Fetches data from a JSON API through HTTP GET (XMLHttpRequest). It does not make a request to the same URL if the previous has not finished. Those requests are coalesced and their callbacks receive the same result.

httpGet.js

// Variable to track requests and their callbacks so that httpGet()
// does not make a request to the same URL if the previous has not
// finished.
var requests = {};

/**
 * Fetches data from a JSON API through HTTP GET (XMLHttpRequest).
 *
 * Requests to the same URL before the previous one completes are
 * coalesced and their callbacks receive the same result.
 * 
 * @param  {String}   url     URL to hit.
 * 
 * @param  {Function} onLoad  Called when the request completes
 *                            with the signature onLoad(err, res),
 *                            where 'res' is the response parsed
 *                            as Object and 'err' null if request
 *                            succeeded. Else, it will contain an
 *                            HTTP errcode or -1.
 */
function httpGet(url, onLoad) {
  var request = new XMLHttpRequest();

  if (!requests[url]) {

    requests[url] = { callbacks: [onLoad] };

    request.open('GET', url, true);

    request.onload = function() {
      var callbacks = requests[url].callbacks;
      requests[url] = null;
      
      if (request.status >= 200 && request.status < 400) {
        while (callbacks.length) {
          callbacks.pop()(null, JSON.parse(request.responseText));
        }
      } else {
        while (callbacks.length) {
          callbacks.pop()(request.status);
        }
      }
    };

    request.onerror = function() {
      var callbacks = requests[url].callbacks;
      requests[url] = null;

      while (callbacks.length) {
        callbacks.pop()(-1);
      }
    };

    request.send();
  } else {
    requests[url].callbacks.push(onLoad);
  }
}