If you want to map objects to something (in this case a property). I think Array.prototype.map is what you're looking for if you want to code functionally.

(fiddle)

If you want to support older browsers, that are not ES5 compliant you can shim it (there is a polyfill on the MDN page above). Another alternative would be to use underscorejs's pluck method:

var users = [
      {name: "Joe", age: 22},
      {name: "Kevin", age: 24},
      {name: "Peter", age: 21}
    ];
var result = _.pluck(users,'name').join(",")

You can use Object.values and map: Objet.values returns an array of values in the object and join converts these values to string by joining them by ',' here.

const users = [{
  name: "Jon",
  age: 34,
  country: "ES"
}];

var result = users.map(u => Object.values(u).join(','));
console.log(result);

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(', ');

If you want to get all the values as a string it should be just a matter of doing this

const PhoneConfirmationFormValues = {
  num1: 1,
  num2: 2,
  num3: 3,
  num4: 4,
};

const result = Object.values(PhoneConfirmationFormValues).join("")

console.log(result)

Object.values, return all the values from all the keys in your object as an array, and with that you can use the Array.join method to transform your array items in and string which in this case the separator is a blank string.

If you want the result as a number you can do

Number(result)

Use map instead of forEach and drop the parenthesis and the curly braces in the lambda:

var userNames = objectArr.map(o => o.userName).join(', ');

One possible approach:

var tagsArray = {};
tagsArray["1"] = "one";
tagsArray["2"] = "two";
tagsArray["Z"] = "zed";

var result = Object.values(tagsArray).join(",");
console.log(result); // "one,two,zed"

More on Array.prototype.join and Object.values.