Recursive types in TypeScript to defining a type that describes any allowable JSON value.

recursive-types.ts

// https://www.typescriptlang.org/docs/handbook/release-notes/typescript-4-1.html#recursive-conditional-types

type Primitive =  string | number | boolean | null
type JSONObject = {[k: string] : JSONValue}
type JSONArray = JSONValue[]
type JSONValue = 
| Primitive
| JSONObject
| JSONArray

function isJSON(arg: JSONValue) {}

isJSON("hello")
isJSON([4, 8, 15, 16, 23, 42])
isJSON({ greeting: "hello" })
isJSON(false)
isJSON(true)
isJSON(null)
isJSON({ a: { b: [2, 3, "foo"] } })

isJSON(() => "")
isJSON(class {})
isJSON(undefined)
isJSON(new BigInt(143))
isJSON(isJSON)