Even Swiftier JSON Handler
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Latest commit e077d1b Jul 14, 2018


OBSOLETED by swift-sion

swift-sion can do all what swift-json can do plus:

Swift 4.1 MIT LiCENSE build status


Handle JSON safely, fast, and expressively. Completely rewritten from ground up for Swift 4 and Swift Package Manager.


import JSON
let json:JSON = ["swift":["safe","fast","expressive"]]


This module is a lot like SwiftyJSON in functionality. It wraps JSONSerialization nicely and intuitively. But it differs in how to do so.

  • SwiftyJSON's JSON is struct. JSON of this module is enum
  • SwiftyJSON keeps the output of JSONSerialization.jsonObject in its stored property and convert its value runtime. JSON of this module is static. Definitely Swiftier.
  • SwiftyJSON's JSON.swift is over 1,500 lines while that of this module is less than 400 (as of this writing). Since it is so compact you can use it without building framework.


You can build JSON directly as a literal…

let json:JSON = [
    "null":     nil,
    "bool":     true,
    "int":      -42,
    "double":   42.195,
    "string":   "漢字、カタカナ、ひらがなと\"引用符\"の入ったstring😇",
    "array":    [nil, true, 1, "one", [1], ["one":1]],
    "object":   [
        "null":nil, "bool":false, "number":0, "string":"" ,"array":[], "object":[:]

…or String…

let str = """
    "null":     null,
    "bool":     true,
    "int":      -42,
    "double":   42.195,
    "string":   "漢字、カタカナ、ひらがなと\\"引用符\\"の入ったstring😇",
    "array":    [null, true, 1, "one", [1], {"one":1}],
    "object":   {
        "null":null, "bool":false, "number":0, "string":"" ,"array":[], "object":{}


…or a content of the URL…


…or by decoding Codable data…

import Foundation
struct Point:Hashable, Codable { let (x, y):(Int, Int) }
var data = try JSONEncoder().encode(Point(x:3, y:4))
try JSONDecoder().decode(JSON.self, from:data)


once you have the JSON object, converting to other formats is simple.

to JSON string, all you need is stringify it. .description or "(json)" would be enough.

"\(json)"				// JSON is CustomStringConvertible

If you need Data, simply call .data.


If you want to feed it (back) to Foundation framework, call .jsonObject

let json4plist = json.pick{ !$0.isNull }    // remove null
let plistData = try PropertyListSerialization.data (
print(String(data:plistData, encoding:.utf8)!)


a blank JSON Array is as simple as:

var json = JSON([])

and you can assign elements like an ordinary array

json[0] = nil
json[1] = true
json[2] = 1

note RHS literals are NOT nil, true and 1 but .Null, .Bool(true) and .Number(1). Therefore this does NOT work

let one = "one"
json[3] = one // error: cannot assign value of type 'String' to type 'JSON'

In which case you do this instead.

json[3].string = one

They are all getters and setters.

json[1].bool   = true
json[2].number = 1
json[3].string = "one"
json[4].array  = [1]
json[5].object = ["one":1]

As a getter they are optional which returns nil when the type mismaches.

json[1].bool    // Optional(true)
json[1].number  // nil

Therefore, you can mutate like so:

json[2].number! += 1            // now 2
json[3].string!.removeLast()    // now "on"
json[4].array!.append(2)        // now [1, 2]
json[5].object!["two"] = 2      // now ["one":1,"two":2]

When you assign values to JSON array with an out-of-bound index, it is automatically streched with unassigned elements set to null, just like an ECMAScript Array

json[10] = false	// json[6...9] are null

As you may have guessed by now, a blank JSON object(dictionary) is:

json = JSON([:])

And manipulate intuitively like so.

json["null"]    = nil		// not null
json["bool"]    = false
json["number"]  = 0
json["string"]  = ""
json["array"]   = []
json["object"]  = [:]		// not {}

deep traversal

JSON is a recursive data type. For recursive data types, you need a recursive method that traverses the data deep down. For that purpuse, JSON offers .pick and .walk.

.pick is a ".deepFilter" that filters recursively. You've already seen it above. It takes a filter function of type (JSON)->Bool. That function is applied to all leaf values of the tree and leaves that do not meet the predicate are pruned.

// because property list does not accept null
let json4plist = json.pick{ !$0.isNull }

.walk is a deepMap that transforms recursively. This one is a little harder because you have to consider what to do on node and leaves separately. To make your life easier three different versions of .walk are provided. The first one just takes a leaf node.

// square all numbers and leave anything else 
JSON([0,[1,[2,3,[4,5,6]]], true]).walk {
    guard let n = $0.number else { return $0 }
    return JSON(n * n)

The second forms just takes a node. Instead of explaining it, let me show you how .pick is implemented by extending JSON with .select that does exactly the same as .pick.

extension JSON {
    func select(picker:(JSON)->Bool)->JSON {
        return self.walk{ node, pairs, depth in
            switch node.type {
            case .array:
                return .Array(pairs.map{ $0.1 }.filter({ picker($0) }) )
            case .object:
                var o = [Key:Value]()
                pairs.filter{ picker($0.1) }.forEach{ o[$0.0.key!] = $0.1 }
                return .Object(o)
                return .Error(.notIterable(node.type))

And the last form takes both. Unlike the previous ones this one can return other than JSON. Here is a quick and dirty .yaml that emits a YAML.

extension JSON {
    var yaml:String {
        return self.walk(depth:0, collect:{ node, pairs, depth in
            let indent = Swift.String(repeating:"  ", count:depth)
            var result = ""
            switch node.type {
            case .array:
                guard !pairs.isEmpty else { return "[]"}
                result = pairs.map{ "- " + $0.1}.map{indent + $0}.joined(separator: "\n")
            case .object:
                guard !pairs.isEmpty else { return "{}"}
                result = pairs.sorted{ $0.0.key! < $1.0.key! }.map{
                    let k = $0.0.key!
                    let q = k.rangeOfCharacter(from: .newlines) != nil
                    return (q ? k.debugDescription : k) + ": "  + $0.1
                    }.map{indent + $0}.joined(separator: "\n")
                break   // never reaches here
            return "\n" + result
            if $0.isNull { return  "~" }
            if let s = $0.string {
                return s.rangeOfCharacter(from: .newlines) == nil ? s : s.debugDescription
            return $0.description

Protocol Conformance

  • JSON is Equatable so you can check if two JSONs are the same.
JSON(string:foo) == JSON(urlString:"https://example.com/whereever")
  • JSON is Hashable so you can use it as a dictionary key.

  • JSON is ExpressibleBy*Literal. That's why you can initialize w/ variable:JSON construct show above.

  • JSON is CustomStringConvertible whose .description is always a valid JSON.

  • JSON is Codable. You can use this module instead of JSONEncoder.

  • JSON is Sequence. But when you iterate, be careful with the key.

let ja:JSON = [nil, true, 1, "one", [1], ["one":1]]
// wrong!
for v in ja {
// right!
for (i, v) in ja {
	// i is NOT an Integer but KeyType.Index.
	// To access its value, say i.index
let jo:JSON = [
    "null":nil, "bool":false, "number":0, "string":"",
    "array":[], "object":[:]
for (k, v) in jo {
	// k is NOT an Integer but KeyType.Key.
	// To access its value, say i.key

That is because swift demands to return same Element type. If you feel this counterintuitive, you can simply use .array or .object:

for v in ja.array! {
	// ...
for (k, v) in jo.object! {
	// ...

Error handling

Once inited, JSON never fails. That is, it never becomes nil. Instead of being failable or throwing exceptions, JSON has a special value .Error(.ErrorType) which propagates across the method invocations. The following code examines the error should it happen.

if let e = json.error {
	if let nsError = e.nsError {
		// do anything with nsError



$ git clone https://github.com/dankogai/swift-json.git
$ cd swift-json # the following assumes your $PWD is here
$ swift build



$ scripts/run-repl.sh


$ swift build && swift -I.build/debug -L.build/debug -lJSON

and in your repl,

  1> import JSON
  2> let json:JSON = ["swift":["safe","fast","expressive"]]
json: JSON.JSON = Object {
  Object = 1 key/value pair {
    [0] = {
      key = "swift"
      value = Array {
        Array = 3 values {
          [0] = String {
            String = "safe"
          [1] = String {
            String = "fast"
          [2] = String {
            String = "expressive"


Xcode project is deliberately excluded from the repository because it should be generated via swift package generate-xcodeproj . For convenience, you can

$ scripts/prep-xcode

And the Workspace opens up for you with Playground on top. The playground is written as a manual.

iOS and Swift Playground

Unfortunately Swift Package Manager does not support iOS. To make matters worse Swift Playgrounds does not support modules. But don't worry. This module is so compact all you need is copy JSON.swift.

In case of Swift Playgrounds just copy it under Sources folder. If you are too lazy just run:

$ scripts/ios-prep.sh

and iOS/JSON.playground is all set. You do not have to import JSON therein.

From Your SwiftPM-Managed Projects

Add the following to the dependencies section:

  url: "https://github.com/dankogai/swift-json.git", from: "4.0.0"

and the following to the .target argument:

  name: "YourSwiftyPackage",
  dependencies: ["JSON"])

Now all you have to do is:

import JSON

in your code. Enjoy!


Swift 4.1 or better, OS X or Linux to build.