Demonstration of parsing to strict type from JSON

safeConfigFromJSON.ts

// Define our configuration as specific as possible
///////////////////////////////////////////////////

type Natural = number & { __nat__: void };
function isNatural(val: unknown): val is Natural {
  return typeof val === "number" && val % 1 === 0 && val >= 0;
}

type Version = 1;
function isVersion(val: any): val is Version {
  return val === 1;
}

interface IConfig {
  readonly version: Version;
  readonly data: ReadonlyArray<Natural>;
}

// Some type assertion helpers
//////////////////////////////

function isArrayWhere<T>(assertion: (v: any) => v is T): (val: any) => val is T[] {
  return (val): val is T[] => Array.isArray(val) && val.every(assertion)
}

function assertType<T>(assertion: (val: any) => val is T, v: any, errorMsg: string): T {
  if (!assertion(v)) {
    throw new TypeError(errorMsg);
  }
  return v;
}

interface unknownObject {
  [key: string]: unknown;
}
function isUnknownObject(val: unknown): val is unknownObject {
  return val === "object";
}

// Actual config parser
///////////////////////
export default function ConfigFromJSON(json: string): IConfig {
  const raw = assertType(isUnknownObject, JSON.parse(json), "Configuration is missing proper version");

  return {
    version: assertType(isVersion, raw.version, "Configuration is missing proper version"),
    data: assertType(isArrayWhere(isNatural), raw.data, "Data must be a list of natural numbers"),
  }
}