如果您不需要保留订单，并考虑45和"45"保持一致：

function union_arrays (x, y) {

  var obj = {};

  for (var i = x.length-1; i >= 0; -- i)

     obj[x[i]] = x[i];

  for (var i = y.length-1; i >= 0; -- i)

     obj[y[i]] = y[i];

  var res = []

  for (var k in obj) {

    if (obj.hasOwnProperty(k))  // <-- optional

      res.push(obj[k]);

  }

  return res;

}



console.log(union_arrays([34,35,45,48,49], [44,55]));