Async I/O in Javascript

async_io.js

const https = require("https");

const urlList = [
  "https://reqres.in/api/users?page=1",
  "https://reqres.in/api/users?page=2",
  "https://reqres.in/api/users?page=3"
];

function getSiteContents(url) {
  return new Promise(function (resolve, reject) {
    https.get(url, function (res) {
      var bodyData = "";
      res.on("data", function (chunk) {
        bodyData += chunk;
      });
      res.on("end", function () {
        resolve(bodyData);
      });
      res.on("error", function (error) {
        reject(error.message);
      });
    }); 
  });
}

// One way we can proceed with execution
// Make one Promise out of a list of Promises
Promise.all(urlList.map(getSiteContents))
  .then(function (siteContents) {
    console.log("Promise based execution --->");
    console.log(siteContents);
  });

// Another way we can proceed with execution
// "async" is an ES7 feature that makes our Promise/async I/O code look 
// more synchronous
async function main () {
  const siteContents = await Promise.all(urlList.map(getSiteContents))
  console.log("Main() based execution --->");
  console.log(siteContents);
}
main();

// As Promises will happen in some future time, this will happen first
console.log("This console.log will most likely happen first");