Cannot read property 'Symbol(Symbol.iterator)' of undefined
The only answer here is that you didn't use "function", so your "this" is not your "users". This would work:
Array.prototype.any = function(comparator) { for(let item of this){ if(comparator(item)){ return true; } } return false;};And then, of course, just use "some".
Using Array.prototype.any() was unnecesary as I was using mongoose to get the users so I changed it to have mongoose try to get a single user that has the secified username and checking if that was defined. Like:
const checkUniqueUserName = (user, res, done) => { User.findOne({"userName": user.userName}, (err, foundUser) => { if(err){ res.sendStatus(500); console.log(err); } else if(foundUser){ res.status(400).send('Username already in use'); } else{ done(user); } });};