JSFiddle - React, Tailwind, and code Playground

JavaScript

var arr = [{
"id":"5",
"topic_id":"4",
"message_from":"24",
"message":"how do you do?",
"date":"2015-01-13 15:34:59"
},
{
"id":"6",
"topic_id":"1",
"message_from":"33",
"message":"go go go!!",
"date":"2015-01-13 13:35:06"
},
{
"id":"7",
"topic_id":"4",
"message_from":"33",
"message":"Je suis charlie",
"date":"2015-01-14 16:24:46"
}]

function getLastMsg(id, arr) {
  return arr.filter(function (el) {
    return el.topic_id === id;
  }).sort(function (a, b) { return +b.id - +a.id; })[0];
}

function getTopicIds(arr) {
  var out = [];
  arr.forEach(function (el) {
    if (out.indexOf(el.topic_id) === -1) out.push(el.topic_id);
  });
  return out;
}

function getLastMsgs(arr) {
  return getTopicIds(arr).map(function (id) {
    return getLastMsg(id, arr);
  });
}

var result = getLastMsgs(arr);

console.log(result);