That's Javascript for you. Summing arrays calls `.toString()` on them, and two empty string gives you a new empty string. [].toString() === '' [] + []

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