Question

What is the difference, pros/cons (if any) between these constructs?

 new function(obj) {
     console.log(obj);
 }(extObj);

vs

 (function(obj) {
     console.log(obj);
 })(extObj);

Answer 1

If you’re looking at dealing expressly with an immediately-invoked function, then there isn’t any real difference between the two, except that the first one will return this inside the function, to the outside world, automatically (that is what new does), and by default if a different return value is not specified (regular functions return undefined).

The rest of the major differences are not really going to matter — access to the prototype chain is going to be pointless, having the returned this.constructor point to the anonymous function would give you access to caching the anonymous function for use at a later time (like removing it from an event listener, if you somehow managed to stick the enclosed function in… …that would be a trick in and of itself).

Allowing people to cache your immediately-invoked function as a constructor property of the returned this object might be a security-risk… …or it might be really useful… …in very specific scenarios.

Every-day purposes of firing code in-line — no real difference.