Look at array filter function.
Something like...
var filtered = msg.payload.filter(e => e.is_wired === true)
msg.payload = filtered
That will return an array of items that are is_wired == true
Note, no spitting necessary, the payload is an array, just filter it.
update...
[{"id":"a31f7aa.4de2b88","type":"function","z":"486d43a.6a75dbc","name":"filter is_wired","func":"var filtered = msg.payload.filter(e => e.is_wired === true)\nmsg.payload = filtered\nreturn msg;","outputs":1,"noerr":0,"initialize":"","finalize":"","x":870,"y":160,"wires":[["441a8c64.a3ad54","1e46c4ce.30b14b"]]},{"id":"9ebcb1a0.345ab","type":"inject","z":"486d43a.6a75dbc","name":"faking your data","props":[{"p":"payload"},{"p":"topic","vt":"str"}],"repeat":"","crontab":"","once":false,"onceDelay":0.1,"topic":"","payload":"[{\"site_id\":\"some_data\",\"assoc_time\":1597038040,\"latest_assoc_time\":1597524359,\"oui\":\"Logitech\",\"user_id\":\"some_data\",\"_id\":\"some_data\",\"mac\":\"some_data\",\"is_guest\":false,\"first_seen\":\"some_data\",\"last_seen\":\"some_data\",\"is_wired\":false,\"hostname\":\"HarmonyHub1\",\"use_fixedip\":true,\"network_id\":\"some_data\",\"fixed_ip\":\"192.168.0.153\",\"noted\":false},{\"site_id\":\"some_data\",\"assoc_time\":1597038040,\"latest_assoc_time\":1597524359,\"oui\":\"Logitech\",\"user_id\":\"some_data\",\"_id\":\"some_data\",\"mac\":\"some_data\",\"is_guest\":false,\"first_seen\":\"some_data\",\"last_seen\":\"some_data\",\"is_wired\":true,\"hostname\":\"HarmonyHub2\",\"use_fixedip\":true,\"network_id\":\"some_data\",\"fixed_ip\":\"192.168.0.153\",\"noted\":false},{\"is_guest\":false,\"first_seen\":1562066511,\"last_seen\":1597569302,\"is_wired\":true,\"use_fixedip\":true,\"network_id\":\"some_data\",\"fixed_ip\":\"192.168.0.8\",\"noted\":true,\"usergroup_id\":\"some_data\",\"name\":\"Homeseer\",\"fingerprint_override\":true,\"hostname\":\"Homeseer1\"},{\"is_guest\":false,\"first_seen\":1562066511,\"last_seen\":1597569302,\"is_wired\":false,\"use_fixedip\":true,\"network_id\":\"some_data\",\"fixed_ip\":\"192.168.0.8\",\"noted\":true,\"usergroup_id\":\"some_data\",\"name\":\"Homeseer\",\"fingerprint_override\":true,\"hostname\":\"Homeseer2\"}]","payloadType":"json","x":640,"y":160,"wires":[["a31f7aa.4de2b88","410b4594.b07b8c"]]},{"id":"441a8c64.a3ad54","type":"debug","z":"486d43a.6a75dbc","name":"Filtered data","active":true,"tosidebar":true,"console":false,"tostatus":false,"complete":"payload","targetType":"msg","statusVal":"","statusType":"auto","x":1070,"y":160,"wires":[]},{"id":"1e46c4ce.30b14b","type":"debug","z":"486d43a.6a75dbc","name":"first items hostname","active":true,"tosidebar":false,"console":false,"tostatus":true,"complete":"payload[0].hostname","targetType":"msg","statusVal":"payload[0].hostname","statusType":"auto","x":1100,"y":220,"wires":[]},{"id":"410b4594.b07b8c","type":"debug","z":"486d43a.6a75dbc","name":"All data","active":true,"tosidebar":true,"console":false,"tostatus":false,"complete":"payload","targetType":"msg","statusVal":"","statusType":"auto","x":860,"y":100,"wires":[]}]
