code.js

string = new Promise(function(resolve) {
  // original code was just:
  // resolve(ctx.processString(contents, relativePath));
  // 
  // I want to emulate that and also invalidate cache in catch()
  
  try {
    resolve(ctx.processString(contents, relativePath));
  } catch(e) {
    return cache.remove(key).then(function(){
      throw e;
    });
  }
}).then(function(result) {
  if (typeof result === 'string') {
    return { output: result };
  } else {
    return result;
  }
}).then(function(value) {
  var stringValue = JSON.stringify(value);

  return cache.set(key, stringValue).then(function() {
    return value;
  });
});