1.1. Classes as first-class objects: We saw that functions are first class objects, but did you know classes also are? (Though, of course, speaking of classes as objects does sound weird...) Study this example and see what makes it tick! Be careful: there's some purposefully weird code in it:

      const makeSaluteClass = term =>
          class {
              constructor(x) {
                 this.x = x;
              }

              salute(y) {
                 console.log(`${this.x} says "${term}" to ${y}`);
               }
          };

      const Spanish = makeSaluteClass("HOLA");
      new Spanish("ALFA").salute("BETA");
      // ALFA says "HOLA" to BETA

      new (makeSaluteClass("HELLO"))("GAMMA").salute("DELTA");
      // GAMMA says "HELLO" to DELTA

      const fullSalute = (c, x, y) => new c(x).salute(y);
      const French = makeSaluteClass("BON JOUR");
      fullSalute(French, "EPSILON", "ZETA");
      // EPSILON says "BON JOUR" to ZETA

1.2. Factorial errors: Factorials, as we defined them, should only be calculated for non-negative integers. However, the function we wrote doesn't verify if its argument is valid or not. Can you add the necessary checks? Try to avoid repeated, redundant tests!

1.3. Climbing factorial: Our implementation of factorial starts multiplying by n, then by n-1, then n-2, and so on., in what we could call a downward fashion. Can you write a new version of the factorial function, that will loop upwards?