Convert array to JSON string in swift

As it stands you're converting it to data, then attempting to convert the data to to an object as JSON (which fails, it's not JSON) and converting that to a string, basically you have a bunch of meaningless transformations.

As long as the array contains only JSON encodable values (string, number, dictionary, array, nil) you can just use NSJSONSerialization to do it.

Instead just do the array->data->string parts:

Swift 3/4

let array = [ "one", "two" ]

func json(from object:Any) -> String? {
    guard let data = try? JSONSerialization.data(withJSONObject: object, options: []) else {
        return nil
    }
    return String(data: data, encoding: String.Encoding.utf8)
}

print("\(json(from:array as Any))")

Original Answer

let array = [ "one", "two" ]
let data = NSJSONSerialization.dataWithJSONObject(array, options: nil, error: nil)
let string = NSString(data: data!, encoding: NSUTF8StringEncoding)

although you should probably not use forced unwrapping, it gives you the right starting point.

If you're already using SwiftyJSON:

https://github.com/SwiftyJSON/SwiftyJSON

You can do this:

// this works with dictionaries too
let paramsDictionary = [
    "title": "foo",
    "description": "bar"
]
let paramsArray = [ "one", "two" ]
let paramsJSON = JSON(paramsArray)
let paramsString = paramsJSON.rawString(encoding: NSUTF8StringEncoding, options: nil)

SWIFT 3 UPDATE

 let paramsJSON = JSON(paramsArray)
 let paramsString = paramsJSON.rawString(String.Encoding.utf8, options: JSONSerialization.WritingOptions.prettyPrinted)!

JSON strings, which are good for transport, don't come up often because you can JSON encode an HTTP body. But one potential use-case for JSON stringify is Multipart Post, which AlamoFire nows supports.

Swift 3.0 - 4.0 version

do {

    //Convert to Data
    let jsonData = try JSONSerialization.data(withJSONObject: dictionaryOrArray, options: JSONSerialization.WritingOptions.prettyPrinted)

    //Convert back to string. Usually only do this for debugging
    if let JSONString = String(data: jsonData, encoding: String.Encoding.utf8) {
       print(JSONString)
    }

    //In production, you usually want to try and cast as the root data structure. Here we are casting as a dictionary. If the root object is an array cast as [Any].
    var json = try JSONSerialization.jsonObject(with: jsonData, options: JSONSerialization.ReadingOptions.mutableContainers) as? [String: Any]


} catch {
    print(error.description)
}

The JSONSerialization.WritingOptions.prettyPrinted option gives it to the eventual consumer in an easier to read format if they were to print it out in the debugger.

Reference: Apple Documentation

The JSONSerialization.ReadingOptions.mutableContainers option lets you mutate the returned array's and/or dictionaries.

Reference for all ReadingOptions: Apple Documentation

NOTE: Swift 4 has the ability to encode and decode your objects using a new protocol. Here is Apples Documentation, and a quick tutorial for a starting example.