Predicate functions & satisfies operator

function isNumber(x: unknown): x is number {
  return typeof x === "number"
}

function demo(a: unknown) {
  if (isNumber(a)) {
    // inside: a is number
    return a.toFixed(2)
  }
  return "not a number"
}

type User = { id: number; name: string }

function isUser(v: unknown): v is User {
  return typeof v === "object" && v !== null &&
    "id" in v && "name" in v &&
    typeof (v as any).id === "number" &&
    typeof (v as any).name === "string"
}

function printUser(u: unknown) {
  if (isUser(u)) {
    console.log(u.name.toUpperCase())
  } else {
    console.log("invalid user")
  }
}

function fetchAndPrint(json: string) {
  const value: unknown = JSON.parse(json)
  if (isUser(value)) {
    // value narrowed to User
    console.log(`User #${value.id}: ${value.name}`)
  } else {
    console.log("Bad payload")
  }
}

type Config = { mode: "dev" | "prod"; retries: number }

const cfg = {
  mode: "prod",
  retries: 3,
  // timeout: 1000, // Error with satisfies if not in type
} satisfies Config

// cfg.mode is still the literal "prod" (not widened to string)
// and adding unknown keys raises an error at compile-time

type Route = { path: `/${string}`; method: "GET" | "POST" }

const routes = [
  { path: "/", method: "GET" },
  { path: "/login", method: "POST" }
] as const satisfies ReadonlyArray<Route>

// each element is checked against Route, literals preserved for DX