A simple Swift library for working with generic JSON structures

What's New

GenericJSON 3.0


Generic JSON

Generic JSON makes it easy to deal with freeform JSON strings without creating a separate, well-typed structure.

Codable and freeform JSON

Swift 4 introduced a new JSON encoding and decoding machinery represented by the Codable protocol. The feature is very nice and very type-safe, meaning it’s no longer possible to just willy-nilly decode a JSON string pulling random untyped data from it. Which is good™ most of the time–but what should you do when you do want to just willy-nilly encode or decode a JSON string without introducing a separate, well-typed structure for it?

For example:

/* error:
 *  heterogeneous collection literal could only be inferred to '[String : Any]';
 *  add explicit type annotation if this is intentional. */
let json = [
   "foo": "foo",
   "bar": 1,

/* Okay then: */
let json: [String: Any] = [
   "foo": "foo",
   "bar": 1,

/* But: fatal error: Dictionary<String, Any> does not conform to Encodable because Any does not conform to Encodable. */
let encoded = try JSONEncoder().encode(json)

So this doesn’t work very well. Also, the json value can’t be checked for equality with another, although arbitrary JSON values should support equality. Enter JSON.


Create a JSON structure

let json: JSON = [
   "foo": "foo",
   "bar": 1,

/* #"{"bar":1,"foo":"foo"}"# */
let str = try String(data: try JSONEncoder().encode(json), encoding: .utf8)!
let hopefullyTrue = (json == json) /* true! */

Convert Encodable objects into a generic JSON structure

struct Player: Codable {
   let name: String
   let swings: Bool

let val = try JSON(encodable: Player(name: "Miles", swings: true))
val == [
   "name": "Miles",
   "swings": true,
] /* true */

Query Values

Consider the following JSON structure:

let json: JSON = [
   "num": 1,
   "str": "baz",
   "bool": true,
   "obj": [
      "foo": "jar",
      "bar": 1,

Querying values can be done using optional property accessors, subscripting or dynamic member subscripting:

/* Property accessors. */
if let str = json.objectValue?["str"]?.stringValue {  }
if let foo = json.objectValue?["obj"]?.objectValue?["foo"]?.stringValue {  }

/* Subscripting. */
if let str = json["str"]?.stringValue {  }
if let foo = json["obj"]?["foo"]?.stringValue {  }

/* Dynamic member subscripting. */
if let str = json.str?.stringValue {  }
if let foo = json.obj?.foo?.stringValue {  }

You may even drill through nested structures using a dot-separated key path:

let val = json[keyPath: ""] /* "jar" */


  • Swift Tools 5.8.0
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Last updated: Mon Apr 15 2024 15:09:00 GMT-0900 (Hawaii-Aleutian Daylight Time)