What do I misunderstand about instance methods?

So I have a schema with this method:

UserSchema.methods.comparePassword = (candidatePassword) => {
    let candidateBuf = Buffer.from(candidatePassword, 'ascii');
    if (sodium.crypto_pwhash_str_verify(this.password, candidateBuf)) {
        return true;
    };
    return false;
};

it's called thusly:

User.find({ username: req.body.username }, function(err, user) {
    isValid = user[0].comparePassword(req.body.password);
    // more stuff
}

This results in Error: argument hash must be a buffer

I am able to verify that user[0] is a valid user, obviously because it sucessfully calls the comparePassword method, and it's the libsodium function that fails.

Further test has shown that this.password is undefined. In fact, this is undefined in the comparePassword method. My understanding was that this would refer to the object from which the method is called, in this case, user[0].

So what's the correct way to refer to the object calling it's own instance method?

via The E