What is JSON and why would I use it?
I've looked on wikipedia and Googled it and read the official documentation, but I still haven't got to the point where I really understand what JSON is, and why I'd use it.
I have been building applications using PHP, MySQL and Javascript / HTML for a while, and if JSON can do something to make my life easier or my code better or my user interface better, then I'd like to know about it. Can someone give me a succinct explanation?
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:
JSON Structure
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
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.
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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