Question

I have been using coffeescript classes, and as far as I was aware, functions should always return the last statement automatically. I am finding however, that functions defined in the constructor object, are not returning anything by default. Why is this?

CoffeeScript

constructor: ->
  @ # <~~ returned as expected

class MyClass
  constructor: ->
    @ # <~~ not returned - why?

class MyClass
  constructor: ->
    return ->
      @ # <~~ returned like normal!

JavaScript

var MyClass;

({
  constructor: function() {
    return this;
  }
});

MyClass = (function() {

  function MyClass() {
    this;

  }

  return MyClass;

})();

MyClass = (function() {

  function MyClass() {
    return function() {
      return this;
    };
  }

  return MyClass;

})();

Answer 1

The “constructor” method is called when you use the “new” keyword on a class – therefore the return value is always supposed to be the new instance, not some other value that you’d like to return. However, it seems like CoffeeScripts lets you force a different return value via the “return” keyword.

class Foo
  constructor: -> {}

f = new Foo
f instanceof Foo   # expected to be very true...

When you name a normal function “constructor” (not a method = outside of a class definition), it’s not a constructor but a regular function and therefore returns as expected.