error-handling.js

function someLibraryCode(callback, dataProvider) {
  dataProvider(function (error, data) {
    if (error) {
      callback(error);
      return;
    }
    var newData = mungeData(data);
    callback(null, newData);
  });
}

function handleObjects() {
  someLibraryCallback(
    function (error, mungedData) {
      if (error) {
        displayDialog("error", error);
        return;
      }
      handleMungedData(mungedData);
    },
    getObjects);
}

function getObjects(callback, data) {
  req = new XMLHttpRequest();
  req.open('POST', '/some-data.json');
  req.onreadystatechange = function () {
    if (req.readyState != 4) {
      return;
    }
    if (req.status != 200) {
      callback('Not a 200 response: ' + req.status);
    }
    callback(null, JSON.parse(req.responseText));
  };
  req.send(JSON.stringify(data));
}