cd src npm install tsc main.ts node main.js 

import * as Json from "./JsonDecoder"; type SomeDataType = { id: string, name: { firstname: string, lastname: string }, hobbies: Array<string>, numbersAndStrings: Array<number|string>, optionalBoolean?: boolean }; const decodeSomeDataType = Json.object<SomeDataType>({ id: Json.string, name: Json.object({ firstname: Json.string, lastname: Json.string }), hobbies: Json.array(Json.string), numbersAndStrings: Json.array(Json.oneOf<number|string>([Json.number, Json.string])), optionalBoolean: Json.maybe(undefined, Json.boolean) }); console.log(decodeSomeDataType.run({ id: "xy-12342", name: { firstname: "John", lastname: "Doe" }, hobbies: [ "Tennis", "Sleeping" ], numbersAndStrings: [ 1, 2, "foo", 3, "bar" ] })); 

string : Decoder<string> number : Decoder<number> boolean : Decoder<boolean> maybe: <a>(defaultValue: a, decoder: Decoder<a>): Decoder<a> nullable: <a>(decoder: Decoder<a>): Decoder<null|a> isnull: <a>(defaultValue:a): Decoder<a> isundefined: <a>(defaultValue:a): Decoder<a> isexactly: <a>(value:a): Decoder<a> array: (decoder: Decoder<a>): Decoder<Array<a>> keyValues: <a>(decoder: Decoder<a>): Decoder<{[name:string]: a}> oneOf: (decoders: Array<Decoder<a>>): Decoder<a> constant: <a>(value:a): Decoder<a> lazy: <a>(mkDecoder: () => Decoder<a>): Decoder<a> object: <a>(decoders: DecoderObject<a>): Decoder<a> type DecoderObject<a> = {[t in keyof a]: Decoder<a[t]>} decoder.map(f) decoder.then(f) decoder.run(json) decoder.runThen(json, onOk, onErr) decoder.runPromise(json) : Promise<a>