How can I control asynchronous nature of node.js for following code snippet

Question

How can we implement synchronized code for following code snippet. We are to write code in call-back function of previous function. Which created callback-hell (as they said). How can we write separately males and females record fetching separately which can execute synchronously. How can we retain the value for console.log(id[0]); in for loop. Please correct me if I am in wrong direction.

var male_arr = []; var female_arr = [];

/*************   Fetching record from database with condition gender=0
***************/ user.find({gender: 0}, {_id: 1}, function (err_male, male) {

    if (err_male) {
        console.log(err_male);
    }

    male_arr = male;    // initializing males array
    looplength = male.length;
    //
    // With CAALBACK function, we have fetched records with condition gender = 1
    //
    user.find({gender: 1}, {_id: 1}, function (err_female, female) {
        if (err_female) {
            console.log(err_female);
        }

        female_arr = female;     // initializing females array

            // Populate the male array
            for (var loop = 0; loop < looplength; loop++) {

                var id = male_arr.splice(0, 1);
     //#1          /*****here is the spliced element********/

                console.log("Outside request api");
                console.log(id[0]);


                // Get all records of females which are in request table of male at id[0]
                request.find({male: id[0]._id}, {female: 1}, function (err, data) {
    //#2           /*****This value of male in id[0] is not retained for match ********/
                    console.log("Inside request api");
                    console.log(id[0]);
                    if (err) {
                        console.log(err);
                    }
                });

            }

    }); });

Juan Stiza · Accepted Answer · 2016-06-06 18:38:36Z

Although Caio's answer might fit your problem, might I suggest using Promises?

Using Bluebird you can do something like this:

Note: this is an example and it might not be actually functional.

var Promise = require('bluebird');

var getMaleFn() {
    return new Promise(function(resolve, reject){
        // Make the call to your API.
        // Handle errors, if any, return reject(err);
        return resolve(males);
    });
}

var getFemaleFn() {
    return new Promise(function(resolve, reject){
        // Make the call to your API.
        // Handle errors, if any, return reject(err);
        return resolve(females);
    });
}

var males = [];
var females = [];

// Fetch all males
getMaleFn()
    //Iterate each male and put it in the array
    .map(function(male) {
        males.push(male);
    })
    // Fetch all females
    .then(function(){
        return getFemaleFn()
    })
    //Iterate each females and put it in the array
    .map(function(female) {
        females.push(female);
    })
    .then(function(){
        console.log('done!');
    });

Caio Tomazelli · Accepted Answer · 2016-06-06 18:23:47Z

1

Async waterfall may be the answer to your problems!

https://github.com/caolan/async#waterfall

answered Jun 6, 2016 at 18:23

Caio Tomazelli

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