Perhaps it is simpler to extend javascript's native Array, so that there is no need for keeping around an extra Vector.arr property. Here is a simple implementation called for learning purposes that boils down to this, in modern JS:

class Vector extends Array {
  // example methods
  add(other) {
    return this.map((e, i) => e + other[i]);
  }
}

// example usage
let v = new Vector(1, 2, 3);
console.log(v.add(v));

This class inherits Array's constructor. Note passing in a single value creates an empty Array of that length and not a length 1 Array. Vector would require a super call in the constructor to inherit exotic Array behavior, such as having a special length property, but this shouldn't be needed for a math vector of fixed length. You can include fancier constructor behavior here, such as being able to construct from an Array as input.