What is the difference between immediately executed functions and using the keyword new?

Question

While I am learning about javascript closure, I ran into an example on w3schools that uses immediately executed functions to provide private variables for functions:

var add = (function () {
    var counter = 0;
    return function () {return counter += 1;}
})();

but to me, all immediately executed function does is executing the statements in the body, which, if I recall correctly can be accomplished by the keyword new

var add = new function () {
    var counter = 0;
    return function () {return counter += 1;}
};

document.write("return" + add()+ "<br>"); // return 1
document.write("return" + add() +"<br>"); // return 2, works the same as the example on w3school

new is obviously more complicated, it creates a new object...but I think you are not obligated to use that object (add a return statement at the end).

My question is: what can be done with immediately executed function but not by keyword new? or can you always replace the 1st using the later?

Answer 1

f() and new f are almost identical (compare this and that). Of course, new does a lot more work preparing an object you're going to throw away.

You cannot blindly replace f() with new f if

• you're using this in f (obviously)
• you're returning non-objects

For example, these two calls will produce different results:

x = 5 + (function () {
    return 123;
})();

y = 5 + new function () {
    return 123;
};

In other cases, if you don't mind wasting resources and validators shouting at you, new is fine.