How to wait for each iteration in for loop and return response as API response in nodeJS

Question

I'm using for loop to iterate over an array of elements and to call the same function with different parameters inside the for loop. Here is my code:

exports.listTopSongs = function(query) {
    return new Promise(function(resolve, reject) {
        var str = query.split(","), category,
        for(var i=0; i<str.length; i++) {
           sampleFn(str[i], 'sample', resolve, reject);
        }
    });
};

function sampleFn(lang, cat, resolve, reject) {
        client.on("error", function (err) {
            console.log(err);
            var err = new Error('Exception in redis client connection')
            reject(err);                                
        });
        client.keys(lang, function (err, keys){
            if (err) return console.log(err);
            if(keys.length != 0) {
                client.hgetall(keys, function (error, value) {
                    var objects = Object.keys(value);
                    result['title'] = lang;
                    result[cat] = [];
                    var x =0;
                    for(x; x<objects.length; x++) {
                        var val = objects[x];
                            User.findAll({attributes: ['X', 'Y', 'Z'],
                                where: {
                                    A: val
                                }
                            }).then(data => {
                                if(data != null) {
                                    //some actions with data and stored it seperately in a Json array
                                    if(result[cat].length == objects.length) {
                                        resolve(result);
                                    }
                                } else {
                                    console.log(""+cat+" is not avilable for this value "+data.dataValues['X']);
                                }
                            });
                    }
               });
         });
   }

Here it won't wait for completion of first iteration. It just run asyncronously before completing first iteration function. I need to return the result as result:[{ 1, 2}, {3,4}]. but it runs seamlessly and returns empty or only one object before completing all. How to resolve it.

I used node-async-loop. But it uses next and i can't able to send my parameteres while using that package. Please help me

Check "aync" package. It has many functions that make for loop run synchronously. — Shubham Jain, Sep 19 '17 at 07:16

score 1 · Answer 1 · answered Sep 19 '17 at 07:22

Async provides control flow methods allowing to do so.

Using async.each:

async.each(openFiles, function(file, callback) {

    // Perform operation on file here.
    console.log('Processing file ' + file);

    if( file.length > 32 ) {
      console.log('This file name is too long');
      callback('File name too long');
    } else {
      // Do work to process file here
      console.log('File processed');
      callback();
    }
}, function(err) {
    // if any of the file processing produced an error, err would equal that error
    if( err ) {
      // One of the iterations produced an error.
      // All processing will now stop.
      console.log('A file failed to process');
    } else {
      console.log('All files have been processed successfully');
    }
});

Théophile Pace · Answer 2 · 2017-09-19T08:19:27.337

If you don't want to use a library, you can code it yourself. It would also be very instructive. I took your issue and coded a dummy async loop :

function listTopSongs(query) {
    return new Promise(async(resolve, reject) => { //add async here in order to do asynchronous calls
        const str = query.split(",") //str is const, and the other variable was not used anyway
        
        for( let i = 0;i < str.length; i++) {
            const planet = await sampleFn(str[i], 'sample', resolve, reject)
            console.log(planet)
        }
    });
};

function sampleFn(a, b, c, d) {
    return fetch(`https://swapi.co/api/planets/${a}/`)
        .then(r => r.json())
        .then(rjson => (a + " : " + rjson.name))
}

listTopSongs("1,2,3,4,5,6,7,8,9")

I used some dummy star wars API to fake a long promise but it should work with your sampleFn. Be careful, it is very, very slow if you have network call like the one in the example.

EDIT: I ran your code and I noticed there are a few mistakes: there is no resolve in your promise so it's not a thenable (https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise/resolve see thenable )

Here is a fully working code. The nice part : no library needed, no dependencies.

//for node.js, use node-fetch :
//const fetch = require("node-fetch")

function listTopSongs(query) {
    return new Promise(async(resolve, reject) => { //add async here in order to do asynchronous calls
        const str = query.split(",") //str is const, and the other variable was not used anyway
        const planets = []
        for (let i = 0; i < str.length; i++) {
            const planet = await sampleFn(i + 1, str[i], resolve, reject)
            planets[i] = planet
            console.log(planet)
        }
        resolve(planets)
    });
};

function sampleFn(a, b, c, d) {
    return fetch(`https://swapi.co/api/planets/${a}/`)
        .then(r => r.json())
        .then(rjson => (a + b + " : " + rjson.name))
}

listTopSongs("a,b,c,d").then(planets => console.log(planets))

I can't able to return my json array from function sampleFn to listTopSongs — Saravanan CM, Sep 19 '17 at 08:35

score 0 · Answer 3 · answered Sep 19 '17 at 14:03

Since you are using promise, you can do something like this

exports.listTopSongs = function(query) {
    return Promise.resolve(true).then(function(){
        var str = query.split(",");
        var promises = str.map(function(s){
            return sampleFn(str[i], 'sample');
        });
        return Promise.all(promises);
    }).then(function(results){
       //whatever you want to do with the result
    });
};

For this to work you have to change your sampleFn to not to depend on external resolve and reject functions. I don't see a reason using external resolve and reject. why just not use Promise.Resolve, Promise.Reject;

How to wait for each iteration in for loop and return response as API response in nodeJS

3 Answers3