why got it this result in array.filter? javascript

Question

JAVASCRIPT
var c = new Array(1, 2, 3, 4, 5, 6);
var b = c.filter(function(a) {
return (a % 2) - 1;
});

//output [2,4,6]

i dont understand it

Answer 1

xaralampis_ Why you are going on profile pic?

Answer 2

Pip?