This article was originally published at nicholascloud.com.

Sometimes bugs can be particularly evasive, and today I had such a one.

A module in deep in our codebase was throwing an Error, but only in Mozilla's Firefox browser.

The error was NS_ERROR_ILLEGAL_VALUE.

I did some quick DuckDuckGoing and found that the error occurs when a native DOM function in Firefox is passed a value of a type it does not expect.

The stack trace led back to this line in our application code:

const hit = find( cache, c => c.original === obj );
if ( hit ) {
    return hit.copy;
}
// ...some time later...
return someUncachedObject;

"@-E$!&&@#", I thought. "Why is lodash's find() function passing a bad value to a native function?"

You see, I use lodash all the time. So much, in fact, that I made one fatal error in my diagnosis.

I assumed that because the find() function was defined, that lodash had indeed been imported.

How. Wrong. I. Was.

It turns out that window.find() is, in fact, a non-standard, but nevertheless nearly omnipresent function that is designed to search a DOM document for a particular string. And since any function attached to window is global, a missing import of the same name -- say, a missing lodash/find import -- would not raise any alarms. The code built. The code ran. And it ran without error in every browser but Firefox. Why?

The window.find() function expects a first argument of type String. In modern browsers other than Firefox (pretty much all the Chromium-based browsers), passing a non-String argument to window.find() will simply cause the function to return false. As you can see in the snippet above, though rendering the cache useless, the application nevertheless continued to work. In Firefox, however, window.find() will throw if its first argument is not of type String. Thus my bug.

I am ashamed to say how long it took me to realize lodash/find was not the function being called.

In the end I applied the great wisdom of Spock's ancestors, and started considering things that could not possibly be the case, until it dawned on me that perhaps -- just perhaps -- find() was not what it appeared to be after all.

And a single import find from "lodash/find"; statement fixed the bug.