TypeScript Magic

ts-magic.ts

/* Combined Union Type + Array of allowed values */
const ServerEnvironments = ['development', 'test', 'stage', 'production'] as const;

// equivalent to `type ServerEnvironments = 'development' | 'test' | 'stage' | 'production'
type ServerEnvironments = typeof ServerEnvironments[number];

const serverEnv: ServerEnvironments = 'development';

const isValidServerEnv = (env: string): env is ServerEnvironments => ServerEnvironments.include(env);

const parseServerEnv = (env: string = process.env.SERVER_ENV): ServerEnvironments => isValidServerEnv(env) ? env : ServerEnvironments[2];

/* Non-class constructor + instance combined type + value */
type Person = { name: string; age: number; married: boolean; }

type PersonConstructor = {
  (rawData: Record<string, unknown>) => Person;
  fromJSON(jsonData: Record<string, unknown>) => Person;
};

const Person: PersonConstructor = ({ name, age, married = false }) => ({ name, age, married });
Person.fromJSON = () => {...};