Best way to call an asynchronous function within map?

Question

I'm mapping over an array and for one of the return values of the new object, I need to make an asynchronous call.

var firebaseData = teachers.map(function(teacher) {   return {     name: teacher.title,     description: teacher.body_html,     image: urlToBase64(teacher.summary_html.match(/src="(.*?)"/)[1]),     city: metafieldTeacherData[teacher.id].city,     country: metafieldTeacherData[teacher.id].country,     state: metafieldTeacherData[teacher.id].state,     studioName: metafieldTeacherData[teacher.id].studioName,     studioURL: metafieldTeacherData[teacher.id].studioURL   } }); 

The implementation of that function will look something like

function urlToBase64(url) {   request.get(url, function (error, response, body) {     if (!error && response.statusCode == 200) {       return "data:" + response.headers["content-type"] + ";base64," + new Buffer(body).toString('base64');     }   }); } 

I'm not clear what's the best approach to do this... promises? Nested callbacks? Use something in ES6 or ES7 and then transpile with Babel?

What's the current best way to implement this?

Answer 1

update in 2018: Promise.all async function within map callback is easier to implement:

    let firebaseData = await Promise.all(teachers.map(async teacher => {         return {             name: teacher.title,             description: teacher.body_html,             image: await urlToBase64(teacher.summary_html.match(/src="(.*?)"/)[1]),             city: metafieldTeacherData[teacher.id].city,             country: metafieldTeacherData[teacher.id].country,             state: metafieldTeacherData[teacher.id].state,             studioName: metafieldTeacherData[teacher.id].studioName,             studioURL: metafieldTeacherData[teacher.id].studioURL         }     }));   async function urlToBase64(url) {   return request.get(url, function (error, response, body) {     if (!error && response.statusCode == 200) {       return "data:" + response.headers["content-type"] + ";base64," + new Buffer(body).toString('base64');     }   }); } 

Edit@2018/04/29: I put the general example for everyone:

Edit@2019/06/19 : async/await should have try/catch to handle error: it would throw a warning message if error occur;

let data = await Promise.all(data.map(async (item) => {       try {       item.fetchItem = await fetchFunc(item.fetchParams);        return item;        } catch(err) {          throw err;       }   }));