coffeescript and javascript handle return values in constructors differently

I was once under the false impression that it didn't matter what value a constructor returned with used with new as in p = new Person()

But, I've told by the gurus at the node.js and coffee-script github sites that this is not so.

In general, with coffeescript it's best to use class as in: class Person ...

But, if one tries to create a function that acts as a constructor in CoffeeScript, there is at least one gotcha. The follow three short scripts demonstrate the problem, which apparently the writers of CoffeeScript and JavaScript will simply have to deal with. (The gurus who create/maintain these languages don't think its their problem and closed my bug report immediately.)

So, Mark, as someone who writes books on CoffeeScript, I hope the following might help you warn your readers. (BTW: like your book)

#--------------------------   person.coffee -----------------------------------

Person = (name) ->
    @name = name
    @greet_friend = (friend) ->
        console.log("Hello #{friend}. My name is #{@name}.")
    ###
    wierd CoffeeScript/Node.js error:
    You will get and error if you comment the next line out
    and explicitly return the greet_friend method above.
    ###
    return null



Person.prototype.hello_world =  ->
    console.log("#{@name} says hello world.")


p = new Person('Fred')
p.greet_friend('Mary')
p.hello_world()

#-------------------------   bad_person.coffee ---------------------
Person = (name) ->
    @name = name
    @greet_friend = (friend) ->
        console.log("Hello #{friend}. My name is #{@name}.")
    ###
    wierd CoffeeScript v1.6.1/Node.js v0.8.22 error:
    This will bomb because we've commented out
    the 'return null' line below, which means
    that CoffeeScript will explicitly return
    the greet_friend method above.

    return null
    ###

Person.prototype.hello_world =  ->
    console.log("#{@name} says hello world.")


p = new Person('Fred')
p.greet_friend('Mary')
p.hello_world()

#------------------- redeemed_bad_person.js -----------------------

// Generated by CoffeeScript 1.6.1 
// but modified by me. I removed the
// explicit return of the greet_friend
// method.

(function() {
  var Person, p;

  Person = function(name) {
    this.name = name;
    this.greet_friend = function(friend) {
      return console.log("Hello " + friend + ". My name is " + this.name + ".");
    };
    /*
        wierd CoffeeScript v1.6.1/Node.js v0.8.22 error:
        This version works because we do not
      explicitly return the greet_friend 
      method above.
    */

  };

  Person.prototype.hello_world = function() {
    return console.log("" + this.name + " says hello world.");
  };

  p = new Person('Fred');

  p.greet_friend('Mary');

  p.hello_world();

}).call(this);

Comece comparando person.coffee, bad_person.coffee e redeemed_bad_person.js, concentrando-se em como os valores de retorno dos construtores afetam new. Confirme o comportamento com as versões descritas do CoffeeScript e do Node.js; considera-se concluído quando a armadilha e o padrão de construtor recomendado estiverem documentados no material relevante do livro.