与PHP不同，如果对象不存在，则必须创建对象：

  var monthNames = [Jan，Feb，Mar，Apr，May，Jun，Jul， Aug，Sep，Oct，Nov，Dec] ar = [{id：15，heading：Post1，content：Post 1 Content， date：2016-11-09 08:51:37}，{id：16，heading：Post2，content：Post 2 Content，date： 2016-11-09 08:52:09}，{id：17，heading：Post3，content：Post 3 Content，date：2015-06-09 08:52:09}] obj = {} ar.forEach（function（v）{d = new Date（v.date）; m = monthNames [d.getMonth（）] if（！obj.hasOwnProperty（d。 getFullYear（）））{obj [d.getFullYear（）] = {}} if（！obj [d.getFullYear（）]。hasOwnProperty（m））{obj [d.getFullYear（）] [m] = [] } OBJ [d.getFullYear（）] [M] .push（v.heading）}）的console.log（OBJ） 

[{"id":"15","heading":"Post1","content":"Post 1 Content","date":"2016-11-09 08:51:37"},
{"id":"16","heading":"Post2","content":"Post 2 Content","date":"2016-11-09 08:52:09"},
{"id":"17","heading":"Post3","content":"Post 3 Content","date":"2015-06-09 08:52:09"}]

I have above JSON array. I am trying to convert it into a JSON Object as

2016
    Nov
        Post1
        Post2
2015
   June
        Post3

I was able to achieve this in PHP by

while ($row = mysqli_fetch_row($result)) {
    $year = date('Y', strtotime($row['2']));
    $month = date('M', strtotime($row['2']));
    $navarray[$year][$month][] = array($row[0], $row[1], $row[3]);
}

But can't figure it out in JS.

Unlike PHP you must create the objects if they don't exists:

var monthNames = ["Jan", "Feb", "Mar", "Apr", "May", "Jun", "Jul", "Aug", "Sep", "Oct", "Nov", "Dec"]

ar = [{"id":"15","heading":"Post1","content":"Post 1 Content","date":"2016-11-09 08:51:37"},
{"id":"16","heading":"Post2","content":"Post 2 Content","date":"2016-11-09 08:52:09"},
{"id":"17","heading":"Post3","content":"Post 3 Content","date":"2015-06-09 08:52:09"}]

obj = {}
ar.forEach(function(v) {
  d = new Date(v.date);
  m = monthNames[d.getMonth()]
  if (!obj.hasOwnProperty(d.getFullYear())) {
    obj[d.getFullYear()] = {}
  }
  if (!obj[d.getFullYear()].hasOwnProperty(m)) {
    obj[d.getFullYear()][m] = []
  }
  obj[d.getFullYear()][m].push(v.heading)
})
console.log(obj)