How to debug "uncaught exception: undefined (unknown)" in Firefox

Question

I have this line come up in the console, only in Firefox, from my JavaScript application I'm developing:

It seems relatively harmless, but I'm curious if there's any way to deduce its origin, because it must come from somewhere, even if it claims 'unknown'. Wrapping the entire script in a try/catch block and toggling Firefox's "Pause on Exception" setting doesn't do anything, which seems to imply it's a special exception? I have some ideas what parts of my code might be causing it that are using Working Draft APIs, but I'm more interested in why it reports this way and what I can do about it. Does Firefox not provide any more detail?

Answer 1

There's a few ways you could try to squash this bug.

One thing that's very tedious but will get you the line number of the exception is code that looks like:

foo();
console.log("Line 1");
bar();
console.log("Line 2");
baz();
console.log("Line 3");

and so on, and if you get this in the console:

Line 1
Line 2
Uncaught exception: undefined

then you know that baz() was causing the error. Another way is to use the debugger, like so:

debugger;
foo();
bar();
baz();

and you can use firefox's debugger to go over each line and see which one throws the error to the console.

If you have a lot of code, you could try the divide-and-conquer trick, like this:

var fooStr = foo();
var fooArr = fooStr.split("");
fooArr = fooArr.reverse();
foo(fooArr.join(""));
console.log("Block one");

var barStr = bar();
var barArr = barStr.split("");
barArr = barArr.reverse();
bar(barArr.join(""));
console.log("Block two");

var bazStr = baz();
var bazArr = bazStr.split("");
bazArr = bazArr.reverse();
baz(bazArr.join(""));
console.log("Block three");

Then, if the console looks like this:

Block one
Uncaught exception: undefined

Then the problem is in block 2. Then, you could do this:

var barStr = bar();
console.log("Line 1");
var barArr = barStr.split("");
console.log("Line 2");
barArr = barArr.reverse();
console.log("Line 3");
bar(barArr.join(""));
console.log("Line 4");
console.log("Block two");
console.log("Line 5");

And if you see:

Line 1
Uncaught exception: undefined

Then you know that var barArr = barStr.split(""); is your problem. From then, you might want to log variable values, like this:

console.log(barStr);
var barArr = barStr.split("");

And if you see this in the console:

undefined
Uncaught exception: undefined

Then you know that bar() is returning undefined (instead of a string), which does not have a split method. Then you look at the code of bar to determine if, say you forgot a parameter? Mabey bar looks like this:

function bar(value){
    return strings[value];
}

and strings is an object with something in it. Therefore, strings[undefined] will return undefined, which does not have a split method. Bug squashed!