javascript - Using Bluebird Promise in For Loop to build and return an Array of Objects -

i trying build , return object using bluebird promises. promise http request gets additional data add object.

i created function carries out request in loop (i using framework carries out middleware - z. about)

const getwebappcustomfielddetails  = (z, url) => {     const responsepromise = z.request({         url:url,         headers:{           'content-type': 'application/json'         }      });     return responsepromise      .then(response =>{       return json.parse(response.content);      }); }; 

this function called within following code:

const webappfields  = (z, bundle) => { //this section carries creates initial request gets bulk of data const responsepromise = z.request({   url: webappurl(bundle) + '/' + encodeuri(bundle.inputdata.webapp),   headers:{     'content-type': 'application/json'   }, }); //this array hold objects created response var fields = []; return responsepromise   .then(response => {      response = json.parse(response.content);      //from response, append core fields     response.systemfields.foreach( function (systemfield) {       fields.push({         'key': systemfield.name,         'required': systemfield.required,         'type': systemfield.type.tolowercase()       });     });     return response;   })     .then(response => {     //sometimes there custom fields need retrieved individually     const customfieldcount = response.fields.length;     var customfieldappend = '';     (var = 0; < customfieldcount; i++){        getwebappcustomfielddetails(z, response.fields[0].links[0].uri)         .then(response =>{           customfieldappend = {             'key': response.name,             'required': response.required,             'type': response.type.tolowercase()           };           //this push doesn't updated fields array!           fields.push(customfieldappend);         });     }   //this return not include custom fields!   return fields;   }); }; 

i cannot figure out how return value nested promise

you can use promise.reduce reduce list of promises creating inside for loop single promise:

...     .then(response => {         const customfieldcount = response.fields.length;         var customfieldappend = '';          // promice.reduce return accumulator "totalfields"         return promise.reduce(response.fields, (totalfields, field) => {             getwebappcustomfielddetails(z, field.links[0].uri) // using "field" variable provided reducer function                             .then(response => {                                 customfieldappend = {                                     'key': response.name,                                     'required': response.required,                                     'type': response.type.tolowercase()                                 };                                 // add data accumulator "totalfields"                                 totalfields.push(customfieldappend);                             });         }, []); // third argument initial value of accummulator. in example array, initial value empty array.     }); ... 

promise.reduce takes in input 3 arguments: list of arguments loop on (response.fields), reducer function , initial value accumulator. accumulator (which called totalfields) first argument of reducer function, variable used reduce values in list in single value. in case accumulator array (the fields array used in example) initial value empty array.

inside reduce function can access single elements of list (second argument) , can call async operation , fill accumulator ad every step. promise.reduce function return accumulator wrapped inside promise. reason function can directly returns promise.reduce's returned promise.