Demystifying Async Programming in Javascript - fetchJson callbacks

fetchJson-callbacks.js

// Suppose that we have a method fetchJson() that does GET requests and has an interface that looks
// like this, where callback is expected to take error as its first argument and the parsed response
// data as its second.
function fetchJson(url, callback) { ... }
fetchJson('/api/user/self', function(e, user) {
  fetchJson('/api/interests?userId=' + user.id, function(e, interests) {
    var recommendations = [];
    interests.forEach(function () {
      fetchJson('/api/recommendations?topic=' + interest, function(e, recommendation) {
        recommendations.push(recommendation);
        if (recommendations.length == interests.length) {
          render(profile, interests, recommendations);
        }
      });
    });
  });
});