Javascript Vector constructor

vector.js

function Vector(x, y) {
  this.x = x;
  this.y = y;
}

Vector.prototype.plus = function(vector) {
  var x_new = this.x + vector.x;
  var y_new = this.y + vector.y;
  return new Vector(x_new, y_new);
}

Vector.prototype.minus = function(vector) {
  var x_new = this.x - vector.x;
  var y_new = this.y - vector.y;
  return new Vector(x_new, y_new);
}

Object.defineProperty(Vector.prototype, "length", {
  get: function() { return Math.sqrt(this.x*this.x + this.y*this.y); }
});