How can i access variable inside callback nodejs?

Question

I'm unable to get the values in meals object although i have create new object at the top can any one tell which is the best procedure access variable inside callback function

var meals = new Object();

passObj.data = _.map(passObj.data, (x)=> {               

    x.mealImageUrl = !_.isNull(x.image_url) ? `${config.image_path}${x.image_url}` : x.image;

    dbHelpder.query(`select * from meals where meal_category = ${x.category_id}`,(error,result)=>{

        meals = x.result;

        passObj.total = 555
    });

    return x;
});

Yarco · Accepted Answer · 2017-12-26 12:45:34Z

1

You need to use callback again inside the callback function. :) You are doing something asynchronous, it means, there are no sequence codes. (At least, I keep this in my mind, don't know how others think about this.) So, the code should be:

function somehow(callback) { // you get the result from callback
  var meals = new Object();
  passObj.data = _.map(passObj.data, (x)=> {
    dbHelpder.query(`select * from meals where meal_category = ${x.category_id}`,(error,result)=>{
      meals = x.result;
      passObj.total = 555;
      callback(meals); // Here you get the result
    });
  }
  return x;
}

So, when you are going to use this function, it should be

function afterMeals(resultMeals) {
  // do something on the meals
}
somehow(afterMeals);

Use some other technology can make it a bit clear (like promise), but you can not avoid callback actually.

answered Dec 26, 2017 at 12:45

Yarco

81310 silver badges22 bronze badges

Sign up to request clarification or add additional context in comments.

3 Comments

Yarco Over a year ago

That's how they called it callback hell

MäHį JįlläNį Over a year ago

Thanks i got it (y)

Yarco Over a year ago

@MäHį JįlläNį If I solved your problem, please make it as the answer, so I can get a bit reputation. ;) Aha

Koenigsberg · Accepted Answer · 2017-12-26 12:57:18Z

First of all, I cannot see what passObj exactly is, apparently it is defined elsewhere.

Secondly, callback functions don't function the way you seem to think they do. Typically one reason to use them is to implement asynchronous calls, so returning a value is not of use.

The idea is as follows. Usually you have a call like this:

var myFunc1 = function(){

   return 42;

}

var x = myFunc1(); 

myFunc2(x);

However when myFunc1 is an asynchronous call returning a value is impossible without using some sort of promise, which is a topic on its own. So if myFunc1 was an asynchronous call and the 42 was returned e.g. by a server, then just returning a value caused the value to be null, because the return value is not calculated and received yet, when you arrive at return.

This is a reason for callbacks to be introduced. They work in a way, that allows for asynchronous calls and proceeding the way you want to after the call has finished. To show on the example above:

var myFunc1 = function( myFunc2, params ){

   // do async stuff here, then call the callback function from myFunc1
   ...
   myFunc2(x);

}

So the asynchronous function doesn't return anything. It makes the calls or calculations it needs to make and when those are done (in the example that is when x has been declared and assigned a value) myFunc2, which is the callback function in our example, is called directly from the asynchronous function.

Long story short - do what you need to do with x directly inside the callback function.

Collectives™ on Stack Overflow

How can i access variable inside callback nodejs?

2 Answers 2

3 Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

3 Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related