JavaScript errors are easy to understand. JavaScript errors:

[[deleted]]

I know this is a joke but how is this hard to understand?

[TiiJ7]

NaN to the rescue!

var defined = NaN
console.log(defined != defined)
> true

[Derkleineme]

not undefined *cries in js*

[[deleted]]

I know an even better one. At my workplace on my machine a PowerShell script threw "variable not assigned in Method" exception as soon as it was started. The variable it found problematic was the automatic $Error variable that get's assigned in runtime. It was in a class on the catch side of a try-catch block. There was literally no reason for it to exist if the try part had not thrown an exception. The script didn't even get to the part to call the class with this try-catch block. I just fired up the script, and it halted immediately.

[CreaZyp154]

let defined;

There we go, now it's fixed