var a = [1,4,78,98];

var b= [45,56,3];

var c = [2,3,45];

​

function union() {

  var arrs = [].concat.apply([], arguments);

  return arrs.filter(function(v, i) {

    return arrs.indexOf(v) == i;

  });

}

​

console.log('Custom:', union(a,b,c));

console.log('Unders:', _.union(a,b,c));