Javascript + Firebug : "cannot access optimized closure" What does it mean? - javascript

I just got the following error in a piece of javascript (in Firefox 3.5, with Firebug running)
cannot access optimized closure
I know, superficially, what caused the error. I had a line
options.length()
instead of
options.length
Fixing this bug, made the message go away. But I'm curious. What does this mean? What is an optimized closure? Is optimizing an enclosure something that the javascript interpretter does automatically? What does it do?

I had this issue too when Firebug is running.
It seems to happen sometimes, when an exception is raised (for whatever reason) and when there's a recursive function call somewhere in the call stack. The exception gets re-raised as the mysterious "InternalError: cannot access optimized closure"
Changing the way I define the recursive function, seems to make this issue go away. eg changing from
function foo(bar) {... foo(recursively); ...}
to
var foo = function(bar) {... foo(recursively); ...}
Hope that helps.

It is a bug in Firefox happening with Firebug open:
https://bugzilla.mozilla.org/show_bug.cgi?id=505001
[An earlier answer mentioned this was due to this other bug, which I think is incorrect as that other problem was not related to Firebug.]

Seems like a Firefox bug:
https://bugzilla.mozilla.org/show_bug.cgi?id=496790

A closure is a function with context. If you dynamically create a new function, then you create a closure.
function makeAdder(int num) {
return function(int num2) { return num + num2; }
}
adder = makeAdder(5);
adder(7) // returns (5+7) = 12
adder(2) // returns (5+2) = 7
Here, the closure is the inner function, as returned by makeAdder, along with the '5' that was passed.
The javascript engine might choose to optimize away the function shown above, to make things run faster, never generating or running that code, so it can't be debugged or referenced. Optimizers are supposed to be very careful to ensure there's no impact, so I'd guess this one made a mistake.

http://ludex-marketing.com/blog/2009/12/google-analytics-javascript-error-cannot-access-optimized-closure-in-ga-js/

This can also be caused by a simple race condition. I was just refactoring a 'startup' object that does a few things before the document is ready. As soon as I tried to access a second object defined immediately below the startup object I received this error.
I was under the impression that script execution waited until all of the code was compiled. Clearly that's not the case. Waiting for document ready to call methods on the second object fixed the problem. Also, using this nice 'dump()' function confirms that the second object is only partially defined when the error occurs: http://www.openjs.com/scripts/others/dump_function_php_print_r.php

I encountered the same error today. In my case this occurred because I was referencing an object's attribute or function that did not exist or was not available. I'm guessing that since the object was available via a closure that was optimized, firebug could not access metadata on that object and thus the cryptic error message.

This also happened to me today. Firebug error'd at line 2 of this function:
function IsValidDate(objName) {
re = new RegExp('^( +|today|pdate|- *\\d+ *(day(s|)|week(s|))+ *$', 'i');
if (re.test(objName.value)) return 2;
return (chkdate(objName));
}
When I added "var " before the declaration of "re" in line 1, the error went away.

There is an exception being raised somewhere else in your code within the function that has this error. It could be as simple trying to access a variable that doesn't exist.
I think we need to get a Firebug dev in here to answer why it doesn't give a more specific error as to where in the closure that raised the exception to prompt the error.
You pasted options.length(), but it is not what prompted the error. What caused the error is the fact that your bug was inside a closure.
function(){
array.length()
}
that gives the error

Related

Debugging javascript TypeError in heavily used function, when console trace doesn't indicate full trace

I have a utility function that I use 93 times, let's call it function add(a,b){return a + b}
I am getting a TypeError: a is undefined in the console at the moment.
Usually when something similar happens, Firefox provides a trace of the line numbers of the code that calls add below the error message, allowing me to get back to the cause. However, this time there is no further information.
I've considered adding a break point in the add function, however because it gets called so much, it is impractical.
How should one trace the source of a TypeError in a heavily used javascript function?
Edit:
As pointed out by Jonas Wilms, the simplifed example will not produce a TypeError. Consider this instead:
function add(a,b){
if (typeof a != 'string'){
throw Error('my TypeError')
}
return a + b
}
Edit:
I solved this by doing ctrl-F on add( and stepping through each call until I spotted where undefined inputs were. I still have a feeling there's a better way! Or perhaps a way to arrange code so that this doesn't happen.

Is assigning new properties to built-in types actually illegal?

I had the following code that used to work fine in Chrome but has suddenly stopped working in the latest version (38.0.2125.104).
(function() {
"use strict";
var stringPrototype = String.prototype;
stringPrototype.foo = function() { this.$foo = 1234; };
var s = "abcd";
s.foo();
})();
Now I get the error (if you run the snippet, you'll see the error in the console):
Uncaught TypeError: Cannot assign to read only property '$foo' of abcd
I am interested to know if this is a new bug in Chrome or if what I had been doing is actually illegal. If it is illegal, is there any reference specification that would describe why this is so?
This has never caused an error in previous versions of Chrome.
The comments above helped me solve my problem. To sum up, the assignment I was making:
this.$foo = 1234
Was never doing anything. In the newer version of Chrome this spurious assignment actually results in an error. This seems like an improvement since it causes a failure sooner and should surface a potential bug earlier.
Thus, though I thought the newer version of Chrome was causing an error in previously working code, since my code had not ever been doing anything, I can safely remove the assignment, since the behavior must will be exactly the same.
(Answering this as a community wiki since all the heavy lifting was done by Pointy and Derek.)

No line number specified in console error message. What to do?

In a recent day I faced with this kind of error message, that has no position specified. Debugging becomes much more difficult. What did I do wrong?
When I started re-define JSON.parse functions of my objects, after that appears it first time.
I replaced them to normal functions, but the error message is still strange.
Maybe because I'm writing prototype functions, and I call them before constructing a new object?
Is it because I call a function from console?
I managed to simplify the error:
var o = {g : false};
function f(){
console.log(o.g());
}
If I call it from Chrome and form console, than I will not get any line number.

javascript - Uncaught ReferenceError: keys is not defined

I am getting an error when I run the following command in an included script. But if I run the command from the google chrome console, it works properly.
var a = {};
console.log(keys(a));
Error:
Uncaught ReferenceError: keys is not defined
What's going on here? How can I use the keys function in an included script?
console.log(keys(a))
keys() is not function provided by the browser for use in your code. You probably want Object.keys()
a = {};
console.log(Object.keys(a));
Sometimes the console has extra functions exposed to it for ease of use debugging that aren't available in your actual code. keys() sounds like one, and copy('some text') is another.
I'm failing to find a link which lists them, sadly. But I'm quite sure there are more than those 2 functions.
Whenever you get an error like this, try to search for a definition of the function/variable that's been reported as undefined. If it is defined, try looking for a reason this might not be working. Did you know that the keys function is apart of the Object constructor? You can't call it as if it's a free-standing function. Though if you get into the habit of doing this, try making your own function to allow this:
function key( object ) {
return Object.keys( object );
}
Your code should pass given a definition like this.

Using JavaScript console modifies outcome of script

In Chrome, this fiddle prints false. However, the same code typed in the JavaScript console prints true:
a = 1;
var a = 2;
console.log(delete a);​
Why do I get different results depending on whether or not I'm using the Chrome console?
Because the Chrome console runs inside an eval construct or something similar, rather than running in the global scope.
There is a lot of discussion about the delete operator here on StackOverflow. A search for [javascript] delete will help answer other questions that come up.
var defined variables can not be deleted.

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