var jsonobj={"name":"George", "age":29, "friends":["John", "Sarah", "Albert"]}
var jsonstr=JSON.stringify(jsonobj)
alert(typeof jsonstr) //string

This method supports two optional parameters:

• replacer: A user defined function or array of String or Number objects that lets you screen each member within the JSON object to conditionally include as part of the string output.
   
• space: A String or Number object that inserts white space into the JSON string for readability purposes. A Number inserts "x" number of spaces (up to 10) before each name/value pair within the string. A String inserts the specified string (up to 10 characters) before each name/value pair within the string.

The following adds 10 white spaces to the beginning of each member inside the generated the JSON string:

var jsonobj={"name":"George", "age":29, "friends":["John", "Sarah", "Albert"]}
var jsonstr=JSON.stringify(jsonobj, null, 10)

More about the replacer parameter

The replacer parameter can either be a function or an array of String/Numbers. It steps through each member within the JSON object to let you decide what value each member should be changed to. As a function it can return:

• A number, string, or Boolean, which replaces the property's original value with the returned one.
• An object, which is serialized then returned. Object methods or functions are not allowed, and are removed instead.
• Null, which causes the property to be removed.

As an array, the values defined inside it corresponds to the names of the properties inside the JSON object that should be retained when converted into a JSON object.

In the below, I use a replacer function to add 1 to all numeric properties of the resulting JSON string:

var jsonobj={"name":"George", "age":29, "friends":["John", "Sarah", "Albert"]}
var jsonstr=JSON.stringify(jsonobj, function(key, value){
 if (typeof value=="number")
  return value+1
})
//OUTPUT: '{"name":"George", "age":30, "friends":["John", "Sarah", "Albert"]}'

var jsonstr='{"name":"George", "age":29, "friends":["John", "Sarah", "Albert"]}'
var george=JSON.parse(jsonstr) //convert JSON string into object
alert(george.age) //alerts 29

parse() supports the optional reviver parameter which is a user defined function to make further changes to the resurrected JSON object. When defined, the reviver function is applied recursively to every member of the object and replaces each one with the value returned by the function. If the reviver function returns null, the member is deleted. The reviver function is commonly used to transform ISO date strings within a JSON string into actual Date objects upon the string is converted to a JSON object.

In the following example, I store today's date (in milliseconds) inside an object's property:

var jobduty={"thedate":new Date().getTime(), "thetask": "Take out garbage"}

When we convert this object to a JSON string then back into a JSON object, the property "thedate" by default returns a numeric representation of the date in milliseconds, not an actual date object:

var jobstr=JSON.stringify(jobduty)
var jobjson=JSON.parse(jobstr)
alert(jobjson.thedate+" "+typeof jobjson.thedate) //12424353534566 number

Using a reviver function, we can change that so the property returns an actual date representation of the date:

var jobstr=JSON.stringify(jobduty)
var jobjson=JSON.parse(jobstr, function(key, value){
if (key=="thedate") //if "thedate" property
 return new Date(value)
else
 return value
})
alert(jobjson.thedate+" "+typeof jobjson.thedate) //alerts Tue Mar 09 2010 00:02:23 GMT-0800 (Pacific Standard Time) object

In this case I check to see if the property currently being worked on is "thedate", and if so, call new Date() on the property's value to return a date object representation of it.

The below example defines a toJSON() method inside the JSON object we wish to serialize to sort any arrays within the object when it's converted into a JSON string:

var company={}
company.name="Tasties"
company.employees=["George", "Edward", "Sarah"] //unsorted array
company.products=["Twinkies", "Hoo Hoos", "Ding Dongs"] //unsorted array

company.toJSON=function(){
 var companyfinal={} //new object
 for (var prop in this){ //loop through each property within company
  if (this[prop] instanceof Array) //if this property is an array
   companyfinal[prop]=this[prop].sort() //sort it
  else
   companyfinal[prop]=this[prop] //just return original property plus value
 }
 return companyfinal //return modified company object to be serialized
}

var jsontext = JSON.stringify(company)
alert(jsontext)

//ALERTS: {"name":"Tasties","employees":["Edward","George","Sarah"],"products":["Ding Dongs","Hoo Hoos","Twinkies"]}

The toJSON() object also exists natively on the Date, Number, String, and Boolean objects.