That's Javascript for you.

Summing arrays calls `.toString()` on them, and two empty string gives you a new empty string.

    [].toString() === ''
    [] + [] 
        === '' + '' 
        === ''

https://github.com/denysdovhan/wtfjs#adding-arrays

The + operator doesn’t work on arrays, but tries to cast the sides of it to a string. Arrays implement the toString function, which shows the toString of the individual values comma separated. An empty array returns an empty string. Which means that this is equivalent to empty string + empty string.

[ ‘foo’, ‘bar ] + [ ‘baz’ ] would return ‘foo,barbaz’