A Simplistic TCP Finite State Machine (FSM) - Javascript Solution

solution.js

/*
The "transition table" from the instructions:

CLOSED: APP_PASSIVE_OPEN -> LISTEN
CLOSED: APP_ACTIVE_OPEN  -> SYN_SENT

LISTEN: RCV_SYN          -> SYN_RCVD
LISTEN: APP_SEND         -> SYN_SENT
LISTEN: APP_CLOSE        -> CLOSED

SYN_RCVD: APP_CLOSE      -> FIN_WAIT_1
SYN_RCVD: RCV_ACK        -> ESTABLISHED

SYN_SENT: RCV_SYN        -> SYN_RCVD
SYN_SENT: RCV_SYN_ACK    -> ESTABLISHED
SYN_SENT: APP_CLOSE      -> CLOSED

ESTABLISHED: APP_CLOSE   -> FIN_WAIT_1
ESTABLISHED: RCV_FIN     -> CLOSE_WAIT

FIN_WAIT_1: RCV_FIN      -> CLOSING
FIN_WAIT_1: RCV_FIN_ACK  -> TIME_WAIT
FIN_WAIT_1: RCV_ACK      -> FIN_WAIT_2

CLOSING: RCV_ACK         -> TIME_WAIT

FIN_WAIT_2: RCV_FIN      -> TIME_WAIT

TIME_WAIT: APP_TIMEOUT   -> CLOSED

CLOSE_WAIT: APP_CLOSE    -> LAST_ACK

LAST_ACK: RCV_ACK        -> CLOSED
*/

// this isn't really code it's just the instructions above but represented as an object
const data = {
  CLOSED: { APP_PASSIVE_OPEN: "LISTEN", APP_ACTIVE_OPEN: "SYN_SENT" },
  LISTEN: { RCV_SYN: "SYN_RCVD", APP_SEND: "SYN_SENT", APP_CLOSE: "CLOSED" },
  SYN_RCVD: {
    APP_CLOSE: "FIN_WAIT_1",
    RCV_ACK: "ESTABLISHED",
  },
  SYN_SENT: {
    RCV_SYN: "SYN_RCVD",
    RCV_SYN_ACK: "ESTABLISHED",
    APP_CLOSE: "CLOSED",
  },
  ESTABLISHED: { APP_CLOSE: "FIN_WAIT_1", RCV_FIN: "CLOSE_WAIT" },
  FIN_WAIT_1: {
    RCV_FIN: "CLOSING",
    RCV_FIN_ACK: "TIME_WAIT",
    RCV_ACK: "FIN_WAIT_2",
  },
  CLOSING: { RCV_ACK: "TIME_WAIT" },
  FIN_WAIT_2: { RCV_FIN: "TIME_WAIT" },
  TIME_WAIT: { APP_TIMEOUT: "CLOSED" },
  CLOSE_WAIT: { APP_CLOSE: "LAST_ACK" },
  LAST_ACK: { RCV_ACK: "CLOSED" },
}

function traverseTCPStates(eventList) {
  let state = "CLOSED"
  
  for (event of eventList) {
    let transitions = data[state]

    if (transitions == undefined) {
      return "ERROR"
    }

    state = transitions[event]

    if (state === undefined) {
      return "ERROR"
    }
  }
  
  return state
}

You can actually write traverseTCPStates one line:

function traverseTCPStates(eventList) {
  return eventList.reduce((current, event) => data[current]?.[event] || "ERROR", "CLOSED")
}

But it's kinda hard to read because reduce & ?. are kinda advanced.