jQuery-style Instantiation: Decorate a Constructor Function to Enforce `new` and `return this`

decorateCtor.js

// Takes a function and wraps it, making sure:
//
// * It enforces `new`
// * It returns `this`
//
// Doing this enables a jQuery coding style for object creation,
// where constructor functions become chain-able immediately after
// construction, and don't have to be called with `new`. For example:
//
//   var F = decorateCtor(function (a, b, c) { ... });
//   F.prototype.rock = 'on';
//   F(1, 2, 3).rock;
//   >> 'on'
var decorateCtor = (function () {
  // Fast lookups.
  var bind = Function.prototype.bind,
      slice = Array.prototype.slice;

  return function (callback) {
    // Return a new function -- the "decoration" wrapper function.
    return function Ctor () {
      if (!(this instanceof Ctor)) {
        // Capture the arguments in array format.
        var args = slice.call(arguments);
        // Add `null` to the beginning.. this is our "context"
        // for bind -- i.e. no context, since calling new overrides
        // bound contexts but not bound arguments.
        // See <http://dmitrysoshnikov.com/notes/note-1-ecmascript-bound-functions/#constructor-with-various-number-of-arguments>
        args.unshift(null);
        // Bind the arguments (in array format) to the constructor,
        // creating a new bound function.
        var boundCtor = bind.apply(Ctor, args);
        // Initialize the constructor with bound arguments.
        return new boundCtor();
      }
      callback.apply(this, arguments);
      return this;
    };
  };
})();

If you use this in older browsers, you'll also want to include a shim for Function.prototype.bind (don't worry, it's pretty small).

Some background on the technique:

• MDN: Bound functions as constructors
• Experimenting with Function.prototype.construct