whatAmI.ts

function whatAmI(thing: string|RegExp|null|number[]|Date) {
  if (thing instanceof RegExp || typeof thing === "string") {
    // Here, TypeScript knows that thing is RegExp|string
    if (typeof thing !== "string")
      console.log("Ahh, so it's a RegExp: " + thing.toString());
    else
      console.log("It's a string of length " + thing.length);
  } else {
    // Here, TypeScript knows that thing is null|number|Date
    if (thing == null)
      console.log("Aha! You just gave me a null.");
    else if (Array.isArray(thing))
      console.log("Oh, it's an array of numbers: " + 
        thing.map(n => n.toPrecision(3)).join(", "));
    else
      console.log("So, it's a date in " + thing.getFullYear());
  }
}