使用_this引用父类方法

Question

I'm using the prefix _this to refer to a parent class function when calling it from a callback function. However, when I run the node server I get the error message '_this is not defined'. I've actually pulled this code directly from a MEAN stack book I'm learning from but I can't seem to get around this.

Parent Function

var getErrorMessage = function(err){
var message = '';
if(err.code){
    switch(err.code){
        case 11000:
        case 11001:
        message = 'Username already exists';
        break;
        default:
        message = 'Something went Wrong';
    }
} else{
    for(var errName in err.errors){
        if(err.errors[errName].message){
            message = err.errors[errName].message;
        }
    }
}

return message;
};

Function being called in a callback later on

exports.saveOAuthUserProfile = function(req, profile, done){
User.findOne({provider: profile.provider, providerId: profile.providerId}, function(err,user){
    if(err){
        return done(err);
    } else{
        if(!user){
            var possibleUsername = profile.username || ((profile.email) ? profile.email.split('@')[0] : '');

            User.findUniqueUsername(possibleUsername, null, function(availableUsername){
                profile.username = availableUsername;

                user = new User(profile);

                user.save(function(err){
                    if(err){
                        var message = _this.getErrorMessage(err);
                        req.flash('error',message);

                        return res.redirect('/signup');
                    } else{
                        return done(err,user);
                    }
                });
            });
        } else{
            return done(err, user);
        }
    }
});
};

Answer 1

In continuation with the comment

Declare as below after the parent function in global scope.

var _this = {};
_this.getErrorMessage = getErrorMessage; // Passing the function reference

Later in callback

_this.getErrorMessage(/*YOUR_PARAMETERS_HERE*/);

---

Alternatively you can remove _this completely and call the function directly as the parent function is in global scope.

Note: the understanding is Both the parent function and callback is in the same .js file with parent function declared on global scope with respect to that .js file and not declared as GLOBAL.getErrorMessage which makes it explicitly global as window object in client side.