client.getServerList(NULL, 100, (servers) => {
console.log(servers);
})
You can find information about filters here: https://developer.valvesoftware.com/wiki/Master_Server_Query_Protocol#Filter
The shorter one is used with the item's market id wich (if i got it right) is hard to get. The long one is in the tradeoffer object that is sent and therefore a lot easier to use