javascript - How to make this piece of code asynchronous via promises? -

any glue. please not give example of settimeout.

my supervisor said unnecessary having more then() statement if then()'s not return new promise.i not completely, have looked @ exploring es6 there no example of situation.

let readdata = new promise(function (resolve, reject) {      fs.readfile('/home/geek/desktop/activity-logs.csv', 'utf8', (err, data) => {          if (err)              reject();          resolve(data);        });    });    readdata      .then((data) => {          banklogs = splitstring(data, ';')      })      .then(() => {          (let index = 2; index < banklogs.length; index += 5) {              objectofusers[banklogs[index]] = {}              temporarysymbols.push(banklogs[index].concat(banklogs[index + 1]));          }          object.keys(objectofusers).foreach(function (element) {              objectkeys.push(element)          });          (let index = 0; index < objectkeys.length; index++)              createobject(index);          console.log(objectofusers)      })      .catch((err) => console.log('error happened : ' + err));

from code snippet implemented promise chain resolving 1 promise initializes , returns new one.

something like:

let readdata = new promise(/* ... */);  readdata.then((data) => {         let promise2 = new promise(/* ... */);         return promise2;     }).then(() => {         // ...     })     .catch((err) => console.log('error happened : ' + err)); 

however after resolving readdata wrote simple logic , there no new promises: banklogs = splitstring(data, ';')

so should be:

let readdata = new promise(function (resolve, reject) {     fs.readfile('/home/geek/desktop/activity-logs.csv', 'utf8', (err, data) => {         if (err)             reject();         resolve(data);         });     });  readdata.then((data) => {     var banklogs = splitstring(data, ';')      (let index = 2; index < banklogs.length; index += 5) {         objectofusers[banklogs[index]] = {}         temporarysymbols.push(banklogs[index].concat(banklogs[index + 1]));     }     object.keys(objectofusers).foreach(function (element) {         objectkeys.push(element)     });     (let index = 0; index < objectkeys.length; index++)         createobject(index);     console.log(objectofusers) }) .catch((err) => console.log('error happened : ' + err)); 

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if use banklogs inside promise only, make private.

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promise chain approach use code readability , handling 1 error callback promises.

edit

you can pass value through promise chain using wrapper object or list of items (example):

'use strict';  var fs = require('fs');  let readdata = new promise(function (resolve, reject) {     fs.readfile('/users/maxim/appsflyer/projects/zevel/file1.txt', 'utf8', (err, data) => {         if (err)             reject();          var obj = {             data: data,             banklogs: {} // in 1st promise initialize banklogs         };             resolve(obj);         });     });  // 2nd dummy promise  let newreaddata = new promise(function (resolve, reject) {     fs.readfile('/users/maxim/appsflyer/projects/zevel/file2.txt', 'utf8', (err, data) => {         if (err)             reject();         resolve(data);      });  });  readdata.then((obj1) => {         obj1.banklogs.value = "somevalue";      return newreaddata.then(function(data2){                obj1.data2 = data2;         return obj1;     }); }).then((datatotal) => {        console.log(datatotal);  }) .catch((err) => console.log('error happened : ' + err));