The code below calls PHP service using AJAX

var xmlhttp = new XMLHttpRequest();
xmlhttp.onreadystatechange = function() {
  if (this.readyState == 4 && this.status == 200) {
    var myObj = JSON.parse(this.responseText);
    document.getElementById("demo").innerHTML = myObj.name;
  }
};
xmlhttp.open("GET", "demo_file.php", true);
xmlhttp.send();

PHP code that responds to the request

$myArr = array("name"=>"John", "age"=>30, "city"=>"New York");

$myJSON = json_encode($myArr);

echo $myJSON;

Nested arrays inside JSON object

var myObj, i, j, x = "";
myObj = {
  "name":"John",
  "age":30,
  "cars": [
    {"name":"Ford", "models":["Fiesta", "Focus", "Mustang"]},
    {"name":"BMW", "models":["320", "X3", "X5"]},
    {"name":"Fiat", "models":["500", "Panda"] }
  ]
}

To access nested JSON arrays

for (i in myObj.cars) {
  x += "<h1>" + myObj.cars[i].name + "</h1>";
  for (j in myObj.cars[i].models) {
    x += myObj.cars[i].models[j];
  }
}

The

JSON.stringify()

will remove functions from a JavaScript object because

function

is not valid data type in

JSON

var obj = { name: "John", age: function () {return 30;}, city: "New York" };

// Functions and any other unsupported data types will be ignored
var myJSON = JSON.stringify(obj);
document.write(myJSON);

The result will be:

{"name":"John","city":"New York"}

As we can see, the

function

is removed. To solve this issue we can convert the function into

string

before running

stringify

var obj = { name: "John", age: function () {return 30;}, city: "New York" };
obj.age = obj.age.toString();
var myJSON = JSON.stringify(obj);
document.write(myJSON);

The result will be:

{"name":"John","age":"function () {return 30;}","city":"New York"}

When we use JSON.parse() on a JSON

derived from an array

, the method will return a JavaScript array, instead of an object

var xmlhttp = new XMLHttpRequest();
xmlhttp.onreadystatechange = function() {
  if (this.readyState == 4 && this.status == 200) {
    var myArr = JSON.parse(this.responseText);
    document.getElementById("demo").innerHTML = myArr[0];
  }
};
xmlhttp.open("GET", "json_demo_array.txt", true);
xmlhttp.send();

We can request JSON (

derived from an object

) from the server by using an XMLHttpRequest request As long as the response from the server is written in JSON format, we can parse the JSON data into a JavaScript object

var xmlhttp = new XMLHttpRequest();
xmlhttp.onreadystatechange = function() {
  if (this.readyState == 4 && this.status == 200) {
    var myObj = JSON.parse(this.responseText);
    document.getElementById("demo").innerHTML = myObj.name;
  }
};
xmlhttp.open("GET", "json_demo.txt", true);
xmlhttp.send();

Store and retrieve data from local storage

let myObj, myJSON, text, obj;

// Storing data:
myObj = { name: "John", age: 31, city: "New York" };
myJSON = JSON.stringify(myObj);
localStorage.setItem("testJSON", myJSON);

// Retrieving data:
text = localStorage.getItem("testJSON");
obj = JSON.parse(text);
document.getElementById("demo").innerHTML = obj.name + ' is ' + obj.age;

We can convert the object into JSON, and send the data to a server as a value of

x

variable (but it's not ideal way of sending JSON to a server)

var myObj = {name: "John", age: 31, city: "New York"};
var myJSON = JSON.stringify(myObj);
window.location = "demo_json.php?x=" + myJSON;

Most of the time,

JSON.stringify

is used with the first argument only. But if we need to fine-tune the replacement process, like to filter out

circular references

, we can use the second argument of

JSON.stringify

let room = {
  number: 23
};

let meetup = {
  title: "Conference",
  participants: [{name: "John"}, {name: "Alice"}],
  place: room // meetup references room
};

room.occupiedBy = meetup; // room references meetup

document.write( JSON.stringify(meetup, ['title', 'participants']) );

If we pass an array of properties to it, only these properties will be encoded, so the result will be:

{"title":"Conference","participants":[{},{}]}

As the result shows, the property list is applied to the whole object structure. So the objects in

participants

are empty, because

name

is not in the list. Let’s include in the list every property except

room.occupiedBy

that would cause the circular reference:

let room = {
  number: 23
};

let meetup = {
  title: "Conference",
  participants: [{name: "John"}, {name: "Alice"}],
  place: room // meetup references room
};

room.occupiedBy = meetup; // room references meetup

document.write( JSON.stringify(meetup, ['title', 'participants', 'place', 'name', 'number']) );

The result will be:

{"title":"Conference","participants":[{"name":"John"},{"name":"Alice"}],"place":{"number":23}}

Now everything except

occupiedBy

is serialized. But the list of properties is quite long. Fortunately, we can use a function instead of an array as the

replacer

.

Unsupported data types will be converted to

null

when we want to JSON.stringify the JavaScript object

document.write(JSON.stringify({ x: 5, y: 6 }) + "<br />");
// expected output: "{"x":5,"y":6}"

document.write(JSON.stringify([new Number(3), new String('false'), new Boolean(false)]) + "<br />");
// expected output: "[3,"false",false]"

document.write(JSON.stringify({ x: [10, undefined, function(){}, Symbol('d'), new Date(2006, 0, 2, 15, 4, 5)] }) + "<br />");
// expected output: "{"x":[10,null,null,null,"2006-01-02T11:04:05.000Z"]}"