Use of function(callback)

Question

I'm new to mongoose and I'm trying to understand what exactly is happening when 'callback' is being passed. I understand that async.parallel is calling two functions that return the results of the query, but I don't understand the use of 'callback' in this situation.

async.parallel({
        author: function(callback) {
          Author.findById(req.body.authorid).exec(callback)
        },
        authors_books: function(callback) {
          Book.find({ 'author': req.body.authorid }).exec(callback)
        },
    }, function(err, results){
// some function
}

Answer 1

calling callback means you are done with that specific operation.

async.parallel waits until calling all of the callbacks, then call the last function and passes the results to it which obtains from callbacks in every single operation before.

So in your case, async calls both of your operations at the same time. When mongoose find or throw an error by finding the author by req.body.authorid calls the callback, same for authors_books . For the purpose of determining when all of the parallel functions are done, the last function has been called with err and results parameters. you can check the err and results to find out how were done the operations.

Note : You will get the result of the operation in results parameter in the last function. for example, you will get author from the first operation in results.author and also results.authors_books come from the second operation.