What is JSON and what is it used for? What is JSON and what is it used for? json json

What is JSON and what is it used for?

JSON (JavaScript Object Notation) is a lightweight format that is used for data interchanging. It is based on a subset of JavaScript language (the way objects are built in JavaScript). As stated in the MDN, some JavaScript is not JSON, and some JSON is not JavaScript.

An example of where this is used is web services responses. In the 'old' days, web services used XML as their primary data format for transmitting back data, but since JSON appeared (The JSON format is specified in RFC 4627 by Douglas Crockford), it has been the preferred format because it is much more lightweight

You can find a lot more info on the official JSON web site.

JSON is built on two structures:

  • A collection of name/value pairs. In various languages, this is realized as an object, record, struct, dictionary, hash table, keyed list, or associative array.
  • An ordered list of values. In most languages, this is realized as an array, vector, list, or sequence.

JSON Structure

JSON Object diagram

JSON Array diagram

JSON Value diagram

JSON String diagram

JSON Number diagram

Here is an example of JSON data:

{     "firstName": "John",     "lastName": "Smith",     "address": {         "streetAddress": "21 2nd Street",         "city": "New York",         "state": "NY",         "postalCode": 10021     },     "phoneNumbers": [         "212 555-1234",         "646 555-4567"     ] }

JSON in JavaScript

JSON (in Javascript) is a string!

People often assume all Javascript objects are JSON and that JSON is a Javascript object. This is incorrect.

In Javascript var x = {x:y} is not JSON, this is a Javascript object. The two are not the same thing. The JSON equivalent (represented in the Javascript language) would be var x = '{"x":"y"}'. x is an object of type string not an object in it's own right. To turn this into a fully fledged Javascript object you must first parse it, var x = JSON.parse('{"x":"y"}');, x is now an object but this is not JSON anymore.

See Javascript object Vs JSON

When working with JSON and JavaScript, you may be tempted to use the eval function to evaluate the result returned in the callback, but this is not suggested since there are two characters (U+2028 & U+2029) valid in JSON but not in JavaScript (read more of this here).

Therefore, one must always try to use Crockford's script that checks for a valid JSON before evaluating it. Link to the script explanation is found here and here is a direct link to the js file. Every major browser nowadays has its own implementation for this.

Example on how to use the JSON parser (with the json from the above code snippet):

//The callback function that will be executed once data is received from the servervar callback = function (result) {    var johnny = JSON.parse(result);    //Now, the variable 'johnny' is an object that contains all of the properties     //from the above code snippet (the json example)    alert(johnny.firstName + ' ' + johnny.lastName); //Will alert 'John Smith'};

The JSON parser also offers another very useful method, stringify. This method accepts a JavaScript object as a parameter, and outputs back a string with JSON format. This is useful for when you want to send data back to the server:

var anObject = {name: "Andreas", surname : "Grech", age : 20};var jsonFormat = JSON.stringify(anObject);//The above method will output this: {"name":"Andreas","surname":"Grech","age":20}

The above two methods (parse and stringify) also take a second parameter, which is a function that will be called for every key and value at every level of the final result, and each value will be replaced by result of your inputted function. (More on this here)

Btw, for all of you out there who think JSON is just for JavaScript, check out this post that explains and confirms otherwise.


The Concept Explained - No Code or Technical Jargon

What is JSON? – How I explained it to my wifeTM

Me: “It’s basically a way of communicating with someone in writing....but with very specific rules.

Wife: yeah....?

Me: In prosaic English, the rules are pretty loose: just like with cage fighting. Not so with JSON. There are many ways of describing something:

• Example 1: Our family has 4 people: You, me and 2 kids.

• Example 2: Our family: you, me, kid1 and kid2.

• Example 3: Family: [ you, me, kid1, kid2]

• Example 4: we got 4 people in our family: mum, dad, kid1 and kid2.

Wife: Why don’t they just use plain English instead?

Me: They would, but remember we’re dealing with computers. A computer is stupid and is not going to be able to understand sentences. So we gotta be really specific when computers are involved otherwise they get confused. Furthermore, JSON is a fairly efficient way of communicating, so most of the irrelevant stuff is cut out, which is pretty hand. If you wanted to communicate our family, to a computer, one way you could do so is like this:

{    "Family": ["Me", "Wife", "Kid1", "Kid2"] }

……and that is basically JSON. But remember, you MUST obey the JSON grammar rules. If you break those rules, then a computer simply will not understand (i.e. parse) what you are writing.

Wife: So how do I write in Json?

A good way would be to use a json serialiser - which is a library which does the heavy lifting for you.


JSON is basically a way of communicating data to someone, with very, very specific rules. Using Key Value Pairs and Arrays. This is the concept explained, at this point it is worth reading the specific rules above.

In short - JSON is a way of serializing in such a way, that it becomes JavaScript code. When executed (with eval or otherwise), this code creates and returns a JavaScript object which contains the data you serialized. This is available because JavaScript allows the following syntax:

var MyArray = [ 1, 2, 3, 4]; // MyArray is now an array with 4 elementsvar MyObject = {    'StringProperty' : 'Value',    'IntProperty' : 12,    'ArrayProperty' : [ 1, 2, 3],    'ObjectProperty' : { 'SubObjectProperty': 'SomeValue' }}; // MyObject is now an object with property values set.

You can use this for several purposes. For one, it's a comfortable way to pass data from your server backend to your JavaScript code. Thus, this is often used in AJAX.

You can also use it as a standalone serialization mechanism, which is simpler and takes up less space than XML. Many libraries exists that allow you to serialize and deserialize objects in JSON for various programming languages.