为什么回调函数的第一个参数是 null？

Question

I read everything what I could find,but still I do not understand this.

Passportjs authentication(Twitter) authorize

function(token, tokenSecret, profile, done) {
    Account.findOne({ domain: 'twitter.com', uid: profile.id }, function(err, account) {
      if (err) { return done(err); }
      if (account) { return done(null, account); }

Why is the err callback only with oneargument,and the second account callback has also null argument?

Answer 1

done in this code is also a callback function which conforms to a convention called error first

There's really only two rules for defining an error-first callback:

• The first argument of the callback is reserved for an error object. If an error occurred, it will be returned by the first err argument.
• The second argument of the callback is reserved for any successful response data. If no error occurred, err will be set to null and any successful data will be returned in the second argument.

Therefore when an error occurs done is called with a single parameter - the error. When no error occurs done is called with 2 parameters. The first is null (no error) the second is the sucessful response details.