JSON is just a notation; to make the change you want parse it so you can apply the changes to a native JavaScript Object, then stringify back to JSON

var jsonStr = '{"theTeam":[{"teamId":"1","status":"pending"},{"teamId":"2","status":"member"},{"teamId":"3","status":"member"}]}';

var obj = JSON.parse(jsonStr);
obj['theTeam'].push({"teamId":"4","status":"pending"});
jsonStr = JSON.stringify(obj);
// "{"theTeam":[{"teamId":"1","status":"pending"},{"teamId":"2","status":"member"},{"teamId":"3","status":"member"},{"teamId":"4","status":"pending"}]}"

As you asked, here's how to do it:

1. Mapping the array to an object
2. Converting the object to JSON

let array = [{
    Field: 'Key',
    Value: '7'
  },
  {
    Field: 'City',
    Value: 'Some City'
  },
  {
    Field: 'Description',
    Value: 'Some Description'
  }
];

// #1 Mapping the array to an object...
let obj = {};
array.forEach(item => obj[item.Field] = item.Value);

// #2 Converting the object to JSON...
let json = JSON.stringify(obj);

console.log(json);

Bonus (ES6 + reduce):

const obj = array.reduce((acc, { Field, Value }) => ({ ...acc, [Field]: Value }), {});

If you want to modify the original array instead of returning a new array, use .push()...

array1.push.apply(array1, array2);
array1.push.apply(array1, array3);

I used .apply to push the individual members of arrays 2 and 3 at once.

or...

array1.push.apply(array1, array2.concat(array3));

To deal with large arrays, you can do this in batches.

for (var n = 0, to_add = array2.concat(array3); n < to_add.length; n+=300) {
    array1.push.apply(array1, to_add.slice(n, n+300));
}

If you do this a lot, create a method or function to handle it.

var push_apply = Function.apply.bind([].push);
var slice_call = Function.call.bind([].slice);

Object.defineProperty(Array.prototype, "pushArrayMembers", {
    value: function() {
        for (var i = 0; i < arguments.length; i++) {
            var to_add = arguments[i];
            for (var n = 0; n < to_add.length; n+=300) {
                push_apply(this, slice_call(to_add, n, n+300));
            }
        }
    }
});

and use it like this:

array1.pushArrayMembers(array2, array3);

var push_apply = Function.apply.bind([].push);
var slice_call = Function.call.bind([].slice);

Object.defineProperty(Array.prototype, "pushArrayMembers", {
    value: function() {
        for (var i = 0; i < arguments.length; i++) {
            var to_add = arguments[i];
            for (var n = 0; n < to_add.length; n+=300) {
                push_apply(this, slice_call(to_add, n, n+300));
            }
        }
    }
});

var array1 = ['a','b','c'];
var array2 = ['d','e','f'];
var array3 = ['g','h','i'];

array1.pushArrayMembers(array2, array3);

document.body.textContent = JSON.stringify(array1, null, 4);

Observation

• If there is a single object and you want to push whole object into an array then no need to iterate the object.

Try this :

var feed = {created_at: "2017-03-14T01:00:32Z", entry_id: 33358, field1: "4", field2: "4", field3: "0"};

var data = [];
data.push(feed);

console.log(data);

Instead of :

var my_json = {created_at: "2017-03-14T01:00:32Z", entry_id: 33358, field1: "4", field2: "4", field3: "0"};

var data = [];
for(var i in my_json) {
    data.push(my_json[i]);
}

console.log(data);

@Jaromanda X is correct- you are looking for JSON.parse. Something like this would suffice:

responseArray =  "[{"id":"blah", "type":"blah", ...}, {"id":"blah2",.... },...]"
<Item[]> JSON.parse(responseArray)

Obviously, this doesn't do any validation of the response (which is bad practice). You should ideally perform some validation of the result:

responseArray =  "[{"id":"blah", "type":"blah", ...}, {"id":"blah2",.... },...]"

var result;
try {
   itemListResponse = <Item[]>JSON.parse(responseArray);

   if(!itemListResponse.has("id") ||
      !itemListResponse.has("type") ||
      !itemListResponse.has("state")){

      throw "Invalid Item";
   }
} catch (e){

}

Or, alternatively, use a JSON Schema validator like ajv.

Use the Array.prototype.push method to append values to the end of an array:

Copy// initialize array
var arr = [
  "Hi",
  "Hello",
  "Bonjour"
];

// append new value to the array
arr.push("Hola");

console.log(arr);

You can use the push() function to append more than one value to an array in a single call:

Copy// initialize array
var arr = ["Hi", "Hello", "Bonjour", "Hola"];

// append multiple values to the array
arr.push("Salut", "Hey");

// display all values
for (var i = 0; i < arr.length; i++) {
  console.log(arr[i]);
}

Note that the push() method returns the updated length of the array.

Update

If you want to add the items of one array to another array, you can use firstArray.concat(secondArray):

Copyvar arr = [
  "apple",
  "banana",
  "cherry"
];

// Do not forget to assign the result as, unlike push, concat does not change the existing array
arr = arr.concat([
  "dragonfruit",
  "elderberry",
  "fig"
]);

console.log(arr);

Update

Just an addition to this answer if you want to prepend any value to the start of an array (i.e. first index) then you can use Array.prototype.unshift for this purpose.

Copyvar arr = [1, 2, 3];
arr.unshift(0);
console.log(arr);

It also supports appending multiple values at once just like push.

Update

Another way with ES6 syntax is to return a new array with the spread syntax. This leaves the original array unchanged, but returns a new array with new items appended or prepended, compliant with the spirit of functional programming.

Copyconst arr1 = [
  "Hi",
  "Hello",
  "Bonjour",
];
const arr2 = [
  "Ciao",
  "Hej",
  "Merhaba",
];

const newArr1 = [
  ...arr1,
  "Salut",
];
const newArr2 = [
  "Salut",
  ...arr2,
];
const newArr3 = [
  ...arr1,
  ...arr2,
];

console.log(newArr1, newArr2, newArr3);