Named Function Expressions in IE, part 2

I asked this question a while back and was happy with the accepted answer. I just now realized, however, that the following technique:

var testaroo开发者_运维知识库 = 0;
(function executeOnLoad() {
    if (testaroo++ < 5) {
        setTimeout(executeOnLoad, 25);
        return;
    }
    alert(testaroo); // alerts "6"
})();

returns the result I expect. If T.J.Crowder's answer from my first question is correct, then shouldn't this technique not work?

---

A very good question. :-)

The difference:

The difference between this and your detachEvent situation is that here, you don't care that the function reference inside and outside "the function" is the same, just that the code be the same. In the detachEvent situation, it mattered that you see the same function reference inside and outside "the function" because that's how detachEvent works, by detaching the specific function you give it.

Two functions?

Yes. CMS pointed out that IE (JScript) creates two functions when it sees a named function expression like the one in your code. (We'll come back to this.) The interesting thing is that you're calling both of them. Yes, really. :-) The initial call calls the function returned by the expression, and then all of the calls using the name call the the other one.

Modifying your code slightly can make this a bit clearer:

var testaroo = 0;
var f = function executeOnLoad() {
    if (testaroo++ < 5) {
        setTimeout(executeOnLoad, 25);
        return;
    }
    alert(testaroo); // alerts "6"
};
f();

The f(); at the end calls the function that was returned by the function expression, but interestingly, that function is only called once. All the other times, when it's called via the executeOnLoad reference, it's the other function that gets called. But since the two functions both close over the same data (which includes the testaroo variable) and they have the same code, the effect is very like there being just one function. We can demonstrate there are two, though, much the way CMS did:

var testaroo = 0;
var f = function executeOnLoad() {
    if (testaroo++ < 5) {
        setTimeout(executeOnLoad, 0);
        return;
    }
    alert(testaroo); // alerts "6"

    // Alerts "Same function? false"
    alert("Same function? " + (f === executeOnLoad));
};
f();

This isn't just some artifact of the names, either, there really are two functions being created by JScript.

What's going on?

Basically, the people implementing JScript apparently decided to process named function expressions both as function declarations and as function expressions, creating two function objects in the process (one from the "declaration," one from the "expression") and almost certainly doing so at different times. This is completely wrong, but it's what they did. Surprisingly, even the new JScript in IE8 continues this behavior.

That's why your code sees (and uses) two different functions. It's also the reason for the name "leak" that CMS mentioned. Shifting to a slightly modified copy of his example:

function outer() {
    var myFunc = function inner() {};

    alert(typeof inner); // "undefined" on most browsers, "function" on IE

    if (typeof inner !== "undefined") { // avoid TypeError on other browsers
        // IE actually creates two function objects: Two proofs:
        alert(inner === myFunc); // false!
        inner.foo = "foo";
        alert(inner.foo);        // "foo"
        alert(myFunc.foo);       // undefined
    }
}

As he mentioned, inner is defined on IE (JScript) but not on other browsers. Why not? To the casual observer, aside from the two functions thing, JScript's behavior with regard to the function name seems correct. After all, only functions introduce new scope in Javascript, right? And the inner function is clearly defined in outer. But the spec actually went to pains to say no, that symbol is not defined in outer (even going so far as to delve into details about how implementations avoid it without breaking other rules). It's covered in Section 13 (in both the 3rd and 5th edition specs). Here's the relevant high-level quote:

The Identifier in a FunctionExpression can be referenced from inside the FunctionExpression's FunctionBody to allow the function to call itself recursively. However, unlike in a FunctionDeclaration, the Identifier in a FunctionExpression cannot be referenced from and does not affect the scope enclosing the FunctionExpression.

Why did they go to this trouble? I don't know, but I suspect it relates to the fact that function declarations are evaluated before any statement code (step-by-step code) is executed, whereas function expressions — like all expressions — are evaluated as part of the statement code, when they're reached in the control flow. Consider:

function foo() {

    bar();

    function bar() {
        alert("Hi!");
    }
}

When the control flow enters function foo, one of the first things that happens is that the bar function is instantiated and bound to the symbol bar; only then does the interpreter start processing the statements in foo's function body. That's why the call to bar at the top works.

But here:

function foo() {

    var f;

    f = function() {
        alert("Hi!");
    };

    f();
}

The function expression is evaluated when it's reached (well, probably; we can't be sure some implementations don't do it earlier). One good reason the expression isn't (or shouldn't be) evaluated earlier is:

function foo() {

    var f;

    if (some_condition) {
        f = function() {
            alert("Hi (1)!");
        };
    }
    else {
        f = function() {
            alert("Hi! (2)");
        };
    }

    f();
}

...doing it earlier leads to ambiguity and/or wasted effort. Which leads to the question of what should happen here:

function foo() {

    var f;

    bar();

    if (some_condition) {
        f = function bar() {
            alert("Hi (1)!");
        };
    }
    else {
        f = function bar() {
            alert("Hi! (2)");
        };
    }

    f();
}

Which bar gets called at the beginning? The way the specification authors chose to address that situation was to say that bar is not defined in foo at all, hence side-stepping the issue entirely. (It's not the only way they could have addressed it, but it seems to be the way they chose to do so.)

So how does IE (JScript) process that? The bar called at the beginning alerts "Hi (2)!". This, combined with the fact we know two function objects are created based on our other tests, is the clearest indication that JScript processes named function expressions as function declarations and function expressions, because that's exactly what is supposed to happen here:

function outer() {

    bar();

    function bar() {
        alert("Hi (1)!");
    }

    function bar() {
        alert("Hi (2)!");
    }
}

There we have two function declarations with the same name. Syntax error? You'd think so, but it isn't. The specification clearly allows it, and says that the second declaration in source code order "wins." From Section 10.1.3 of the 3rd edition spec:

For each FunctionDeclaration in the code, in source text order, create a property of the variable object whose name is the Identifier in the FunctionDeclaration...If the variable object already has a property with this name, replace its value and attributes...

(The "variable object" is how symbols get resolved; that's a whole 'nother topic.) It's just as unambiguous in the 5th edition (Section 10.5), but, um, a lot less quotable.

So it's just IE, then?

Just to be clear, IE isn't the only browser that has (or had) unusual handling of NFEs, although they're getting pretty lonely (a pretty big Safari issue has been fixed, for instance). It's just that JScript has a really big quirk in this regard. But come to that, I think it actually is the only current major implementation with any really big issue — be interested to know of any others, if anyone knows of them.

Where we stand

Given all of the above, for the moment, I stay away from NFEs because I (like most people) have to support JScript. After all, it's easy enough to use a function declaration and then refer to it later (or indeed, earlier) with a variable:

function foo() { }
var f = foo;

...and that works reliably across browsers, avoiding issues like your detachEvent problem. Other reasonable people solve the problem differently, just accepting that two functions will get created and trying to minimize the impact, but I don't like that answer at all because of exactly what happened to you with detachEvent.

---

Well, it will work, the problem with JScript (IE), is that the identifier of the function expression (executeOnLoad) will leak to its enclosing scope, and actually creating two function objects..

(function () {
  var myFunc = function foo () {};
  alert(typeof foo); // "undefined" on all browsers, "function" on IE

  if (typeof foo !== "undefined") { // avoid TypeError on other browsers
    alert( foo === myFunc ); // false!, IE actually creates two function objects
  }
})();