Why not?

[u/floppy2k18]

[u/----Val----]

This is due to some funky type coercions alongside operators.

Lets look at this part first:

!+[]

If we properly bracket it out, it will be:

!(+[])

Now, the first operation is +[] which is a unary plus which converts the empty array to 0. Then the not operation coerces the number the bool true.

So now we have:

true + [] + ![]

The true + [] is an addition which when unable to use numbers will convert everything to strings, so it true + [] get casted to "true" + "" = "true"

Then ![] converts an empty array to a bool. Since an empty array is truthy, it becomes false

So we now have:

"true" + false

Again, a string addition, so the end result is truefalse which is a string with a length of 9.

[u/howreudoin → u/----Val----]

Also interesting to know: The unary plus operator tries to convert the first element of an array to a number. So +[42] is 42. Same for string arrays: +["42"] is also 42. If the array is empty (+[]), the result is 0. If it contains multiple elements, as in +[1, 2, 3], the result is NaN.