This looks like a job for $.map!

var data = {
    "category": [{
          "id": 28,
          "name": "Dogs"
      },
      {
          "id": 14,
          "name": "Cats"
      },
      {
          "id": 878,
          "name": "Sheep"
      }]
}

var cats = $.map(data.category, function(v){
    return v.name;
}).join(', ');

Based on your description in the comments, you'd simply do an array concat():

var jsonArray1 = [{'name': "doug", 'id':5}, {'name': "dofug", 'id':23}];
var jsonArray2 = [{'name': "goud", 'id':1}, {'name': "doaaug", 'id':52}];

jsonArray1 = jsonArray1.concat(jsonArray2);

console.log(jsonArray1)
// [{'name': "doug", 'id':5}, {'name': "dofug", 'id':23}, {'name': "goud", 'id':1}, {'name': "doaaug", 'id':52}];

If i understood correctly, you can do the following:

var json= {"Title":"Movie1","Year":"2013"};
var result="";
for( key in json){
  result+= json[key];
}

You don't have to know the number of properties or it's names before hand. This should work for simple scenarios.

There are couple of different solutions to achieve this:

1 - Native javascript for-in loop:

const result = {};
let key;

for (key in obj1) {
  if(obj1.hasOwnProperty(key)){
    result[key] = obj1[key];
  }
}

for (key in obj2) {
  if(obj2.hasOwnProperty(key)){
    result[key] = obj2[key];
  }
}

2 - Object.keys():

const result = {};

Object.keys(obj1)
  .forEach(key => result[key] = obj1[key]);

Object.keys(obj2)
  .forEach(key => result[key] = obj2[key]);

3 - Object.assign():
(Browser compatibility: Chrome: 45, Firefox (Gecko): 34, Internet Explorer: No support, Edge: (Yes), Opera: 32, Safari: 9)

const result = Object.assign({}, obj1, obj2);

4 - Spread Operator:
Standardised from ECMAScript 2015 (6th Edition, ECMA-262):

Defined in several sections of the specification: Array Initializer, Argument Lists

Using this new syntax you could join/merge different objects into one object like this:

const result = {
  ...obj1,
  ...obj2,
};

5 - jQuery.extend(target, obj1, obj2):

Merge the contents of two or more objects together into the first object.

const target = {};

$.extend(target, obj1, obj2);

6 - jQuery.extend(true, target, obj1, obj2):

Run a deep merge of the contents of two or more objects together into the target. Passing false for the first argument is not supported.

const target = {};

$.extend(true, target, obj1, obj2);

7 - Lodash _.assignIn(object, [sources]): also named as _.extend:

const result = {};

_.assignIn(result, obj1, obj2);

8 - Lodash _.merge(object, [sources]):

const result = _.merge(obj1, obj2);

There are a couple of important differences between lodash's merge function and Object.assign:

1- Although they both receive any number of objects but lodash's merge apply a deep merge of those objects but Object.assign only merges the first level. For instance:

_.isEqual(_.merge({
  x: {
    y: { key1: 'value1' },
  },
}, {
  x: {
    y: { key2: 'value2' },
  },
}), {
  x: {
    y: {
      key1: 'value1',
      key2: 'value2',
    },
  },
}); // true

BUT:

const result = Object.assign({
  x: {
    y: { key1: 'value1' },
  },
}, {
  x: {
    y: { key2: 'value2' },
  },
});
_.isEqual(result, {
  x: {
    y: {
      key1: 'value1',
      key2: 'value2',
    },
  },
}); // false
// AND
_.isEqual(result, {
  x: {
    y: {
      key2: 'value2',
    },
  },
}); // true

2- Another difference has to do with how Object.assign and _.merge interpret the undefined value:

_.isEqual(_.merge({x: 1}, {x: undefined}), { x: 1 }) // false

BUT:

_.isEqual(Object.assign({x: 1}, {x: undefined}), { x: undefined })// true

Update 1:

When using for in loop in JavaScript, we should be aware of our environment specially the possible prototype changes in the JavaScript types. For instance some of the older JavaScript libraries add new stuff to Array.prototype or even Object.prototype. To safeguard your iterations over from the added stuff we could use object.hasOwnProperty(key) to mke sure the key is actually part of the object you are iterating over.

Update 2:

I updated my answer and added the solution number 4, which is a new JavaScript feature but not completely standardized yet. I am using it with Babeljs which is a compiler for writing next generation JavaScript.

Update 3:
I added the difference between Object.assign and _.merge.

why don't you do it like this?

var x = {
    order: {
        currency: 'eur',
        order_items: []
    }
};
for (var i = 0; i < 10; i++) {
    var newObject = {
        i: i,
        Text: 'Some Text'
    };
    x.order.order_items.push(newObject);
}

var str = JSON.stringify(x);

look at this fiddle: http://jsfiddle.net/9V6Vb/

Here is the generated JSON:

{
    "order": {
        "currency": "eur",
        "order_items": [{
            "i": 0,
            "Text": "Some Text"},
        {
            "i": 1,
            "Text": "Some Text"},
        {
            "i": 2,
            "Text": "Some Text"},
        {
            "i": 3,
            "Text": "Some Text"},
        {
            "i": 4,
            "Text": "Some Text"},
        {
            "i": 5,
            "Text": "Some Text"},
        {
            "i": 6,
            "Text": "Some Text"},
        {
            "i": 7,
            "Text": "Some Text"},
        {
            "i": 8,
            "Text": "Some Text"},
        {
            "i": 9,
            "Text": "Some Text"}]
    }
}​

basically access the individual properties then join them. However since there might be a few of them, we need a little loop.

var obj = {
  "0": {
    "error": "ERROR_SPECIAL_CHAR",
    "message": "Missing a special character"
  },
  "1": {
    "error": "ERROR_CHAR_UPPERCASE",
    "message": "Must contain at least one uppercase letter"
  }
}

var messages = []
for (var key in obj) {
  messages.push(obj[key].message)
}
var str = messages.join(", ");
console.log(str)

Alasql JavaScript SQL library does exactly what you need in one line:

 <script src="alasql.min.js"></script>
 <script>
    var data = { COLORS: [[1,"red"],[2,"yellow"],[3,"orange"]],            
                 FRUITS: [[1,"apple"],[2,"banana"],[3,"orange"]]};

    data.NEW_FRUITS = alasql('SELECT MATRIX COLORS.[0], COLORS.[1], FRUITS.[1] AS [2] \
         FROM ? AS COLORS JOIN ? AS FRUITS ON COLORS.[0] = FRUITS.[0]',
         [data.COLORS, data.FRUITS]);
 </script>

You can play with this example in jsFiddle.

This is a SQL expression, where:

• SELECT - select operator
• MATRIX - modifier, whci converts resultset from array of objects to array of arrays
• COLORS.[0] - first column of COLORS array, etc.
• FRUITS.1 AS 2 - the second column of array FRUITS will be stored as third column in resulting recordset
• FROM ? AS COLORS - data array from parameters named COLORS in SQL statement
• JOIN ? ON ... - join
• [data.COLORS, data.FRUITS] - parameters with data arrays

When you do

"interests = " + arr

the toString method in that array object will be invoked. Quoting the Array.prototype.toString documentation,

The Array object overrides the toString method of Object. For Array objects, the toString method joins the array and returns one string containing each array element separated by commas. For example, the following code creates an array and uses toString to convert the array to a string.

var monthNames = ['Jan', 'Feb', 'Mar', 'Apr'];
var myVar = monthNames.toString(); // assigns 'Jan,Feb,Mar,Apr' to myVar.

To fix your test case, you need to convert the Array manually, like this

str += "interests = [" + monthNames.map(function(currentString) {
    return "'" + currentString + "'";
}).join(", ") + "]";

Note JSON stands for JavaScript Object Notation. JSON is always a string. It is a string representation of your encoded data. There is no such thing as "a JSON Object".

Don't attempt to write JSON by joining strings and other values together. Instead build an object and pass it to JSON.stringify -

const dept = "sci"
const avg = 77.09

// make an object
const myobject = { courses_dept: dept, courses_average: avg }

// use JSON.stringify to encode myobject to JSON
const myjson = JSON.stringify(myobject)
console.log("encoded", myjson)

// test JSON.parse to decode the JSON and get the object back
console.log("decoded", JSON.parse(myjson))

encoded {"courses_dept":"sci","courses_average":77.09}

decoded {
  "courses_dept": "sci",
  "courses_average": 77.09
}