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Aayush Swodari
Beginner
Asked: 2022-06-26T22:14:52+05:45 In:

Where do the callback is comming from

I followed the mdn docs for learning nodejs. In the code i get confused what is the callback argument passed in funtion and where it is comming from i have not writtern any callback named function

exports.book_detail = function (req, res,next) {
  async.parallel({
    book: function(callback){
      Book.findById(req.params.id).populate(‘author’).populate(‘genre’).exec(callback);
    },
    book_instances: function(callback){
      BookInstance.find({ ‘book’: req.params.id })
      .exec(callback);
    }
  },function(err,results){
    if (err) { return next(err) }
    if (results.book == null){
      let err = new Error(‘Book Not found’);
      err.status =404;
      returnnext(err);
    }
    res.render(‘book_detail’,{title: results.book.title, book:results.book, book_instances: results.book_instances})
  })
};


 


in the code we have book,and book_instance as key and we have function as value with callback as argument






 


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Pundit 
      


      

      

      

      

      

      Please study about ways you can define function.
      

      Basically there are multiple ways to do it.
      

       
      

        

      1. function name (arguments) { … }
        2. 

      2. const name = (arguments ) => { … }
        3. 

      3. const name = function (arguments) { … }
        4. 

      

      The 3rd approach is called defining anonymous functions (make function without giving them name but assign to a variable.
      

      Read more: https://javascript.info/function-expressions
      

       
      

      

      

      

      

      

        

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