NodeJS callback with null as first argument

Question

I've just started out with NodeJS and trying to get the hang of callbacks.

Today I've seen null passed by as the first argument to the callback in many examples. Please help me understand why it's there and why I need it.

Example 1

UserSchema.methods.comparePassword = function(pwd, callback) {
    bcrypt.compare(pwd, this.password, function(err, isMatch) {
        if (err) return callback(err);
        callback(null, isMatch);
    });
};

Example 2

example.method = {
    foo: function(callback){
        setTimeout(function(){
            callback(null, 'foo');
        }, 100);
    }
}

Answer 1

By convention in node, the first argument to a callback is usually used to indicate an error. If it's something other than null, the operation was unsuccessful for some reason -- probably something that the callee cannot recover from but that the caller can recover from. Any other arguments after the first are used as return values from the operation (success messages, retrieval, etc.)

This is purely by convention and there is nothing to stop you from writing a function that passes success as the first argument to a callback. If you plan to write a library that is adopted by other node users, you will probably want to stick with convention unless you have a very good reason not to.