn't exist anywhere yet (it's temporary) it throws an error instead.

We could do something like this:

    var a = [];
    ++a; // 1

But we can't actually assign the result to a variable because we can't use alphanumeric characters. So what's another way in which we can assign the results to a variable? We can stick it into an array.

    var a = [ [] ];
    ++a[ 0 ]; // 1

This also works:

    ++[ [] ][ 0 ];

Now, we can't use 0 because it's alphanumeric so we swap 0 for +[] (+ turns the string "" into the number 0, [] turns into the string ""). Thus we get the final result:

    ++[ [] ][ +[] ]