to get formatted JSON string.

The solution is to create JSON Object from JSON string,

and then convert the JSON object back to JSON String, using .PrettyPrinted option.

The code is

let jsonString = "[{\"person\": {\"name\":\"Dani\",\"age\":\"24\"}},{\"person\": {\"name\":\"ray\",\"age\":\"70\"}}]"

var error: NSError?

//1. convert string to NSData
let jsonData = jsonString.dataUsingEncoding(NSUTF8StringEncoding)!

//2. convert JSON data to JSON object
let jsonObject:AnyObject = NSJSONSerialization.JSONObjectWithData(jsonData, options: nil, error: &error)!

//3. convert back to JSON data by setting .PrettyPrinted option
let prettyJsonData = NSJSONSerialization.dataWithJSONObject(jsonObject, options: .PrettyPrinted, error: &error)!

//4. convert NSData back to NSString (use NSString init for convenience), later you can convert to String.
let prettyPrintedJson = NSString(data: prettyJsonData, encoding: NSUTF8StringEncoding)!

//print the result
println("\(prettyPrintedJson)")

The result will look like this