Javascript object Vs JSON

Question

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 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.

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).

Problems solved by JSON

Let's say you want to exchange regular JavaScript objects between two computers, and you set two rules:

• The transmitted data must be a regular string.
• Only attributes can be exchanged, methods are not transmitted.

Now you create two objects on the first host:

var obj1 = { one: 1,"two":2,"three":3 }; // your example
var obj2 = { one: obj1.one, two: 2, three: obj1.one + obj1.two };

How can you convert those objects into strings for transmission to the second host?

• For the first object, you could send this string obtained form the literal definition '{ one: 1,"two":2,"three":3 }', but actually you can't read the literal in the script portion of the document (at least not easily). So obj1 and obj2 must actually be processed the same way.
• You need to enumerate all attributes and their value, and build a string similar to the object literal.

JSON has been created as a solution to the needs just discussed: It is a set of rules to create a string equivalent to an object by listing all attributes and values (methods are ignored).

JSON normalizes the use of double-quotes for attribute names and values.

Remember that JSON is a set of rules only (a standard).

How many JSON objects are created?

Only one, it is automatically created by the JS engine.

Modern JavaScript engines found in browsers have a native object, also named JSON. This JSON object is able to:

• Decode a string built using JSON standard, using JSON.parse(string). The result is a regular JS object with attributes and values found in the JSON string.

• Encode attributes / values of a regular JS object using JSON.stringify(). The result is a string compliant with the JSON set of rules.

The (single) JSON object is similar to a codec, it's function is to encode and decode.

Note that:

• JSON.parse() doesn't create a JSON object, it creates a regular JS object, there is no difference between an object created using an object literal and an object created by JSON.parse() from a JSON-compliant string.

• There is only one JSON object, which is used for all conversions.

Going back to the questions:

• Q1: The use of single of double quotes is allowed for object literals. Note that the quotes are used optionally for attributes names, and are mandatory for string values. The object literal itself is not surrounded by quotes.

• Q2: Objects created from literals and using JSON.parse() are strictly the same. These two objects are equivalent after creation:

  var obj1 = { one: 1, "two": 2, "three": 3 };
var obj2 = JSON.parse('{ "one": "1", "two": "2", "three": "3" }');

• Q3: On modern browsers JSON.parse() is used to create a JS object from a JSON-compliant string. (jQuery has also an equivalent method that can be used for all browsers).

Q1 - in JS you only need to use quotes if the key is a reserved word or if it would otherwise be an illegal token. In JSON you MUST always use double quotes on key names.

Q2 - the jsonString is a serialised version of the input object ...

Q3 - which may be deserialised to an identical looking object using JSON.parse()

Question already has good answers posted, I am adding a small example below, which will make it more easy to understand the explanations given in previous answers. Copy paste below snippet to your IDE for better understanding and comment the line containing invalid_javascript_object_no_quotes object declaration to avoid compile time error.

// Valid JSON strings(Observe quotes)
valid_json = '{"key":"value"}'
valid_json_2 = '{"key 1":"value 1"}' // Observe the space(special character) in key - still valid


//Valid Javascript object
valid_javascript_object_no_quotes = {
    key: "value"  //No special character in key, hence it is valid without quotes for key
}


//Valid Javascript object
valid_javascript_object_quotes = {
    key:"value",  //No special character in key, hence it is valid without quotes for key
    "key 1": "value 1" // Space (special character) present in key, therefore key must be contained in double quotes  - Valid
}



console.log(typeof valid_json) // string
console.log(typeof valid_javascript_object_no_quotes) // object
console.log(typeof valid_javascript_object_quotes) // object

//Invalid Javascript object 
invalid_javascript_object_no_quotes = {
   key 1: "value"//Space (special character) present in key, since key is not enclosed with double quotes "Invalid Javascript Object"
}