gistfile1.js

function thingLoaded() {
  console.log('loaded');
}

function thingFailed() {
  console.log('failed');
}

function thingOnLoad() {
  this.removeEventListener('load', thingOnLoad);
  this.removeEventListener('error', thingOnError);
  thingLoaded();
}

function thingOnError() {
  this.removeEventListener('load', thingOnLoad);
  this.removeEventListener('error', thingOnError);
  thingFailed();
}

if (thing.complete) {
  if (thing.looksSuccessful) {
    thingLoaded();
  }
  else {
    thingFailed();
  }
}
else {
  thing.addEventListener('load', thingOnLoad);
  thing.addEventListener('error', thingOnError);
}