// mining spots defined and stored in memory as =>      miners = ['5a3ae7deca2d336eb2969ea5',...]
// string is the ID of STRUCTURE_CONTAINER
let miners = this.memory.miners;
// get all "miner" creeps
let miningCreeps = _.filter(Game.creeps, (c) => c.memory.role === 'miner');
// populate busyMiners with ID of their assigned STRUCTURE_CONTAINER
// array for mining spots that are busy (have "miner" creep on them)
let busyMiners = _.map(miningCreeps, (c) => c.memory.job.id);
// get ids of not busy miners (get all "miners" not in "busyMiners")
let ids = _.difference(miners, busyMiners);
// select random target from collection "ids"
let object = Game.getObjectById(_.sample(ids));