slack-style-join-left-message-run.js

// Run it online at:
// https://algorithm-pad.now.sh/?gist=https://gist.githubusercontent.com/huytd/7ed3521bc1bbe4b5086d176fda85fec8/raw/6019ff8e19ec8278ac55a21b3cbf43ef54ff1882/slack-style-join-left-message-run.js
const Join  = 1;
const Leave = -1;
const JoinedAndLeft = 2;
const Rejoined = -2;

const toStatus = n => {
  switch (n) {
    case Join: return 'joined';
    case Leave: return 'left';
    case JoinedAndLeft: return 'joined and left';
    case Rejoined: return 'rejoined';
  }
};

const data = [
  {
    action: Join,
    user: "Huy"
  },
  {
    action: Leave,
    user: "Cam"
  },
  {
    action: Leave,
    user: "Huy"
  },
  {
    action: Join,
    user: "Cam"
  },
  {
    action: Join,
    user: "Noob"
  },
  {
    action: Join,
    user: "Kcjpop"
  }
];

debug(data);

const groupByUser = data => data.reduce((r, i) => {
  if (!r[i.user]) r[i.user] = i.action;
  else r[i.user] -= i.action;
  debug(r);
  return r;
}, {});

const groupByAction = data => {
  return Object.keys(data).map(user => ({
    user: user,
    action: data[user]
  })).reduce((r, i) => {
    const act = toStatus(i.action);
    if (!r[act]) r[act] = {
      action: act,
      users: []
    };
    r[act].users.push(i.user);
    debug(r[act]);
    return r;
  }, {});
};

const entryToStr = e => {
  if (e.users.length > 1) {
    const first = e.users.shift();
    return `${first} ${e.action} along with ${e.users.join(', ')}`;
  } else {
    return `${e.users.join(', ')} ${e.action}`;
  }
}

const translate = arr => {
  let r = [];
  for (const e in arr) { 
    r.push(entryToStr(arr[e]));
  }
  const last = r.pop();
  return `${r.join(', ')}, also, ${last}`;
};

log(translate(groupByAction(groupByUser(data))));
// Huy joined and left, Cam rejoined, also, Noob joined along with Kcjpop

https://gist.github.com/huytd/7ed3521bc1bbe4b5086d176fda85fec8#file-slack-style-join-left-message-run-js-L3-L4

You should use past tense too.