Question

In javascript, what is the difference between:

var singleton = function(){ ... }

and

var singleton = new function(){ ... }

?

Declaring priviliged functions as described by crockford (http://www.crockford.com/javascript/private.html) only works using the latter.

Answer 1

The difference is mainly that in your second example, you are using the Function Expression as a Constructor, the new operator will cause the function to be automatically executed, and the this value inside that function will refer to a newly created object.

If you don’t return anything (or you don’t return a non-primitive value) from that function, the this value will be returned and assigned to your singleton variable.

Privileged methods can also be used in your second example, a common pattern is use an immediately invoked function expression, creating a closure where the private members are accessible, then you can return an object that contains your public API, e.g.:

var singleton = (function () {
  var privateVar = 1;

  function privateMethod () {/*...*/}

  return { // public API
    publicMethod: function () {
      // private members are available here
    }
  };
})();