Damn...i forgot about for...of operator. Used classic for

This mutates arr outside the function ( not just the output but the original reference too! )

You can avoid this with by first duplicating arr ( with something like array.slice() )

FYI this won't work in strict mode without let [a,b]. See my fork.

Wow, a pithy solution !

I think the same can be done like this:
return root%1 ? -1 : Math.pow(++root, 2);

since root%1 would return a non zero value (i.e. it's remainder) if it not completely divisible.