Question

I’m trying to re-use a variable named user in the following function:

UserModel.prototype.authenticate = function (doc, callback) {

    // check to see if the username exists
    this.users.findOne({ username: doc.username }, function (err, user) {

        if (err || !user)
            return callback(new Error('username not found'));

        // hash the given password using salt from database
        crypto.pbkdf2(doc.password, user.salt, 1, 32, function (err, derivedKey) {

            if (err || user.password != derivedKey)
                return callback(new Error('password mismatch'));

            // explicitly define the user object
            var user = {

                _id: user._id,
                type: user.type,
                username: user.username,
                displayname: user.displayname

            };

            return callback(err, user);

        });

    });

};

I attempt to redefine the user variable inside of the pbkdf2 callback function. This does not work as I would expect. The lines where I compare user.password != derivedKey breaks, because user is undefined here during run-time. Shouldn’t user still be the instance from the findOne callback method parameter? If I change either of the two user variables to be called something else, it works.

I could just rename the variables, but that would still leave me wondering.

Answer 1

The problem is, you declare a variable called user within the function-context:

var user = { };

This will overwrite / overlap the user which was declared as formal parameter by the outer function-context. It does not help to declare that variable after your if-statement. Variables declared by var and function declarations get hoisted up at parse-time, so infact, that var user statement is placed just on top of your inner function.