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);

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.

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

You could reduce the data to an array, then join that

let obj = {"item": [{"_id": "5a48e0c100a5863454c0af2a","name": "Maths","created_by": "5a43ee3231ad5a6b0850d961","__v": 0,"created_on": "2017-12-31T13:06:09.957Z","active": 1,"grade": [],"syllabus": [{"_id": "5a47a5faed12d92d0c2449f4","name": "CBSE","description": "CBSE Syllabus","created_by": "5a43ee3231ad5a6b0850d961","__v": 0,"created_on": "2017-12-30T14:43:06.305Z","banner": 1,"active": 1},{"_id": "5a47a615ed12d92d0c2449f5","name": "State Board","description": "State Board Syllabus","created_by": "5a43ee3231ad5a6b0850d961","__v": 0,"created_on": "2017-12-30T14:43:33.328Z","banner": 1,"active": 1}]}]};

let l = obj.item.reduce((a, {name, syllabus}) =>
    [name].concat(syllabus.map(({name}) => name))
, []);

console.log(l);
console.log(`${l.shift()} - ${l.join(', ')}`);