javaScriptMergeObject.js

var array1 = [{
  agentID: 28,
  bookingDates: ["2019-02-01,2019-03-04"],
  roomNo: [101],
  roomID: [1]
}, {
  agentID: 29,
  bookingDates: ["2019-02-01,2019-03-02"],
  roomNo: [301],
  roomID: [3]
}, {
  agentID: 28,
  bookingDates: ["2019-02-01,2019-03-03"],
  roomNo: [102],
  roomID: [2]
}];

var output = [];

array1.forEach(function(item) {
  var existing = output.filter(function(v, i) {
    return v.agentID == item.agentID;
  });
  if (existing.length) {
    var existingIndex = output.indexOf(existing[0]);
    output[existingIndex].roomID = output[existingIndex].roomID.concat(item.roomID);
    output[existingIndex].roomNo = output[existingIndex].roomNo.concat(item.roomNo);
    output[existingIndex].bookingDates = output[existingIndex].bookingDates.concat(item.bookingDates);
  } else {
    if (typeof item.roomNo == 'string')
      item.bookingDates = [item.bookingDates];
    output.push(item);
  }
});

console.log(output);