It'd be nice to be able to blur the lines. Maybe implement multireturn by declaring variables public inside your closure scope.

  function Person(firstName, lastName) {
    let fName = firstName; 
    let lName = lastName;

    public getFullName = function() { 
      return fName + ' ' + lName;
    };
    
    public setFirstName = function(firstName) { 
      fName = firstName;
    };
    
    return this;
  }

("return this" is probably not the best syntax at the end there, especially if bolting this onto javascript.)