Cannot read property 'Symbol(Symbol.iterator)' of undefined

I'm trying to loop through an array to check if it contains any item that passes a specified function. I do this by adding a .any() prototype to the Array object:

Array.prototype.any = (comparator) => {
    for(let item of this){
        if(comparator(item)){
            return true;
        }
    }
    return false;
};

Then calling Array.any() like:

else if(users && users.any((user) => user.userName === user.userName)){
        res.status(400).send('Username already in use');
}

This however gives me the following error:

TypeError: Cannot read property 'Symbol(Symbol.iterator)' of undefined
at Array.any (C:\dev\nodejs\testproject\dist\routes\userRoutes.js:29:39)
at C:\dev\nodejs\testproject\dist\routes\userRoutes.js:87:56
at Query.<anonymous> (C:\dev\nodejs\testproject\node_modules\mongoose\lib\model.js:3748:16)
at C:\dev\nodejs\testproject\node_modules\kareem\index.js:277:21
at C:\dev\nodejs\testproject\node_modules\kareem\index.js:131:16
at _combinedTickCallback (internal/process/next_tick.js:67:7)
at process._tickCallback (internal/process/next_tick.js:98:9)

The error seems to me like it is suggesting 'this' in the prototype function is undefined, but 'this' is the users array for which i checked for undefined.

Not realy sure what is actually causing the issue, hope you can help.

via Ids van der Zee