Here's a quick extension for [String: AnyObject] dictionaries in Swift. The output needs to be in the following format (order and spacing don't matter):

{"key": "val", "keywdict": {"anotherKey": 100, "Key2": "Val2"}, "strings": ["string", "another"]}

I am wondering if there's a better way, or if this can be improved.

extension Dictionary //...

func toJSONString() -> String {
  var pcs = [String]()
  for (key, val) in self {
    var valStr = ""
    if let val = val as? String {
      valStr = "\"\(val)\""
    } else if let val = val as? Dictionary<String, AnyObject> {
      valStr = val.toJSONString() // recursion for nested dictionaries
    } else if let val = val as? Array<String> {
      let tmpStr = "\",\"".join(val)
      valStr = "[\"\(tmpStr)\"]"
    } else if let val = val as? NSNumber {
      valStr = "\(val)"
  return "{" + ",".join(pcs) + "}"
  • 1
    \$\begingroup\$ Welcome to CodeReview, Andrew. I hope you get some fine answers. \$\endgroup\$
    – Legato
    Apr 17, 2015 at 16:27

1 Answer 1


Your dictionary to JSON converter has some serious issues:

  • The special characters " and \in strings are not properly escaped:

    [ "a\"b" : "c\\d"].toJSONString()

    produces "{"a"b":"c\d"}", correct would be {"a\"b":"c\\d"} (see for example http://json.org).

  • Only arrays of strings are implemented, but not arrays of numbers or arrays of arrays or dictionaries.
  • Value types which are not covered by your code just print as an empty string instead of causing an error.
  • Boolean values are simply treated as numbers:

    [ "key" : true].toJSONString()

    gives {"key":1}, correct would be {"key":true}

While this could all be fixed, it probably isn't worth the effort because there is a Foundation class NSJSONSerialization which already does exactly this:

var dict : [String: AnyObject] = [:]
dict["a\"b"] = "c\\d"
dict["strings"] = ["string", "another"]
dict["keywdict"] = [ "anotherKey": 100, "Key2": "Val2"]
dict["numbers"] = [ 1, 2, 3]
dict["bools"] = [ true, false]

var error : NSError?
if let jsonData = NSJSONSerialization.dataWithJSONObject(dict, options: nil, error: &error) {
    let jsonString = NSString(data: jsonData, encoding: NSUTF8StringEncoding)! as String
} else {
    println("Error in JSON conversion: \(error!.localizedDescription)")


  • \$\begingroup\$ Thanks for the input, @Martin R. For my implementation (which I should have stated) I knew that I only needed to handle [String] arrays and numerics, though I totally goofed on escaping characters! The NSJSONSerialization implementation is CLEARLY the better solution, and I totally missed it. Thanks again. \$\endgroup\$ Apr 17, 2015 at 20:00

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