Javascript object Vs JSON

Q1: When defining object literals in javascript, the keys may include quotes or not. There is no difference except that quotes allow you to specify certain keys that would cause the interpreter to fail to parse if you tried them bare. For example, if you wanted a key that was just an exclamation point, you would need quotes:

a = { "!": 1234 } // Valid
a = { !: 1234 } //  Syntax error

In most cases though, you can omit the quotes around keys on object literals.

Q2: JSON is literally a string representation. It is just a string. So, consider this:

var testObject = { hello: "world" }
var jSonString = JSON.stringify(testObject);

Since testObject is a real object, you can call properties on it and do anything else you can do with objects:

testObject.hello => "world"

On the other hand, jsonString is just a string:

jsonString.hello => undefined

Note one other difference: In JSON, all keys must be quoted. That contrasts with object literals, where the quotes can usually be omitted as per my explanation in Q1.

Q3. You can parse a JSON string by using JSON.parse, and this is generally the best way to do it (if the browser or a framework provides it). You can also just use eval since JSON is valid javascript code, but the former method is recommended for a number of reasons (eval has a lot of nasty problems associated with it).

1. Is the key/property name valid both with/without quotes ?

The only time you need to enclose a key in quotes when using Object Literal notation is where the key is a reserved word or contains a special character (if, :, - etc). It is worth noting that a key in JSON must be enclosed in double quotes.

2. If I convert the above object to JSON using var jSonString = JSON.stringify(testObject);, what is the difference between the 2 (JS obj and JSON)?

JSON is a data interchange format. It's a standard which describes how ordered lists and unordered maps, strings, booleans and numbers can be represented in a string. Just like XML and YAML is a way to pass structured information between languages, JSON is the same. A JavaScript object on the other hand is a physical type. Just like a PHP array, a C++ class/ struct, a JavaScript object is a type internal to JavaScript.

Here's a story. Let's imagine you've purchased some furniture from a store, and you want it delivered. However the only one left in stock is the display model, but you agree to buy it.

In the shop, the chest-of-drawers you've purchased is a living object:

    var chestOfDrawers = {
        color: "red",
        numberOfDrawers: 4
    }

However, you can't send a chest-of-drawers in the post, so you dismantle it (read, stringify it). It's now useless in terms of furniture. It is now JSON. Its in flat pack form.

    {"color":"red","numberOfDrawers":4}

When you receive it, you then rebuild the chest-of-drawers (read, parse it). Its now back in object form.

The reason behind JSON, XML and YAML is to enable data to be transferred between programming languages in a format both participating languages can understand; you can't give PHP or C++ your JavaScript object directly; because each language represents an object differently under-the-hood. However, because we've stringified the object into JSON notation; i.e. a standardised way to represent data, we can transmit the JSON representation of the object to another language (C++, PHP), they can recreate the JavaScript object we had into their own object based on the JSON representation of the object.

It is important to note that JSON cannot represent functions or dates. If you attempt to stringify an object with a function member, the function will be omitted from the JSON representation. A date will be converted to a string;

    JSON.stringify({
        foo: new Date(),
        blah: function () { 
            alert('hello');
        }
    }); // returns the string "{"foo":"2011-11-28T10:21:33.939Z"}"

3. For parsing a JSON string, is the method below recommended? var javascriptObj = JSON.parse(jSonString);

Yes, but older browsers don't support JSON natively (IE <8). To support these, you should include json2.js.

If you're using jQuery, you can call jQuery.parseJSON(), which will use JSON.parse() under the hood if it's supported and will otherwise fallback to a custom implementation to parse the input.