CODEWITHSUNDEEP
javascriptnode.jscallbackasync-await
Yilmaz
Yilmaz

Reputation: 49581

What is "done" callback function in Node.js?

Most of npm packages always use "done" callback function in their page. I have hard time to understand it. For example:

passport.serializeUser(function(user, done) {
  done(null, user.id);
});

This is my understanding:

"done" is a callback function so its mission is to hold the result which is "null" and "user.id". If so, what is the point of holding "null"? Can't we just say done(user.id)?

If my understanding is wrong, what is the interpretation of above code?

Also, that code is written in old javascript. How can we write it in modern javascript with arrow function or if possible with async/await?

I try to write with async/await. Is this correct?

passport.serializeUser(async (user)=>{
return await user.id})

Upvotes: 2

Views: 4639

Answers (2)

Medet Tleukabiluly
Medet Tleukabiluly

Reputation: 11950

passport.serializeUser(function(user, done) {
  done(null, user.id);
});

same as 

passport.serializeUser(function(user, callback) {
  callback(null, user.id);
});

In node.js, it is considered standard practice to handle errors in asynchronous functions by returning them as the first argument to the current function's callback. If there is an error, the first parameter is passed an Error object with all the details. Otherwise, the first parameter is null.

For that reason, done's first argument is possible error, second is actual value

docs

Upvotes: 1

Jack Bashford
Jack Bashford

Reputation: 44135

It's just a success callback. Nothing special.

You can rewrite it in ES6 like so, using destructuring and arrow functions:

passport.serializeUser(({ id }, done) => done(null, id));

Upvotes: 0

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