Callbacks and Promises

getURL.js

// EXAMPLE 1: Simple AJAX call with callback
// Adapted slightly from https://eloquentjavascript.net/2nd_edition/17_http.html

function getURL(url, callback) {
  var req = new XMLHttpRequest();
  req.open("GET", url, true);
  req.addEventListener("load", function() {
    if (req.status < 400)
      callback(req.response);
    else
      callback(null, new Error("Request failed: " +
                               req.statusText));
  });
  req.addEventListener("error", function() {
    callback(null, new Error("Network error"));
  });
  req.send(null);
}

// EXAMPLE 2: The gates of callback hell. Nested calls of getURL() to different endpoints.
function getRestaurants(zipcode, callback) {
  getURL(
    `https://www.ourserver.com/api/get-restaurants-by-zip?code=${zipcode}`,
    function (data) {
      var { restaurants } = data;
      restaurants.forEach(function (restaurant) {
        var { id } = restaurant;
        getURL(
          `https://www.ourserver.com/api/get-reviews-for-restaurant?id=${id}`,
          callback
        );
      });
    }
  );
}

// EXAMPLE 3: Our getURL function re-written with Promises
function get(url) {
  return new Promise(function (succeed, fail) {
    var req = new XMLHttpRequest();
    req.open("GET", url, true);
    req.addEventListener("load", function () {
      if (req.status < 400) {
        succeed(req.response);
      } else {
        fail(new Error("Request failed: " + req.statusText));
      }
    });
    req.addEventListener("error", function () {
      fail(new Error("Network error"));
    });
    req.send(null);
  });
}

// EXAMPLE 4. Promises allow us to chain asynchronous code together
function getRestaurantsWithPromises(zipcode) {
  return get(
    `https://www.ourserver.com/api/get-restaurants-by-zip?code=${zipcode}`
  ).then((data) => {
    var { restaurants } = data;
    return Promise.all(
      restaurants.map((restaurant) => {
        var { id } = restaurant;
        return get(
          `https://www.ourserver.com/api/get-reviews-for-restaurant?id=${id}`
        );
      })
    );
  });
}

getRestaurantsWithPromises(79912)
  .then((data) => doSomething(data))
  .catch((error) => console.error(`Something went wrong: ${error.message}`));