JSFiddle - React, Tailwind, and code Playground

HTML

<pre id='o'></pre>

JavaScript

var obj1 = { data: [ 
                      {id:1, comment:"comment1"},
                      {id:2, comment:"comment2"},
                      {id:3, comment:"comment3"}
                   ] };

var obj2 = { data: [ 
                      {id:2, comment:"comment2"},
                      {id:3, comment:"comment3"},
                      {id:4, comment:"comment4"}
                   ] };

function merge(one, two){
  if (!one.data) return {data:two.data};
  if (!two.data) return {data:one.data};
  var final = {data:one.data};
  // merge
  for(var i = 0 ; i < two.data.length;i++){
      var item = two.data[i];
      insert(item, final);
  }
  return final;
}


function insert(item, obj){
    var data = obj.data;
    var insertIndex = data.length;
    for(var i = 0; i < data.length; i++){
        if(item.id == data[i].id){
           // ignore duplicates
           insertIndex = -1;
           break;
        } else if(item.id < data[i].id){
           insertIndex = i;
           break;
        }
    }
    if(insertIndex == data.length){
        data.push(item);
    } else if(insertIndex != -1) {
        data.splice(insertIndex,0,item);
    }
}

var final = merge(obj1, obj2);

$('#o').append("\n\n\nUsing merge()\n");
$('#o').append( JSON.stringify(final, undefined, 2) );