Promise version of EloquentJavaScript Excercise 17.1

content-negotiation.js

//Ref:
//https://developers.google.com/web/fundamentals/getting-started/primers/promises

function requestAuthor(type) {
  return new Promise(function(resolve, reject) {
    var req = new XMLHttpRequest();
    req.open("GET", "http://eloquentjavascript.net/author", true);
    req.setRequestHeader("accept", type);
    req.onload = function() {
      if(req.status == 200) {
        resolve(req.responseText);
      } else {
        reject(Error(req.statusText));
      }
    };
    
    req.onerror = function() {
      reject(Error("Network Error"));
    };
    
    req.send(null);
  });
}

var types = ["text/plain",
             "text/html",
             "application/json",
             "application/rainbows+unicorns"];

types.map(requestAuthor).reduce(function(sequence, typePromise, i) {
  return sequence.then(function() {
    return typePromise;
  }).then(function(content) {
    console.log(types[i] + ":\n", content, "\n");
  });
}, Promise.resolve());