var structures = creep.room.find(FIND_STRUCTURES);
var spawnAndExtensions = _.filter(structures, function(structure) {
  return (if(structure == STRUCTURE_SPAWN || structure == STRUCTURE_EXTENSION) && structure.energy < structure.energyCapacity);
});
var containers = _.filter(structures, function(structure) {
  return (if(structure == STRUCTURE_CONTAINER) && structure.store < structure.storeCapacity);
});
var target;
if(spawnAndExtensions.length > 0) {
  target = spawnAndExtensions[0];
} else if(containers.length > 0){
  target = containers[0];
}
if(target) {
  //DO STUFF
}