Question

I was writing some CoffeeScript just now, and getting a strange error:


TypeError: Thing(param) is not a constructor


But it is! And when I try it in the console:

var that = new Thing(param);
that.doesSomething();

After a bit of confusion, I looked through the compiled source and found out that coffee compiles that = new Thing param to that = new(Thing(param));. Weird; I’ve never seen that before. So I promptly try it: and tada! Now I can replicate:

var that = new(Thing(param));
that.previousLineErrorsOut();

(Incidentally, the CoffeeScript generator on its home page generates the new Thing() form. The plot thickens…)

I also try it out with native constructors (new Worker("somefile") and new(Worker("somefile"))), which behave “correctly”, that is, there’s no difference between the two forms.

So I’m thoroughly confused: what’s new()? Why is it failing in some cases? Why does CoffeeScript transform my perfectly fine new into new()?

Answer 1

new takes an expression representing a constructor and optionally a list of arguments enclosed in parentheses. For example:

new Thing;   // equivalent to next line
new Thing(); // most common form
new (function() {})(); // calls the anonymous function as a
                       // constructor with no arguments
new (function() {});   // equivalent to previous; if no arguments are
                       // specified to new, it will call with no arguments

When you do this:

new(Thing(param));

It’s trying to run the result of calling Thing with the argument param as a constructor with no arguments. The parentheses after the new make Thing(param) the expression representing the constructor. As Thing does not return a constructor in your case, it fails. It’s roughly equivalent to this:

var clazz = Thing(param);
var instance = new clazz();

I do not know why CoffeeScript transforms it that way.