Well you can always override the toString method of your objects:

    var arr = [
        {name: "Joe", age: 22, toString: function(){return this.name;}},
        {name: "Kevin", age: 24, toString: function(){return this.name;}},
        {name: "Peter", age: 21, toString: function(){return this.name;}}
    ];
         
    var result = arr.join(", ");
    console.log(result);

In order to perform this through For loop.

result = [];
const array = [{ a: '1', b: '2' }, { a: '3', b: '4' }];
array.forEach(elm => result.push(elm.a));
console.log(result.join(", "));
// 1, 3

I think that you should use rather the following:

data => {
  this.results = this.results.concat(data.results);
  this._next = data.next;
},

From the concat doc:

The concat() method returns a new array comprised of the array on which it is called joined with the array(s) and/or value(s) provided as arguments.

Upon first appearance, the word "merg" leads one to think you need to use .extend, which is the proper jQuery way to "merge" JSON objects. However, $.extend(true, {}, json1, json2); will cause all values sharing the same key name to be overridden by the latest supplied in the params. As review of your question shows, this is undesired.

What you seek is a simple javascript function known as .concat. Which would work like:

var finalObj = json1.concat(json2);

While this is not a native jQuery function, you could easily add it to the jQuery library for simple future use as follows:

;(function($) {
    if (!$.concat) {
        $.extend({
            concat: function() {
                return Array.prototype.concat.apply([], arguments);
            }
        });
    }
})(jQuery);

And then recall it as desired like:

var finalObj = $.concat(json1, json2);

You can also use it for multiple array objects of this type with a like:

var finalObj = $.concat(json1, json2, json3, json4, json5, ....);

And if you really want it jQuery style and very short and sweet (aka minified)

;(function(a){a.concat||a.extend({concat:function(){return Array.prototype.concat.apply([],arguments);}})})(jQuery);

;(function($){$.concat||$.extend({concat:function(){return Array.prototype.concat.apply([],arguments);}})})(jQuery);

$(function() {
    var json1 = [{id:1, name: 'xxx'}],
        json2 = [{id:2, name: 'xyz'}],
        json3 = [{id:3, name: 'xyy'}],
        json4 = [{id:4, name: 'xzy'}],
        json5 = [{id:5, name: 'zxy'}];
    
    console.log(Array(10).join('-')+'(json1, json2, json3)'+Array(10).join('-'));
    console.log($.concat(json1, json2, json3));
    console.log(Array(10).join('-')+'(json1, json2, json3, json4, json5)'+Array(10).join('-'));
    console.log($.concat(json1, json2, json3, json4, json5));
    console.log(Array(10).join('-')+'(json4, json1, json2, json5)'+Array(10).join('-'));
    console.log($.concat(json4, json1, json2, json5));
});

center { padding: 3em; }

<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.9.1/jquery.min.js"></script>
<center>See Console Log</center>

jsFiddle

Use Array.map:

let data = [
  {
    "animal": "cat",
    "name": "Fluffy"
  },
  {
    "animal": "dog",
    "name": "Bowser"
  },
  {
    "animal": "cat",
    "name": "Felix"
  }
]

Now extract the names using .map:

let names = data.map(item => item.name)


let nameString = names.join(' ')

And now nameString contains Fluffy Bowser Felix.