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.
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.
The answer is that because of hoisting, even though you declare the variable users (var users) after you use it in other expressions (user.password != derivedKey), it gets parsed first, leaving the original users reference overwritten.
There are several docs on hoisting out there and it may be a good idea to take a peak at them.