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

https://i.stack.imgur.com/VHjrV.gif

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 server
var 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.

References

JSON.org

Wikipedia

Json in 3 minutes (Thanks mson)

Using JSON with Yahoo! Web Services (Thanks gljivar)

JSON to CSV Converter

Alternative JSON to CSV Converter

JSON Lint (JSON validator)

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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.

Summary

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.

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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 elements
var 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.

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In short, it is a scripting notation for passing data about. In some ways an alternative to XML, natively supporting basic data types, arrays and associative arrays (name-value pairs, called Objects because that is what they represent).

The syntax is that used in JavaScript and JSON itself stands for "JavaScript Object Notation". However it has become portable and is used in other languages too.

A useful link for detail is here:

http://secretgeek.net/json_3mins.asp

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The JSON format is often used for serializing and transmitting structured data over a network connection. It is used primarily to transmit data between a server and web application, serving as an alternative to XML.

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JSON is JavaScript Object Notation. It is a much-more compact way of transmitting sets of data across network connections as compared to XML. I suggest JSON be used in any AJAX-like applications where XML would otherwise be the "recommended" option. The verbosity of XML will add to download time and increased bandwidth consumption ($$$). You can accomplish the same effect with JSON and its mark-up is almost exclusively dedicated to the data itself and not the underlying structure.

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the common short answer is: if you are using AJAX to make data requests, you can easily send and return objects as JSON strings. Available extensions for Javascript support toJSON() calls on all javascript types for sending data to the server in an AJAX request. AJAX responses can return objects as JSON strings which can be converted into Javascript objects by a simple eval call, e.g. if the AJAX function someAjaxFunctionCallReturningJson returned

"{ \"FirstName\" : \"Fred\", \"LastName\" : \"Flintstone\" }"

you could write in Javascript

var obj = eval("(" + someAjaxFunctionCallReturningJson().value + ")");
alert(obj.FirstName);
alert(obj.LastName);

JSON can also be used for web service payloads et al, but it is really convenient for AJAX results.

Update (ten years later): Don't do this, use JSON.parse

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I like JSON mainly because it's so terse. For web content that can be gzipped, this isn't necessarily a big deal (hence why xhtml is so popular). But there are occasions where this can be beneficial.

For example, for one project I was transmitting information that needed to be serialized and transmitted via XMPP. Since most servers will limit the amount of data you can transmit in a single message, I found it helpful to use JSON over the obvious alternative, XML.

As an added bonus, if you're familiar with Python or Javascript, you already pretty much know JSON and can interpret it without much training at all.

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What is JSON?

JavaScript Object Notation (JSON) is a lightweight data-interchange format inspired by the object literals of JavaScript.

JSON values can consist of:

objects (collections of name-value pairs) arrays (ordered lists of values) strings (in double quotes) numbers true, false, or null

JSON is language independent.

JSON with PHP?

After PHP Version 5.2.0, JSON extension is decodes and encodes functionalities as default.

Json_encode - returns the JSON representation of values Json_decode - Decodes the JSON String Json_last_error - Returns the last error occured.

JSON Syntax and Rules?

JSON syntax is derived from JavaScript object notation syntax:

Data is in name/value pairs Data is separated by commas Curly braces hold objects Square brackets hold arrays

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Sometimes technicality is given where none is required, and while many of the top voted answers are accurately technical and specific, I personally don't think they are any more easy to understand, or succinct, as what can be found on Wikipedia, or in official documentation.

The way I like to think of JSON is exactly what it is - a language within a world of different languages. However, the difference between JSON and other languages is that "everyone" "speaks" JSON, along with their "native language."

Using a real world example, let's pretend we have three people. One person speaks Igbo as their native tongue. The second person would like to interact with the first person, however, the first person speaks Yoruba as their first language.

What can we do?

Thankfully, the third person in our example grew up speaking English, but also happens to speak both Igbo and Yoruba as second languages, and so can act as an intermediary between the first two individuals.

In the programming world, the first "person" is Python, the second "person" is Ruby, and the third "person" is JSON, who just so happens to be able to "translate" Ruby into Python and vice versa! Now obviously this analogy isn't a perfect one, but, as someone who is bilingual, I believe it's an easy way to look at how JSON interacts with other programming languages.

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We have to do a project on college and we faced a very big problem, it is called Same Origin Policy. Amog other things, it makes that your XMLHttpRequest method from Javascript can't make requests to domains other than the domain that your site is on.

For example you can't make request to www.otherexample.com if your site is on www.example.com. JSONRequest allows that, but you will get result in JSON format if that site allows that(for example it has a web service that returns messages in JSON). That is one problem where you could use JSON perhaps.

Here is something practical: Yahoo JSON

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The difference between JSON and conventional syntax would be as follows (in Javascript)

Conventional

 function Employee(name, Id, Phone, email){

      this.name = name;
      this.Id = Id;
      this.Phone = Phone;
      this.email = email;
  }

  //access or call it as 

var Emp = new Employee("mike","123","9373849784","mike.Anderson@office.com");

With JSON

if we use JSON we can define in different way as

  function Employee(args){

   this.name = args.name;
   this.Id = args.Id;
   this.Phone = args.Phone;
   this.email = args.email;
}

//now access this as...

var Emp = new Employee({'name':'Mike', 'Id':'123', 'Phone':'23792747', 'email':'mike.adnersone@office.com'});

The important thing we have to remember is that, if we have to build the "Employee" class or modal with 100 elements without JSON method we have to parse everything when creating class. But with JSON we can define the objects inline only when a new object for the class is defined.

so this line below is the way of doing things with JSON(just a simple way to define things)

 var Emp = new Employee({'name':'Mike', 'Id':'123', 'Phone':'23792747', 'email':'mike.adnersone@office.com'});

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It's very simple. JSON stands for Java Script Object Notation. Think of it as an alternative to using XML for transferring data between software components.

For example, I recently wrote a bunch of web services that returned JSON, and some Javascript developers then wrote code which called the services and consumed the information returned in that format.

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JSON(Javascript object notation) is a light weight data format for data exchange/transfer. Its in key value pair as the JavaScript is. For REST API its widely used for data transfer from server to client. Nowadays many of the social media sites are using this. Although I don't see this as robust as XML with respect of data types. XML has very rich datatypes and XSD. JSON is bit lacking in this.

For same amount of string data JSON will be lighter compare to XML as XML has all that opening and closing tags, etc...

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In the Java context, one reason why JSON might want to be used, is that it provides a very good alternative to Java's Serialization framework, which has been shown (historically) to be subject to some fairly serious vulnerabilities.

Joshua Bloch discusses this in depth in Item 85 "Prefer Alternatives to Java Serialization" (Effective Java 3rd Edition)

Java's Serialization was initially meant to translate data structures into a format that could be easily transmitted or stored. JSON meets this requirement, without the serious exploits referred to above.

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Try the following code to parse your php json response: read.php

<script
  src="https://code.jquery.com/jquery-3.2.1.min.js"
  integrity="sha256-hwg4gsxgFZhOsEEamdOYGBf13FyQuiTwlAQgxVSNgt4="
  crossorigin="anonymous"></script>
<script type="text/javascript">  
$.ajax({
    url:'index.php',
    data:{},
    type:"POST",
    success:function(result) {
        jsondecoded = $.parseJSON(result);
        $.each(jsondecoded, function(index, value) {
            $("#servers").text($("#servers").text() + " " + value.servername);
            console.log(value.start);
            console.log(value.end);
            console.log(value.id);
        });
    },
    statusCode: {
    404: function() {
      alert( "page not found" );
    }
  }
});
</script>

server.php

<?php 
echo '[{"start":"2017-08-29","end":"2017-09-01","id":"22"},{"start":"2017-09-03","end":"2017-09-06","id":"23"}]';
?>

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